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    policy of the United Nations Environment Programme, the International
    Labour Organisation, or the World Health Organization.

    Published under the joint sponsorship of
    the United Nations Environment Programme,
    the International Labour Organisation,
    and the World Health Organization

    First draft prepared by Dr M.E. Meek,
    Environmental Health Directorate, Health
    and Welfare, Ottawa, Canada

    World Health Orgnization
    Geneva, 1993

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    WHO Library Cataloguing in Publication Data

    Selected synthetic organic fibres.

        (Environmental health criteria ; 151)

        1.Carbon - adverse effects  2.Nylons - adverse effects
        3.Polyenes - adverse effects 4.Environmental exposure 

        ISBN 92 4 157151 9        (NLM Classification: QV 627)
        ISSN 0250-863X

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    1. SUMMARY

         1.1. Identity, physical and chemical properties
         1.2. Sources of human and environmental exposure
         1.3. Environmental levels and human exposure
         1.4. Deposition, clearance, retention, durability and
         1.5. Effects on experimental animals and  in vitro
               test systems
         1.6. Effects on humans
         1.7. Summary of evaluation


         2.1. Identity, physical and chemical properties
               2.1.1. Carbon/graphite fibres
               2.1.2. Aramid fibres
               2.1.3. Polyolefin fibres
         2.2. Production methods
               2.2.1. Carbon/graphite fibres
               2.2.2. Aramid fibres
               2.2.3. Polyolefin fibres
         2.3. Sampling and analytical methods


         3.1. Production
               3.1.1. Carbon/graphite fibres
               3.1.2. Aramid fibres
               3.1.3. Polyolefin fibres
         3.2. Uses
               3.2.1. Carbon/graphite fibres
               3.2.2. Aramid fibres
               3.2.3. Polyolefin fibres
         3.3. Emissions into the environment
               3.3.1. Fibre emissions
               3.3.2. Decomposition products


         4.1. Occupational environment
               4.1.1. Carbon/graphite fibres
               Processing of composites
               4.1.2. Aramid fibres
               End-use processing and processing
                                of composites
               4.1.3. Polyolefin
         4.2. General environment


         5.1. Introduction
         5.2. Studies in experimental animals
               5.2.1. Carbon/graphite fibres
               5.2.2. Aramid fibres
               5.2.3. Polyolefins
         5.3.  In vitro solubility studies


         6.1. Experimental animals
               6.1.1. Introduction
               6.1.2. Carbon/graphite fibres
               Intratracheal administration
               Intraperitoneal administration
               Dermal administration
               6.1.3. Aramid fibres
               Intratracheal administration
               Intraperitoneal administration
               6.1.4. Polyolefin fibres
               Intratracheal administration
               Intraperitoneal administration
         6.2.  In vitro studies
               6.2.1. Carbon fibres
               6.2.2. Aramid fibres
               6.2.3. Polyolefin fibres


         7.1. Carbon/graphite fibres
         7.2. Aramid fibres


         8.1. Exposure
         8.2. Health effects



         10.1. Sampling and analytical methods
         10.2. Exposure measurement and characterization
         10.3. Human epidemiology
         10.4. Toxicology studies







    Dr D.M. Bernstein, Geneva, Switzerland

    Dr J.M. Dement, Office of Occupational Health and Technical
       Services, National Institute of Environmental Health Sciences,
       Research Triangle Park, North Carolina, USA

    Dr P.T.C. Harrison, Department of the Environment, London, United

    Professor T. Higashi, Department of Work Systems & Health,
       Institute of Industrial Ecological Sciences, University of
       Occupational & Environmental Health, Kitakyishu-shi, Japan

    Dr I. Mangelsdorf, Institute for Toxicology, GSF München,
       München-Neuherberg, Germany

    Dr E. McConnell, Raleigh, North Carolina, USA

    Mrs M. Meldrum, Health and Safety Executive, Bootle, Merseyside,
       United Kingdom

    Professor M. Neuberger, Institute of Environmental Hygiene,
       University of Vienna, Kinderspitalgasse, Vienna, Austria

    Dr R.P. Nolan, Environmental Sciences Laboratory, Brooklyn College,
       Brooklyn, New York, USA

    Dr V. Vu, Health Effects Branch, Health and Environmental Review
       Division, Office of Pollution Prevention and Toxics, US
       Environmental Protection Agency, Washington, DC, USA

    Mr R. Zumwalde, Division of Standards Development and Technology
       Transfer, National Institute for Occupational Safety and Health,
       Robert A. Taft Laboratories, Cincinnati, Ohio, USA


    Dr T. Hesterberg, Health Safety and Environment Department, Mountain
       Technical Center, Schuller International Inc., Littleton,
       Colorado, USA

    Dr E.A. Merriman, DuPont Fibers, Wilmington, Delaware, USA

    Dr J.W. Rothuizen, Rothuizen Consulting, Genolier, Switzerland

    Dr D.B. Warheit, E.I. Du Pont de Nemours and Co., Haskell Laboratory
       for Toxicology and Industrial Medicine, Newark, Delaware, USA


    Dr M.E. Meek, Environmental Health Directorate, Health Protection
       Branch, Health and Welfare, Tunney's Pasture, Ottawa, Ontario,
       Canada  (Rapporteur)

    Professor F. Valic, IPCS Consultant, World Health Organization,
       Geneva, Switzerland;  also Vice-Rector, University of Zagreb,
       Zagreb, Croatia  (Responsible Officer and Secretary)


         Every effort has been made to present information in the
    criteria monographs as accurately as possible without unduly
    delaying their publication.  In the interest of all users of the
    Environmental Health Criteria monographs, readers are kindly
    requested to communicate any errors that may have occurred to the
    Director of the International Programme on Chemical Safety, World
    Health Organization, Geneva, Switzerland, in order that they may be
    included in corrigenda.

                                   *  *  *

         A detailed data profile and a legal file can be obtained from
    the International Register of Potentially Toxic Chemicals, Case
    postale 356, 1219 Châtelaine, Geneva, Switzerland (Telephone
    No. 9799111).

                                   *  *  *

         This publication was made possible by grant number 5 U01
    ES02617-14 from the National Institute of Environmental Health
    Sciences, National Institutes of Health, USA.


         A Task Group on Environmental Health Criteria for Selected
    Synthetic Organic Fibres met at the British Industrial and
    Biological Research Association (BIBRA), Carshalton, Surrey, United
    Kingdom, from 28 September to 2 October 1992.  Dr. S.E. Jaggers
    opened the meeting on behalf of the host institute and greeted the
    participants on behalf of the Department of Health.  Professor F.
    Valic welcomed the participants on behalf of the heads of the three
    cooperating organizations of the IPCS (UNEP/ILO/WHO).  The Task
    Group reviewed and revised the draft monograph, made an evaluation
    of the direct and indirect risks for human health from exposure to
    the synthetic organic fibres reviewed, and made recommendations for
    health protection and further research.

         The first draft was prepared by Dr M.E. Meek, Environmental
    Health Directorate, Health and Welfare, Ottawa, Canada.  Professor
    F. Valic was responsible for the overall scientific content and for
    the organization of the meeting, and Dr P.G. Jenkins, IPCS, for the
    technical editing of the monograph.


    CKSCC          cystic keratinizing squamous cell carcinoma

    FEV            forced expiratory volume

    GC             gas chromatography

    MMAD           mass median aerodynamic diameter

    MMMF           man-made mineral fibre

    MS             mass spectrophotometry

    PAN            polyacrylonitrile

    PCOM           phase contrast optical microscopy

    SEM            scanning electron microscopy

    SPF            specific pathogen free

    TGA            thermogravimetric analysis

    TWA            time-weighted average


         This document is a review of occupational and environmental
    exposure to selected synthetic organic fibres.  The particular
    fibres covered in this document have been selected because they are
    the only ones for which some toxicological data are available, and
    they are probably of most importance in terms of production volumes
    and potential for human exposure.  Considerations relating to
    possible future uses and applications of synthetic organic fibres
    have not been discussed, nor have the possible consequences of
    exposure to secondary products, combustion products, etc.  The Task
    Group noted the complexity of the working environment in the
    manufacture and use of synthetic organic fibres, which may involve
    exposures to a range of chemical substances and non-fibrous dusts.

    1.  SUMMARY

    1.1  Identity, physical and chemical properties

         Carbon/graphite fibres are filamentary forms of carbon produced
    by high-temperature processing of one of three precursor materials:
    rayon (regenerated cellulose), pitch (coal tar or petroleum residue)
    or polyacrylonitrile (PAN). Nominal diameters of carbon fibres range
    between 5 and 15 µm.  Carbon fibres are flexible and electrically
    and thermally conductive, and in high performance varieties have
    high Young's modulus (coefficient of elasticity measuring the
    softness or stiffness of the material) and tensile strength. They
    are corrosion resistant, lightweight, refractile and chemically
    inert (except to oxidation), and have a high degree of stability to
    traction forces, low thermal expansion and density, and high
    abrasion and wear resistance.

         Aramid fibres are formed by the reaction of aromatic diamines
    and aromatic diacid chlorides. They are produced as continuous
    filaments, staple and pulp. There are two main types of aramid
    fibres, para- and meta-aramid, both with a nominal diameter of
    12-15 µm. Para-aramid fibres can have fine-curled, tangled fibrils
    within the respirable size range (< 1 µm in diameter) attached to
    the surface of the core fibre. These fibrils may be detached and
    become airborne upon abrasion during manufacture or use. Generally,
    aramid fibres exhibit medium to very high tensile strength, medium
    to low elongation, and moderate to very high Young's modulus. They
    are resistant to heat, chemicals and abrasion.

         Polyolefin fibres are long-chain polymers composed of at least
    85% by weight of ethylene, propylene or other olefin units;
    polyethylene and polypropylene are used commercially. Except for
    some types such as microfibre, the nominal diameters of most classes
    of polyolefin fibres are sufficiently large that few are within the
    respirable size range.

         Polyolefins are extremely hydrophobic and unreactive. Their
    tensile strengths are considerably less than those of carbon or
    aramid fibres and they are relatively flammable, melting at
    temperatures between 100 and 200 °C.

         Methods developed for counting mineral fibres have been used
    for industrial hygiene monitoring of synthetic organic fibres. 
    However, these methods have not been validated for this purpose. 
    Factors such as electrostatic properties, solubility in mounting
    media and refractive index may cause difficulties when using such

    1.2  Sources of human and environmental exposure

         The estimated worldwide production of carbon and graphite
    fibres was in excess of 4000 tonnes in 1984. For aramid it was more
    than 30 000 tonnes in 1989, and for polyolefin fibres more than
    182 000 tonnes (USA only). Carbon and aramid fibres are used
    principally in advanced composite materials in aerospace, military
    and other industries to improve strength, stiffness, durability,
    electrical conductivity or heat resistance. Polyolefin fibres are
    typically used in textile applications.

         Exposures to synthetic organic fibres have been documented in
    the occupational environment. Synthetic organic fibres can be
    released into the environment during production, processing or
    combustion of composites and during disposal. Very few data are
    available concerning actual releases of these materials into the

         Available data on the transport, distribution and
    transformation of organic fibres in the environment are restricted
    to identification of products of municipal incineration of refuse
    from carbon fibre-containing composites and pyrolysis decomposition
    products of carbon fibre and aramids. During simulation of municipal
    incineration, both the diameters and lengths of carbon fibres were
    reduced. Principal pyrolysis decomposition products of carbon and
    aramid fibres include aromatic hydrocarbons, carbon dioxide, carbon
    monoxide and cyanides.

    1.3  Environmental levels and human exposure

         Synthetic organic fibre dusts are released in the workplace
    during operations such as fibre forming, winding, chopping, weaving,
    cutting, machining and composite formation and handling.

         In the case of carbon/graphite fibres, respirable fibre
    concentrations are generally less than 0.1 fibres/ml but
    concentrations of up to 0.3 fibres/ml have been measured close to
    chopping or winding operations. Fibres may also be released during
    machining (drilling, sawing, etc.) of carbon fibre composites,
    although most of the respirable material thus produced is

         Average airborne concentrations of para-aramid fibrils in the
    workplace are reported to be less than 0.1 fibrils/ml in filament
    operations and less than 0.2 fibrils/ml in floc cutting and pulp
    operations. During staple yarn processing, average airborne fibril
    concentrations are typically below 0.5 fibrils/ml, but levels as
    high as approximately 2.0 fibrils/ml have been reported. Other
    end-use workplace exposures are typically below 0.1 fibrils/ml on
    average with peak exposures of 0.3 fibrils/ml. Special potential for
    exposure was demonstrated for waterjet cutting of composites, levels

    being as high as 2.91 fibrils/ml. Particles of mean aerodynamic
    diameter of 0.21 µm have been generated during laser cutting of
    epoxy plastics reinforced with aramid fibres, but the fibre content
    of the dust was not reported. Certain volatile organic compounds
    (including benzene, toluene, benzonitrile and styrene) and other
    gases (hydrogen cyanide, carbon monoxide and nitrogen dioxide) are
    also produced during such operations.

         Limited air monitoring data from a facility producing
    polypropylene fibres indicate maximum airborne levels for fibres
    longer than 5 µm of 0.5 fibres/ml, with most values being less than
    0.1 fibres/ml. Scanning electron microscopy showed that airborne
    fibre sizes range from 0.25 to 3.5 µm in diameter and 1.7 to 69 µm
    in length. In a single ambient sample collected near a carbon fibre
    weaving plant, a concentration of 0.0003 fibres/ml was detected. 
    The average dimensions of the fibres were 706 µm by 3.9 µm. The
    release of carbon fibre at the crash site of two military aircraft,
    following combustion of carbon fibre composite used in construction,
    has also been reported. No other relevant information on
    concentrations in the environment was identified.

    1.4  Deposition, clearance, retention, durability and translocation

         Few data on specific synthetic organic fibres have been ident
    ified. The data on para-aramid fibres (Kevlar) indicate that, when
    inhaled, these fibres are deposited at alveolar duct bifurcations.
    There is also evidence of translocation to the tracheobronchial
    lymph nodes.

    1.5  Effects on experimental animals and  in vitro test systems

         For the synthetic organic fibre types reviewed here, there is a
    dearth of good quality data from relevant experimental studies.

         There are no adequate studies in which the fibrogenic or
    carcinogenic potential of carbon/graphite fibres has been examined.
    Effects following short-term inhalation exposure (days) of rats to
    respirable-size pitch-based fibres included inflammatory responses,
    increased parenchymal cell turnover and minimal type II alveolar
    cell hyperplasia. Available data from an intratracheal instillation
    and an intraperitoneal injection study are considered inadequate for
    assessment owing to the lack of characterization of the test
    materials and lack of adequate documentation of protocol and
    results. A mouse skin painting study on four carbon fibre types
    suspended in benzene was inadequate for the evaluation of
    carcinogenic activity.

         In the case of paraaramid fibres, the majority of data is
    derived from experiments on Kevlar. Short-term (2 week) inhalation
    studies of Kevlar dust have resulted in a pulmonary macrophage
    response which decreased in severity after exposure ceased.
    Short-term studies of ultrafine Kevlar fibrils have shown a similar

    macrophage reaction and patchy thickening of the alveolar ducts.
    Both lesions again decreased after exposure, but a minimal amount of
    fibrosis was present 3-6 months later. A two-year inhalation study
    of Kevlar fibrils in rats induced exposure-related lung fibrosis (at
    > 25 fibres/ml) and lung neoplasms (11% at 400 fibres/ml and 6% at
    100 fibres/ml in female rats; 3% at 400 fibres/ml in male rats) of
    an unusual type (cystic keratinizing squamous cell carcinoma).
    Increased mortality due to lung toxicity was observed at the highest
    concentration, indicating that the Maximum Tolerated Dose had been
    exceeded. There is consider able debate concerning the biological
    potential of these lesions and their relevance to humans. The full
    carcinogenic potential of the fibrils may not have been revealed in
    this study because it was terminated after 24 months.

         Intratracheal instillation of a single dose of shredded Nomex
    paper (2.5 mg) containing fibres with diameters of 2 to 30 µm
    resulted in a non-specific inflammatory response. A granulomatous
    reaction developed two years post-exposure.  Intratracheal
    instillation of a single dose of 25 mg Kevlar resulted in a
    non-specific inflammatory response which subsided within about one
    week. A granulomatous reaction and a minimal amount of fibrosis were
    observed later.

         In three studies, intraperitoneal injection of Kevlar fibres
    (up to 25 mg/kg) resulted in a granulomatous response but no
    significantly increased incidence of neoplasms. It was suggested by
    the authors of these investigations that the lack of neoplastic
    response was possibly due to the agglomeration of the Kevlar fibrils
    in the peritoneal cavity.

         There are no adequate studies in which the fibrogenic or
    carcinogenic potential of polyolefin fibres has been examined. A
    90-day inhalation experiment in rats with respirable (46% < 1 µm)
    polypropylene fibres (up to 50 fibres/ml) indicated dose- and
    duration-dependent changes characterized by increased cellularity
    and bronchiolitis. No relevant data on intratracheal instillation
    are available. In intraperitoneal injection studies on polypropylene
    fibres or dust in rats, there was no significant increase in
    peritoneal tumours.

         There are inadequate data on which to make an assessment of the
     in vitro toxicity and genotoxicity of synthetic organic fibres. 
    For aramids, studies have shown that short and fine para-aramid
    fibrils have cytotoxic properties. With regard to polyolefin fibres,
    there is some evidence of cytotoxicity of polypropylene fibres. 
    Mutagenicity tests on extracts of polyethylene granules gave
    negative results.

    1.6  Effects on humans

         In a cross-sectional study of 88 out of 110 workers in a
    PAN-based continuous filament carbon fibre production facility,
    there were no adverse respiratory effects, as assessed by
    radiographic and spirometric examination and administration of
    questionnaires on respiratory symptoms. In other less well
    documented studies, adverse effects have been reported in workers
    involved in the production of both carbon and polyamide fibres; data
    presented in the published accounts of these investigations,
    however, were insufficient to assess the validity of the reported

    1.7  Summary of evaluation

         Data concerning the exposure levels of most synthetic organic
    fibres are limited. Those data available generally indicate low
    levels of exposure in the occupational environment. There is a 
    possibility of higher exposures in future applications and uses. 
    Virtually no data are available with respect to environmental fate,
    distribution, and general population exposures.

         On the basis of limited toxicological data in laboratory
    animals, it can be concluded that there is a possibility of
    potential adverse health effects following inhalation exposure to
    these synthetic organic fibres in the occupational environment. The
    potential health risk associated with exposure to these synthetic
    organic fibres in the general environment is unknown at this time,
    but is likely to be very low.


    2.1  Identity, physical and chemical properties

    2.1.1  Carbon/graphite fibres

         Carbon and graphite fibres are filamentary forms of carbon
    produced by high temperature transformation of one of three organic
    precursor materials: rayon fibres (regenerated cellulose), pitch
    (coal tar or petroleum residue) or polyacrylonitrile (PAN) fibres.
    Graphite fibres are materials characterized by a three-dimensional
    polycrystalline structure (Volk, 1979). The typical temperature
    range for graphite fibre production is 1900 to  3000 °C, while
    carbon fibre is manufactured at about 1200 to 1300 °C (Martin  et
     al., 1989). In much of the literature the terms carbon and
    graphite fibre are used interchangeably. Trade names of
    carbon/graphite fibres include WCA, CCA-4, Magnamite, Thornel-T,
    Celion, Panex, HITEX, Fortafil, Thornel-P and Carboflex (ITA, 1985;
    ICF, 1986).

         Rayon-based carbon fibres are very stable at high temperatures
    and consist of over 99% carbon. PAN-based carbon fibres have a lower
    carbon content (92 to 95%) and undergo further changes when heated
    beyond the original process temperature. The carbon content of
    pitch-based carbon fibres varies from 75% upward.  Pitch-based
    fibres with a Young's modulus of over 345 GPa are almost pure (over
    99.5%) carbon.

         The nominal diameters of carbon fibres have been reported to
    range from 5 to 15 µm (Volk, 1979) and from 7 to 10 µm (Jones  et
     al., 1982; ICF, 1986) according to end use. The distribution of
    fibre diameters is not available from the literature. Carbon fibres
    are lightweight, refractile, and have high tensile strengths, high
    Young's modulus (a measure of the stiffness of the material), a high
    degree of dimensional stability and low thermal expansion. These
    fibres are chemically inert and corrosion resistant, but oxidization
    may occur at high temperatures. They have moderate electrical and
    thermal conductivity (Volk, 1979). Carbon fibres are, however,
    vulnerable to shearing due to a decrease in interlamellar shear
    strength with increasing Young's modulus. The Young's modulus,
    electrical and thermal conductivity, and tensile strength increase
    with the degree of molecular orientation along the fibre axis (Volk,
    1979). Strength and modulus values are greater at higher production
    temperatures. However, fibres with the highest Young's modulus may
    not have the highest tensile strength (Jones  et al., 1982).

         Information on the density, tensile strength and Young's
    modulus of carbon and graphite fibres are presented in Table 1
    (Volk, 1979).

    2.1.2  Aramid fibres

         Aramid fibres (aromatic polyamides) are produced by a two step
    process involving production of the polymer followed by spinning.
    The polymer is typically produced by the reaction of aromatic
    diamines and aromatic diacid chlorides in an amide solvent. Aramid
    fibres are defined as fibres in which the base material is a
    long-chain synthetic polyamide in which at least 85% of the amide
    linkages are attached directly to two aromatic rings (Preston,
    1979). Two types of aramid fibres are produced by the DuPont
    Company: Kevlar (para-aramid) and Nomex (meta-aramid), which differ
    primarily in the substitution positions on the aromatic ring
    (Preston, 1979; Reinhardt, 1980; Galli, 1981).  Kevlar is made of
    poly(p-phenyleneterephthalamide) and Nomex is made of
    poly(m-phenyleneisophthalamide) (Preston, 1979).  Fibres named Conex
    and Fenilon, which have compositions similar to that of Nomex, have
    also been developed in Japan (by Teijin) and the Russian Federation,
    respectively (Preston, 1979). Arenka, now marketed as Twaron
    (Holland/Germany), is a para-aramid fibre produced by Akzo. A
    para-aramid fibre called Arimid is produced in the Russian

         Para-aramid fibres such as Kevlar/Twaron are produced as
    continuous filament yarn, cut (staple) fibre (38-100 mm in length),
    short fibre (6-12 mm in length) or pulp (2-4 mm in length), all with
    a nominal diameter of 12-15 µm. However, abrasive processing will
    produce some fibrils from para-aramid fibre. Pulp has many
    fine-curled, tangled fibrils within the respirable size range
    attached to the surface of the core fibre, and some of these fibrils
    will break off and become airborne during manufacture or use. These
    fibrils have a ribbon-like morphology and may have widths less than
    1 µm. Figure 1 shows a scanning electron microscopy of ultrafine
    Kevlar fibrils peeling off a 12-µm Kevlar fibre (Warheit  et al.,

         Meta-aramid fibres such as Nomex/Conex are manufactured as
    continuous filament, staple fibre and short fibre, all with a
    nominal diameter of approximately 12 µm. Unlike para-aramid fibres,
    meta-aramids have no fibrillar substructure and do not tend to
    produce smaller diameter fibrils upon abrasion (ICF, 1986).

        Table 1.  Physical and chemical properties of carbon/graphite fibresa


    Fibre                                          Density (g/cm3)     Tensile strength (MPa)    Young's modulus (GPa)


    Rayon-based carbon fibres (low modulus)           1.43-1.7                345-690                   21-55

    Rayon-based carbon fibres (high modulus)          1.65-1.82                  -                     345-517

    PAN-based carbon fibres                            1.7-1.8               2400-2750                 193-241

    Pitch-based carbon fibres (filament yarn)            2.0                   2000                      345

    a From: Volk (1979)

    FIGURE 1

         Generally, aramid fibres have medium to very high tensile
    strength, medium to low elongation-to-break, and moderate to very
    high modulus (Preston, 1979; Brown & Power, 1982). Meta-aramid fibre
    of low orientation has a density of approximately 1.35 g/cm3 and
    the hot-drawn fibre has a density of approximately 1.38 g/cm3.
    Para-aramid fibres of relatively high crystallinity have a density
    of 1.44 g/cm3. The volume resistivities and dielectric strengths
    of these fibres are high, even at elevated temperatures (Preston,
    1979). Aramid fibres are heat resistant, with mechanical properties
    being retained at temperatures up to 300-350 °C (Preston, 1979).
    Whole aramid fibres are generally resistant to chemicals, with the
    exception of strong mineral acids and bases (Preston, 1979; Galli,
    1981; Chiao & Chiao, 1982). The strength to weight ratio of Kevlar
    is high; on a weight basis, it is five times as strong as steel, ten
    times as strong as aluminum and up to three times as strong as
    E-glass. Aramid fibres have excellent toughness, and withstand
    continuous heat at temperatures in the 160-205 °C range. Aramids
    will not melt or support combustion, and carbonization will not be
    appreciable under 400 °C. Para-aramid fibres have a small negative
    coefficient of longitudinal thermal expansion, similar to graphite,
    making them suitable for joint usage in composites (Galli, 1981;
    Hanson, 1980).  Some physical and chemical properties of aramid
    fibres are presented in Table 2 (Hodgson, 1989).

    2.1.3  Polyolefin fibres

         Polyolefin fibres are long-chain polymers composed (at least
    85% by weight) of ethylene, propylene or other olefin units
    (Buchanan, 1984). Although many different types of polyolefins are
    produced, this term is commonly used only for hydrocarbon
    polyolefins, polyethylene and polypropylene (Hartshorne & Laing,
    1984). These are the fibres considered in this monograph. 
    Approximately 95% of all polyolefin fibres are polypropylene (Ahmed,

         Structurally, the high-density polyethylenes consist of linear
    methylene chains usually terminated by vinyl but occasionally by
    methyl groups. The low-density analogue also contains methylene
    chains, but has frequent branches. Chains are usually terminated by
    methyl groups. An intermediate density polyethylene is also
    available, together with blends of polyethylene with polypropylene
    or polyisobutene for use mainly in ropes or twines. Trade names of
    polyolefin fibres include BDH Low Density, Courlene C3, Courlene X3,
    Courlene Y3, Downspun 82, Dyneema, Fibrite MF, Meraklon BCF,
    Meraklon DO, Novatron, Polyolefine, Sanylene, and Spratra
    (Hartshorne & Laing, 1984).

        Table 2.  Physical and chemical properties of aramid fibresa


    Fibre type         Density (g/cm3)   Tensile strength    Specific strength    Young's modulus     Flammability         Approximate
                                               (MPa)              (MPa/d)              (GPa)          (LOI) at room          thermal
                                                                                                       temperature      degradation (°C)

    Kevlar/Twaron         1.44-1.45          2790-3000             2000               120-124             24.5                > 400

    Nomex                   1.38                 -                   -                   -                 26                 > 370

    a  From: Hodgson (1989)

         Polyolefin fibres and products are produced in eight different
    forms (ICF, 1986):

    1)   monofilament yarn typically having a diameter of 150 µm or 

    2)   multifilament yarn similar in structure to monofilament yarn 
         but for which the diameter of individual filaments is generally 
         in the range of 5 to 20 µm has also been reported;

    3)   staple fibre, which is multifilament yarn cut into varying  
         lengths of up to a few millimeters and which may be used in 
         either woven or discontinuous form;

    4)   tape and fibrillated film yarn with a large rectangular
         diameter resembling a ribbon; the thickness typically varies
         from 2.5 to 12.7 µm for tape and 2.5 to 6.4 µm for fibrillated
         film yarn;

    5)   spun-bonded fabric, i.e. non-woven fibrous structures produced
         in the form of flat fabric in one continuous process directly
         from the molten resin;

    6)   synthetic pulp - a relatively new class of discontinuous fibres 
         with diameters ranging from 5 to 40 µm and maximum lengths of
         2.5 to 3 mm;

    7)   meltblown or microfibre - a relatively new type of polyolefin 
         fibre produced by the meltblown process for which the diameter
         typically ranges from 0.1 to 2 µm and the length is a few

    8)   high strength, high modulus, highly oriented, high molecular 
         weight polyethylene fibres formed by spinning, drawing a
         partially polymerized gel.

         The specific gravities of polyethylene and polypropylene are
    low and they are highly hydrophobic; properties of these materials
    in wet conditions are, therefore, similar to those measured at
    standard temperature and humidity (21 °C; 65% relative humidity). 
    The polyolefins are unaffected by a wide variety of inorganic acids
    and bases and organic solvents at room temperature. The melting
    point of low-density polyethylenes is less than 116 °C and of
    high-density ones is above 131 °C. Medium-density polyethylene is
    reported to melt at 125 °C, whereas polypropylenes melt at between
    167 and 179 °C (Hartshorne & Laing, 1984).  Some physical and
    chemical properties of the polyolefins are presented in Table 3
    (Hodgson, 1989).

    2.2  Production methods

    2.2.1  Carbon/graphite fibres

         In general, the process of carbon and graphite fibre production
    involves three distinct steps: preparation and heat treating,
    carbonization, and optional high-temperature annealing (Volk, 1979).
    Initially, the precursors (PAN, pitch or rayon) are oxidatively
    stabilized and dehydrogenated at moderate tempera tures
    (200-300 °C). For pitch-based fibres, the commercial coal or
    petroleum pitch is converted through heat treatment into a mesophase
    or liquid crystal state. The fibre is then carbonized at
    temperatures of 750-1375 °C in a non-oxidizing atmosphere, and
    production may involve a secondary heating phase, known as
    graphitization, at approximately 1400 °C in an inert atmosphere.

         Mesophase pitch-based fibres having a Young's modulus exceeding
    690 GPa, made by heating without stretching to 3000 °C, are the most
    graphitic in nature (Volk, 1979). To produce high modulus fibres,
    yarn is stretched during the last heat treatment. Depending on the
    configuration of the final product (e.g., chopped fibre, continuous
    strand, felt or fabric) and intended use, the fibre may be treated
    with a sizing material to improve its handling characteristics and
    compatibility with various matrices. The formulation of sizing
    compounds (protective coatings) is considered proprietary; it has
    been reported, however, that they are typically compounds with
    epoxy-based functionalities, resins, epoxides and aqueous systems
    (ICF, 1986).  Fibres may be chopped to lengths in the range of 3 to
    25 mm for use in reinforced composite materials or milled down to
    200 µm for special conductive applications. For composites, the
    fibrous material is embedded in a matrix, such as an epoxy resin, to
    form strong, lightweight engineering materials (Martin  et al.,

    2.2.2  Aramid fibres

         Aramid polymers are made by solution polymerization. This
    involves low temperature polycondensation of diacid chlorides and
    diamines in amide solvents. Meta-aramid fibre is spun from the
    polymerization solution of dimethylacetamide after neutralization. 
    Para-aramid polymer must first be neutralized and isolated from the
    polymerization solution; it is then re-dissolved in a spinning
    solution of concentrated sulfuric acid. This liquid crystalline
    solution is then extruded through a spinneret, and the acid is
    extracted and neutralized to form a highly-oriented fibre.

        Table 3.  Physical and chemical properties of polyolefin fibresa


    Polymer                        Fibre type             Density        Young modulus     Tensile strength     Elongation to
                                                          (g/cm3)            (GPa)               (MPa)            break (%)

    Low-density polyethylene       monofilament            0.92             0.8-1.0                -                  -

    High-density polyethylene      monofilament          0.95-0.96          1.7-4.2                -                  -

    Polyethylene                   high density            0.96             1.7-4.2             290-570             10-45

    Gel-spun polyethylene          filament                  -               44-77             2580-5500              -

    Polypropylene                  staple and tow        0.90-0.96         0.28-3.3                -                  -
                                   monofilament          0.90-0.91          1.6-4.8                -                  -
                                   multifilament         0.90-0.91          1.2-3.2                -                  -

    Polypropylene                  filament                0.91             1.6-4.8             270-540             14-30
                                   high tenacity             -                 -                  811                 -

    a  From: Hodgson (1989)

    2.2.3  Polyolefin fibres

         Low-density polyethylene is manufactured by a high-temperature
    and high-pressure polymerization process, whereas the high-density
    form is made at low temperatures and pressures using a more
    efficient catalyst (e.g., the Marlex process).  Intermediate density
    polyethylene and polypropylene are produced with a catalyst by the
    Ziegler process (Hartshorne & Laing, 1984).

         There are several processes for the production of continuous 
    and discontinuous polyolefin fibres (ICF, 1986). Mono-filament and
    multi-filament yarns are formed by the continuous extrusion of the
    molten polymer through a spinneret, solidification by heat transfer,
    and winding onto packages. Processing may include drawing of the
    fibre to up to six times its original length, as well as heat
    treatment to relieve thermal stress. Staple fibre is cut from
    multi-filament yarn. Yarns are also formed by splitting a highly
    oriented film. Flash spun fibres are formed by dissolving polyolefin
    polymer under pressure and flashing from a spinneret to form fibres.
    If done at high pressure the fibres are much shorter and can be used
    as a synthetic pulp. Melt blown fibres are formed by drawing a spun
    fibre in high pressure steam or air.

    2.3  Sampling and analytical methods

         Sampling and analytical methods for organic fibres include the
    measurement of total airborne or respirable mass concentration and
    the determination of airborne fibre number concentrations by phase
    contrast optical microscopy (PCOM). Sampling methods used for
    organic fibres are similar to those used for inorganic fibres such
    as asbestos or man-made mineral fibres. These methods typically
    involve drawing a measured volume of air through a filter mounted in
    a holder that is located in the breathing zone of the subject. For
    measurement of mass concentrations, either polyvinyl chloride or
    glass fibre filters are normally used. The filters are stabilized in
    air and weighed against control filters, both before and after
    sampling, to permit correction of weight changes caused by varying
    humidity. Cellulose ester membrane filters are usually used for
    assessing fibre number concentrations. In this case, the filter is
    made optically transparent with one of several clearing agents
    (e.g., triacetin, acetone or ethylene glycol monomethyl ether), and
    the fibres present in random areas are counted and classified using
    PCOM (WHO, 1985; NIOSH, 1989 [Method 7400]). Different fibre
    counting rules have been used in various countries and these may
    give somewhat different values.

         Although the basic methods for the determination of total
    airborne mass and fibre number concentrations are similar in most
    countries, differences in the sampling procedure, the filter size
    and type, the clearing agent and the microscope used, and subjective
    errors in sampling and counting all contribute to variations in

    results. Specific reference methods for the determination of organic
    fibres have not been developed.  However, the methods mentioned
    above have been used for routine industrial hygiene monitoring. A
    WHO project is in progress to develop a reference method for the
    measurement of health risk-related fibres in workplace air.

         Several potential problems exist with respect to use of PCOM
    sampling and analytical methods for organic fibres. Firstly, these
    fibres have significant electrostatic charges that could affect
    sampling efficiency. Secondly, some of these fibres may be soluble
    in the microscope slide mounting medium, and their visibility in
    PCOM has not been evaluated. Lastly, PCOM methods lack specificity
    for counting organic fibres.

         The improved resolution of electron microscopy and the
    identification capacity particularly of the analytical transmission
    electron microscope (TEM), with selected area electron diffraction
    (SAED) and energy dispersive X-ray analysis (EDXA), make these
    methods particularly suitable for more complete characterization of
    the fibre size distribution and analysis of fibres of small diameter
    (NIOSH, 1989 [Method 7402]). However, these methods have so far
    rarely been used for analyses of organic fibres.

         For analysis by scanning electron microscopy (SEM), fibres
    collected on polycarbonate filters can be examined directly. This
    avoids the need for transfer techniques that may affect the fibre
    size distribution. For TEM, direct transfer preparation techniques
    involving carbon coating of particles on the surface of a
    polycarbonate or membrane filter and indirect transfer methods in
    which attempts have been made to retain the fibre size distribution
    are the most widely accepted.


    3.1  Production

    3.1.1  Carbon/graphite fibres

         The principal producers of carbon and graphite fibres are
    Japan, the USA and the United Kingdom, although these materials are
    also manufactured in France, Germany and Israel (ITA, 1985; ICF,
    1986). The raw material most commonly used is polyacrylo nitrile
    (PAN); in Japan, the world's largest producer of PAN-based fibres,
    the precursor material is obtained from the excess acrylic fibre
    capacity of the Japanese petrochemical industry. The USA is the main
    world producer of carbon/graphite fibres from rayon and pitch (ICF,

         World production capacity was estimated to be 4173 tonnes in
    1984. Based on information provided by manufacturers in the USA,
    estimated world consumption for 1984 and 1987 was 889 tonnes and
    2046 tonnes, respectively. Similar estimates based on information
    provided by companies in Japan were 1068 tonnes and 1756 tonnes,
    respectively. The USA and Japan are the principal consumers,
    followed by western Europe (ITA, 1985). More recently, it has been
    reported that the total world capacity for carbon fibres is 10 000
    tonnes per annum, although the author noted that it was difficult to
    assess what this means in terms of real annual output (Hodgson,

         The estimated consumption of rayon-based carbon fibres from
    1970 to 1976 was 50 to 125 tonnes (Volk, 1979). It has also been
    estimated that the combined production of PAN-based carbon fibres in
    the three principal producing countries (USA, Japan and the United
    Kingdom) rose from 10 tonnes in 1970 to 250 tonnes in 1976 (Volk,

    3.1.2  Aramid fibres

         Kevlar and Nomex which are both produced by DuPont have been
    sold commercially since 1970 and 1965, respectively (Reinhardt,
    1980). The production capacity for Kevlar in 1978 was reported to be
    approximately 3400 tonnes (Galli, 1981). In 1975, the production
    capacity for Nomex was expanded from 4500 tonnes to more than 9100
    tonnes (Preston, 1979). More recently, it has been reported that the
    production capacity per year of Kevlar (USA) and Twaron
    (Holland/Germany) is 20 000 and 5000 tonnes, respectively (Hodgson,
    1989). Plants in Northern Ireland and Japan with annual production
    capacities of about 5000 tonnes per year each have been brought on
    line since then.

    3.1.3  Polyolefin fibres

         Production of polyolefin fibres has been increasing owing to
    greater demand for continuous filament yarn, monofilaments and
    staple fibres (Buchanan, 1984). Production and consumption in the
    USA in 1983 were approximately 182 500 and 175 000 tonnes,
    respectively (SRI International, 1985). The annual polyolefin fibre
    production capacities of producers in the USA have been reported to
    range from approximately 4500 kg to more than 134 000 kg (SRI
    International, 1987).

    3.2  Uses

         Carbon and aramid fibres are used principally in advanced
    composite materials to improve strength, stiffness, durability,
    electrical conductivity or heat resistance. Since these fibres
    improve properties such as these without adding much weight, they
    are used principally in the aerospace industry, for military reasons
    and in sports equipment manufacture (ITA, 1985).  Polyolefin fibres
    are typically used in textile applications, although gel spun fibres
    are used for high tensile strength, but low temperature,

    3.2.1  Carbon/graphite fibres

         In addition to the general categories mentioned above, carbon 
    fibre in felt form is used in high temperature insulation, 
    principally in inert atmospheres due to the tendency of the 
    material to oxidize.

         Rayon-based carbon fibres are used primarily in aerospace 
    applications, such as phenolic-impregnated heat shields and 
    carbon-carbon composites for missile parts and aircraft brakes 
    (Volk, 1979).  PAN-based carbon fibres are used in structural 
    applications in the aerospace industry, sports goods (golf clubs, 
    fishing rods, tennis rackets, bows and arrows, skis, sailboat masts 
    and spars), reinforcement for moulded plastics, prostheses, 
    artificial joints and dental bridges (Volk, 1979; Bjork  et al.,
    1986).  Pitch-based carbon fibres are used in applications to
    increase electrical conduction and resistance to heat distortion and
    to improve wear and stiffness (e.g., as veil mat for sheet moulding,
    milled mat in injection moulding) (Volk, 1979).

    3.2.2  Aramid fibres

         Properties of aramid fibres, such as heat and flame resistance, 
    dimensional stability, ultra-high strength and high modulus, 
    electrical resistivity, chemical inertness and permselective 
    properties, make them useful in many applications (Preston, 1979).

         Para-aramids are used principally as a strengthening and 
    reinforcing material in composite structures due to their low 
    density, high specific strength and stiffness, greater vibration 
    damping and better resistance to crack propagation and fatigue than
    those of inorganic fibrous materials.  Para-aramids are used
    primarily for tyre cords, protective clothing, industrial fabrics,
    high performance (sports and aerospace) composites, high-strength
    ropes, cables, friction materials and gaskets (ILO, 1989).

         Meta-aramids are primarily used for their heat and corrosion
    resistance (Brown & Power, 1982).  The paper form is used as
    electrical insulation in motors and transformers.  Staple and
    continuous filaments are used in protective clothing and coated
    fabrics, and industrial filter bags for hot gas emissions.

         Aramid fibres are sometimes combined in various applications
    with other fibrous materials such as carbon and graphite to reduce
    costs and increase impact strength (Delmonte, 1981).

    3.2.3  Polyolefin fibres

         Apart from their traditional use in carpet backings, polyolefin
    fibres are being used increasingly in other household furnishings
    such as upholstery, bedding, curtains, wall-covering materials and
    carpet pile (Hartshorne & Laing, 1984; Buchanan, 1984).  The largest
    use of polyolefin fabric in clothing is in disposable nappies
    (diapers) and athletic socks.

         Polyolefin fibres are also used in ropes, cordage and twine,
    webbing, synthetic turf, agricultural fabrics, commercial fishing
    lines and nets, sewing thread and book binders (Buchanan, 1984). 
    They are also used in the medical field in sutures and as matrices
    for tissue ingrowth and anchoring (Hoffman, 1977).

         Microfibres are used in a variety of applications including
    filtration, medical/surgical fabrics, sanitary products, sorbents,
    wipes, apparel insulation, protective clothing and battery

    3.3  Emissions into the environment

    3.3.1  Fibre emissions

         Carbon fibres may be released into the environment during 
    production, processing or combustion of materials made of or 
    containing carbon composites.  These may arise during the 
    incineration of waste material containing carbon fibres and 
    aircraft fires.  Light microscopy analysis of emissions from several 
    key operations in the manufacture and processing of carbon fibres 
    (weaving, prepregging, machining and incineration of composites) 
    showed release rates (fibre mass released per unit of material

    processed) varying over several orders of magnitude, with weaving
    and incineration being the greatest (Gieseke  et al., 1984).  Fibre 
    diameters were found to be reduced only during incineration.

         Based on laboratory studies in which composites containing 
    carbon fibres were burnt, it was concluded that the rate of 
    emissions and lengths of fibres released in municipal incineration 
    are dependent on air flow rates and mechanical agitation within  the
    combustion chamber.  The diameters of emitted fibres were dependant
    upon burning rate, temperature and oxygen levels, and on fibre
    residence time in the incinerator.  It was estimated that a typical
    emission rate of carbon fibres from the burning zone in a municipal
    incinerator would probably be in the order of 1% of the composite
    being burnt.  It was also concluded that composites made with epoxy
    binder materials release more fibres during incineration than those
    made with phenolic binder materials (Gieseke  et al., 1984).

         In studies conducted to simulate municipal incineration of 
    refuse from composites containing carbon fibres (combustion rate  of
    34 kg/h, flame temperature of 925 °C, 450 kg Celion unidirectional
    laminate fed over a 1-min period), stack emission rates were 0.840
    to 0.998 mg/dscm carbon fibre.  Emission rates decreased with time
    (0 to 60 min) following introduction of the refuse from the carbon
    fibre composite (Henry  et al., 1982; Gieseke  et al., 1984).

         In laboratory and field tests conducted by the United States 
    National Aeronautics and Space Administration, it was estimated 
    that approximately 38.4-49.5% of the fibres released during the 
    burning of composite materials containing PAN-based  carbon/graphite
    fibres in aircraft (spoilers, cockpits, and various  structural
    elements) were respirable (i.e. aspect ratio of > 3:1; < 80 µm in
    length and < 3 µm in diameter).  Based on analysis by optical
    microscopy, the average length of emitted fibres was 30 µm 
    (Sussholz, 1980).

         Based on data collected in his previous studies, the same 
    investigator estimated airborne fibre concentrations during the  
    burning of carbon fibre composites.  For a fire resulting from an 
    aircraft crash, it was estimated that 1% of the carbon fibre would 
    be released, constituting about 5 x 1011 respirable fibres per kg.
    Based on these data, it was further estimated that the airborne 
    concentration of fibres in the densest part of the smoke would be
    5 fibres/ml.

         The by-products of carbonization that are captured following 
    incineration of fibres include hydrogen cyanide from PAN-based 
    fibres and polynuclear aromatic compounds from pitch-based fibres. 
    By-products of graphitization include ammonia, cyanide and
    hydrocarbons; these materials are decomposed during the heating
    process (ICF, 1986).

    3.3.2  Decomposition products

         In studies conducted to simulate municipal incineration of 
    refuse from composites containing carbon fibres (combustion rate  of
    34 kg/h, flame temperature of 925 °C, 450 kg Celion unidirectional
    laminate fed over 1-min period), observation by light microscopy
    indicated that fibres are oxidized during incineration and that
    fibre diameters are reduced.  It was also observed that as oxidation
    proceeds, there is notching and subsequent breaking of the fibres. 
    Thus, oxidized fibres that have small diameters are often relatively
    short (Henry  et al., 1982).

         The burning or pyrolysis of organic fibres produces a wide 
    variety and quantity of emitted gases depending on the numerous 
    variables of the decomposition/oxidation.  Simultaneously burning 
    materials, temperature, heating rate, burning time, humidity and 
    oxygen availability are just a few of the important variables.  
    Consequently, the data from a given experiment are uniquely 
    dependent on the conditions of that experiment.

         Pyrolysis decomposition products of carbon and Kevlar fibres
    have been identified by thermogravimetric analysis/gas 
    chromatography/mass spectrometry (TGA/GC/MS) (Razinet  et al.,
    1976).  The products identified in the off-gasses of pyrolysis of 11
    different types of carbon fibre at 1000 °C included carbon dioxide
    (32-100%), methane (2-23%), propane (0.5-19%), propene (2-20%),
    benzene (7-14.4%), toluene (2.0-8.3%), dimethylbenzene (2.9%),
    naphthalene (2.9%) and methylnaphthalene (2.8%).  The carbon dioxide
    content of the off-gasses (expressed as a percentage of the weight
    of the sample used in the decomposition) ranged from 0.02 to 2.34%.

         The products identified after fast pyrolysis of Kevlar (650 °C) 
    were carbon monoxide (63%), benzene (35%) and toluene (2%).  
    Following slow pyrolysis (at 650 °C for 15 min), many compounds, 
    including nitrogen, carbon monoxide, carbon dioxide, methane, 
    ethylene, propene, benzene, toluene, benzonitrile, aniline, 
    methylaniline, phenylacetonitrile, phthalonitrile, phenyldiamine, 
    biphenyl, benzimidazole, fluorene and benzanilide, were  identified.


    4.1  Occupational environment

    4.1.1  Carbon/graphite fibres  Production

         Carbon fibres are released in the workplace during operations 
    such as fibre forming, winding, chopping and composite formation. 
    Analysis by light microscopy of emissions in the  immediate vicinity
    of each of several key operations in the manufacture and processing
    of carbon fibre (including weaving, handling, prepregging, and
    machining of composites) demon strated that release rates (fibre
    mass released per unit of material processed) were greatest for
    weaving (Gieseke  et al., 1984).

         The data on aerosol exposures to carbon fibres in the 
    occupational environment are presented in Table 4.  Due to the lack
    of available data and limited information on methods for sampling
    and analysis, only tentative conclusions concerning airborne
    particle or fibre concentrations can be drawn.  Processing of composites

         Some data are available concerning airborne concentrations and 
    fibre dimensions resulting from the machining of carbon fibre 
    composites.  Following drilling, an analysis of the settled dust was 
    conducted using PCOM and scanning electron microscopy (SEM).  Fibres
    50 to 100 µm in length with diameters 6 to 8 µm were observed and
    found to have dimensions similar to those of fibres in the composite
    material.  Following sawing, settled dust samples were also
    collected and analysed, and this revealed longitudinal breaking of
    fibres.  Some fibres were observed to have fibre  diameters less
    than those in the parent material; fibres were also shorter in
    length than those in the dust produced by drilling (Wagman  et al.,

         Limited data are available concerning the preparation and 
    machining of carbon fibre composites.  PCOM analyses of five 
    samples from one facility yielded concentrations ranging from
    < 0.001 fibres/ml to approximately 0.01 fibres/ml.  The average 
    length of fibres in these samples ranged from approximately 31 µm to
    749 µm and average diameter from 3.9 µm to 6.7 µm (Henry  et al.,

        Table 4.  Airborne concentrations of carbon fibre in the workplacea


            Sampling and analysis                                              Results                                          Reference


    Light microscopic analysis of unspecified         0.0002 fibres/ml in prepregging, mean fibre diameter 6.1 µm and mean      Henry et
    number of samples (possibly one each) of          fibre length 213 µm; 0.0009 fibres/ml in shuttle loom weaving, mean       al. (1982)
    "outside emissions" at prepregging machining      fibre diameter 6.7 µm and mean fibre length 749 µm; 0.003 fibres/ml
    and weaving operations in PAN-based production    in rapier weaving, mean diameter 3.9 µm and mean fibre length 706 µm;
    facility in the USA                               0.01 and 0.001 fibres/ml in machining, mean fibre diameters 6.2 and
                                                      6.6 µm, and mean fibre lengths 32.8 and 30.9 µm, respectively

    Light microscopic analysis of 4 samples of        mean concentration of 0.0005 fibres/ml (range, 0.00002-0.006              Henry et
    either "outside emissions" or worker exposure     fibres/ml); average fibre diameter 6.5 µm and average fibre               al. (1982)
    (not clearly specified) in winding operations     length 48-60 µm
    in PAN-based production facility in the USA

    Gravimetric analysis of 38 samples in             mean concentration of 0.08-0.39 mg/m3 total dust with 40% respirable;     Jones et
    pitch-based continuous filament production        concentrations highest in the laboratory owing to cutting, grinding       al. (1982)
    facility                                          and milling of carbon fibre; reinforced resins (mean, 0.39 mg/m3),
                                                      concentration twice as high in winding (mean, 0.19 mg/m3) as in
                                                      production (mean, 0.08 mg/m3)

    Table 4 (contd).


            Sampling and analysis                                              Results                                          Reference


    Light microscopic analysis of emissions at        0.003-0.029 fibres/ml in prepegging, mean fibre diameter 5.5-6 µm and     Gieseke et
    several locations each in various phases of       mean fibre length 915-2399 µm; 0.001-0.05 fibres/ml in weaving, mean      al. (1984)
    carbon fibre production                           fibre diameter 5.5-7.8 µm and mean fibre length 945-2342 µm

    Gravimetric analysis of 6 breathing zone          0.10-0.80 mg/m3; concentrations much higher for chopping and winding      Gilliam
    samples in PAN-based production facility in       operators (0.54-0.8 mg/m3) than for line operators (0.1-0.12 mg/m3)       (1986)b
    the USA

    PCOM analysis (NIOSH 239) of all areas of a       0.11-0.27 fibres/ml                                                       Familia
    pitch-based production facility in the USA                                                                                  (1986)c
    where exposure was deemed likely (1985 to
    May 1986)

    a This study was designed to estimate emissions from carbon fibre processing.  The Task Group noted that these data may be of
      limited value for predicting occupational exposure levels and airborne fibre characteristics.

    b Gilliam, H.K.  Great Lakes Carbon Corporation, Rockwood, TB 37854-0810.  Letter providing monitoring data and product
      literature to James C. Chang, ICF Inc., Washington, DC, April 28, 1986.

    c Familia, R.A.  Union Carbide/Amoco.  Greenville, SC 29602.  Letter providing sampling data to James Chang, ICF Inc.,
      Washington, DC, (1986).

         The mean airborne concentration determined by PCOM and SEM was
    0.03 fibres/ml in fourteen samples taken during the machining of
    fibre composites at the Air Force Aerospace Materials Research
    Laboratory. Particles with an aspect ratio > 3 constituted less
    than 0.1% of the released material. The diameters (> 7 µm) of the
    airborne fibres were similar to those in the composites (Lurker &
    Speer, 1984).

         Airborne particulate samples were collected during trimming
    operations on a wing panel composed of carbon fibre composites. 
    Based on analysis by PCOM, less than 8% of the airborne particulates
    were fibrous. The diameters of at least 80% of the fibres were
    approximately 7 µm, i.e. similar to those in the composites
    (Dahlquist, 1984).

         The physical, morphological and chemical characteristics of
    machined graphite or fibrous glass composites have been determined.
    Bulk and fractionated samples were examined by light and electron
    microscopy and analysed chemically by GC/MS (Boatman  et al., 1988).
    The test materials were designated as "graphite-p", two graphite
    materials manufactured from PAN, one pitch-based graphite and
    PAN-graphite/Kevlar. The machining operations were spindle shaping
    (at 3450 or 10 000 rpm), hand routing (at 23 000 rpm) and use of a
    saber saw.  Dusts were collected with a high efficiency vacuum
    cleaner and resuspended in a dust generator to create an aerosol in
    which 90% of particles were < 10 µm in aerodynamic diameter. The
    relative proportion of respirable to total mass of bulk samples was
    < 3%; aerodynamic diameters of fractionated samples collected at
    the tool face ranged from 0.8 to 2.0 µm. Particles in bulk samples
    ranged from 7 to 11 µm in diameter, and mean aspect ratios ranged
    from 4:1 to 8:1 (26:1 for the fibrous glass composite). The mean
    diameter of fractionated particles collected at the tool face was
    2.7 µm, with 74% being less than 3.0 µm in diameter. There was no
    evidence of longitudinal splitting of fibres, and volatilization of
    chemicals from machined composites was low.  The authors reported
    that the respirable fractions in dusts collected at the tool face
    were only 2 to 3% by weight. However, since large particles that
    would settle rapidly and not normally reach the breathing zone were
    included in samples taken at the tool face, these values are not
    indicative of personal exposure.

    4.1.2  Aramid fibres  Production

         Available data on airborne concentrations of polyamide fibres 
    in the occupational environment are restricted to brief summaries 
    of unpublished data collected within the industry.  In many cases, 
    methods of sampling and analysis have not been described.

         Verwijst (undated report) described exposure monitoring during
    para-aramid fibre and pulp manufacturing and during laboratory
    operations.  Air concentrations ranged from 0.01 to 0.1 fibrils/ml,
    with the highest values being for pulping.  Verwijst also noted
    relatively high exposure (0.9 fibrils/ml) during water jet cutting
    of composites.

         Since initiation of Kevlar para-aramid fibre production (in 
    about 1971), plant-air and employee exposures have been measured by
    the same phase contrast microscopy techniques used for asbestos
    (PCAM 239 before about 1982 and NIOSH 7400 "A" rules) (Merriman,
    1992).  These data are summarized in Table 5.  For continuous
    filament yarn handling, plant exposures are extremely low
    (0.02 fibres/ml maximum) (Reinhardt, 1980).  Cutting of staple and
    floc fibre produced levels of 0.2 fibres/ml or less with a single
    peak measurement of 0.4 fibres/ml.  Pulp  drying and packaging
    operations led to maximum concentrations  of 0.09 fibres/ml.

         Airborne respirable concentrations of Nomex/Conex have been 
    reported to be less than the limit of detection i.e. 0.01 fibres/ml 
    (Reinhardt, 1980; ILO, 1989).  End-use processing and processing of composites

         Para-aramid end-use plant monitoring data using PCOM are 
    summarized in Table 5 for brake pad production, gasket composite 
    fabrication, and staple yarn spinning processes (Merriman, 1992).  
    No exposures exceeded 0.19 fibres/ml for brake pad manufacturing,
    where dry para-aramid pulp was mixed with powdered fillers and
    resin, pressed, cured, ground and drilled.  Average fibre exposures
    were less than 0.1 fibres/ml.

         In gasket sheet and gasket manufacturing, para-aramid pulp is 
    mixed with fillers and solvated rubber cement, rolled into sheets 
    and die-cut into smaller pieces that may be finished by sanding the
    edges.  A total of 64 samples in four plants gave no personal
    exposures greater than 0.15 fibres/ml and no area concentrations
    greater than 0.25 fibres/ml.  Mean exposures were less than
    0.1 fibres/ml for all operations.  In a single test where heat-aged
    gaskets were scraped from flanges, short-term exposure
    concentrations were less than 0.2 fibres/ml.

         Machining of para-aramid fabric-reinforced organic matrix
    composites produced very low exposures; most were less than
    0.1 fibres/ml although one exposure reached 0.25 fibres/ml during 
    grinding.  Although operator exposure during water-jet cutting was
    only 0.03 fibres/ml, the cutting sludge in a single sample was
    highly enriched with respirable fibrils and produced much higher
    levels (2.9 fibres/ml).

        Table 5.  Airborne fibre concentrations in workplaces handling
              para-aramid fibre pulpa


    Manufacturing     Operations              Samples         Mean          Maximum
    industry                                               (fibres/ml)    (fibres/ml)


    Brake pads        mixing                    20            0.07            0.15
                      preforming                17            0.08            0.19
                      grinding/drilling          8            0.04            0.08
                      finishing/inspecting       3            0.05            0.10

    Gaskets           mixing                    30            0.05            0.15
                      calendering                1            0.02            0.02
                      grinding                   5            0.08            0.25
                      cutting                   15            0.02            0.07

    Composite         sanding/trimming           5            0.08            0.25
                      water jet cutting          1            0.03            2.91

    Staple yarn       grinding                   5            0.18            0.28
                      carding                   16            0.39            0.79
                      drawing                    4            0.32            0.87
                      roving                     6            0.33            0.72
                      spinning                  15            0.18            0.57
                      twisting/winding          13            0.55            2.03
                      finishing                  2            0.30            0.48
                      weaving                    6            0.35            0.58

    From: Merriman (1992)
         The end-use most likely to produce significant para-aramid
    fibril exposure levels has been found to be staple fibre carding and
    subsequent processing into yarn.  Carding is highly abrasive and the
    fibrils produced are immediately entrained in the high air  flows
    created by the spinning cylinders.  Monitoring of operators in six
    yarn-spinning mills (67 personal samples) gave average exposures
    ranging from 0.18 to 0.55 fibres/ml, with one operation reaching a
    maximum of 2.03 fibres/ml.

         Kauffer  et al. (undated report) characterized airborne fibre
    concentrations and size distributions during the monitoring of
    machining of carbon fibre- and aramid-based composites in industry
    and the laboratory.  Concentrations encountered were typically well
    below 1 fibril/ml, as determined by optical microscopy.  SEM showed
    mean lengths from 1.9 to 4.3 µm, and mean length/diameter ratios
    values varied from 4.4 to 8.8.  The authors concluded that most of
    the respirable material consisted of resin debris.

         Particle and gaseous emissions during laser cutting of aramid
    fibre-reinforced epoxy plastics have been studied (Busch  et al.,
    1989).  The mass-median aerodynamic diameter of particles generated
    was 0.21 µm, but the concentration of dust and the fibre content of
    the dust were not reported.  GC/MS analyses of samples on charcoal
    and silica tubes demonstrated the following release of gases per
    gram of material pyrolized during cutting: 5.4 mg benzene, 2.7 mg
    toluene, 0.45 mg phenylacetylene, 1.4 mg benzo nitrile, 1.0 mg
    styrene, 0.55 mg ethylbenzene, 0.15 mg  m-xylene and  p-xylene,
    0.04 mg  o-xylene, 0.28 mg indene, 0.16 mg benzofurane, 0.15 mg
    naphthalene, and 0.73 mg phenol.

         Limited personal exposure monitoring was conducted during 
    laser cutting of Kevlar-reinforced epoxy matrix (Moss & Seitz, 
    1990).  An air sample collected within a few feet of the cutting 
    operation revealed few fibres (0.15-0.25 µm in diameter and < 10 µm
    in length) in TEM analyses.  In addition to fibre measurements,
    hydrogen cyanide concentrations in the cutting area ranged from
    0.03 to 0.08 mg/m3 with a TWA of 0.05 mg/m3.  Carbon monoxide
    concentrations ranged from 10 to 35 ppm and nitrogen dioxide
    concentrations were < 0.5 to 5 ppm.

    4.1.3  Polyolefin  Production

         Limited air monitoring data in a facility producing 
    polypropylene fibres have been reported (Hesterberg  et al., 1991). 
    Samples for PCOM analyses were collected in the breathing zone  of
    workers, and fibres were counted using NIOSH method 7400  ("B"
    rules).  Gravimetric measurements of total dust exposures were also
    made.  Airborne levels of fibres longer than 5 µm ranged from none
    detected to a peak of 0.5 fibres/ml, with most values being less

    than 0.1 fibres/ml.  The total dust exposures ranged from below
    detection limits to 0.7 mg/m3, most values being less than
    0.25 mg/m3.  SEM analyses showed airborne fibre diameters to range
    from 0.25 to 3.5 µm (mean = 1.97) and lengths to range from 1.68 to
    69 µm (mean = 29.4 µm).

    4.2  General environment

         Henry  et al. (1982) reported the results of limited sampling
    and analysis by light microscopy of the concentration of carbon
    fibres in ambient air in the vicinity of carbon fibre facilities. 
    The  concentration of fibres in ambient air downstream of the
    baghouse for rapier weaving was reported to be 0.0003 fibres/ml
    (Henry  et al., 1982).  The average length of fibres was 706 µm and
    the average width 3.9 µm.

         The release of carbon fibres at the crash and burn site of two 
    military aircrafts has been described (Formisano, 1989; Mahar, 
    1990).  Both aircrafts were manufactured using carbon fibre 
    composites and the air sampling was conducted several days after 
    the crash.  At the first site 21 area samples and 11 personal 
    samples were taken over four days.  The mean durations of the  area
    and personal samples, respectively, were 168 ± 86 min and  48 ± 46
    min at 2 litres/min.  Twelve of the 21 area samples  contained low
    fibre concentrations and were considered to be  estimates.  The mean
    fibre concentration of the remaining air  samples was 0.29 ± 0.41
    (range 0.015-1.060) fibres/ml.  In the case  of the 11 personal
    samples, 4 were estimates and the mean  concentration of the others
    was 3.40 ± 2.52 (range 0.063-6.998)  fibres/ml.

         A mixture of floor-wax and water was poured over the aircraft 
    wreckage in both cases to suppress the dust.  The personal air 
    samples were collected while a variety of tasks commonly performed
    at a crash site were performed.  The author reported that fibres may
    have been lost by electrostatic binding to the sampling cassette. 
    While a few interfering fibres were present, the straight fibres
    with clean edges were recognized by the analyst as man-made fibres. 
    Air sampling at the second crash site produced barely detectable
    levels, and the highest concentration determined by PCOM was
    0.58 fibres/ml.


    5.1  Introduction

         It is considered that the potential respiratory health effects 
    related to exposure to fibre aerosols are a function of the internal 
    dose to the target tissue, which is determined by airborne 
    concentrations, patterns of exposure, fibre shape, diameter and 
    length (which affect lung deposition and clearance) and 
    biopersistence.  The potential responses to fibres, once they are 
    deposited in the lungs, are a function of their individual 
    characteristics.  In inhalation studies in rodents, fibres with 
    dimensions similar to those that humans can inhale should be used, 
    provided that a high proportion is within the range respirable for 
    the rat.  In addition, complete characterization (e.g., dimensions, 
    number, mass and aerodynamic diameter) of the rat-respirable  fibre
    aerosol and retained dose is essential.  Methods for aerosol 
    generation should insure that fibre lengths are preserved.

         The following general principles have been derived principally 
    on the basis of results of studies with particulates, man-made 
    mineral fibres and asbestos.  It should be recognized, however, 
    that these parameters have not been evaluated in detail specifically 
    for the synthetic organic fibres.

         Because of the tendency of fibres to align parallel to the 
    direction of airflow, the deposition of fibrous particles in the 
    respiratory tract is largely a function of fibre diameter, with 
    length and aspect ratio being of secondary importance.  In 
    addition, the shape of the fibres as well as their electrostatic 
    charge may have an effect on deposition (Davis  et al., 1988).

         Since most of the data on deposition have been obtained in 
    studies on rodents, it is important to consider comparative 
    differences between rats and humans in this respect; these 
    differences are best evaluated on the basis of the aerodynamic 
    diameter.  The ratio of fibre diameter to aerodynamic diameter is 
    approximately 1:3.  Thus, a fibre measured microscopically to have 
    a diameter of 1 µm would have a corresponding aerodynamic diameter
    of approximately 3 µm.  A comparative review of the regional
    deposition of particles in  humans and rodents (rats and hamsters)
    has been presented by US EPA (1988).  The relative distribution
    between the tracheobronchial, and pulmonary regions of the lung in
    rodents follows a pattern similar to human regional deposition
    during nose breathing for insoluble particles with a mass median
    aerodynamic diameter of less than 3 µm.  Figures 2 and 3 illustrate
    these comparative differences.  As can be seen, particularly for
    pulmonary deposition of particles, the percentage deposition in the
    rodent is considerably less, even within the overlapping region of
    respiratory tract deposition, than in humans.  These data indicate

    that, although particles with an aerodynamic diameter of 5 µm or
    more may have significant deposition efficiencies in man, the same
    particles will have extremely small deposition efficiencies in the

         Fibres of various shapes are more likely than spherical
    particles to be deposited by interception, mainly at bifurcations. 
    Available data also indicate that pulmonary penetration of curly
    chrysotile fibres is less than that for straight amphibole fibres.

         In the nasopharyngeal and tracheobronchial regions, fibres are 
    generally cleared fairly rapidly via mucociliary clearance, whereas 
    fibres deposited in the alveolar space appear to be cleared more 
    slowly, primarily by phagocytosis, to a lesser extent via
    translocation, and possibly by dissolution.  Translocation refers to 
    the movement of the intact fibre after initial deposition at foci in 
    the alveolar ducts and on the ciliated epithelium at the terminal 
    bronchioles.  These fibres may be translocated via ciliated mucous 
    movement up the bronchial tree and removed from the lung, or may be
    moved through the epithelium with subsequent migration to
    interstitial storage sites or along lymphatic drainage pathways or
    transport to pleural regions.  Fibres short enough to be fully 
    ingested are thought to be removed mainly through phagocytosis by
    macrophages, whereas longer fibres may be partially cleared at a
    slower rate either by translocation to interstitial sites, breakage 
    or by dissolution.  A higher proportion of longer fibres is,
    therefore, retained in the lung.

    5.2  Studies in experimental animals

         Fibres administered in studies on animals are only a subset of 
    those normally present in the occupational environment.  Due to the
    above-mentioned limitations in the anatomy of the rat as a model for
    man, wherever possible the fraction of rat-respirable fibres in the
    generated aerosol is specified.

    5.2.1  Carbon/graphite fibres

         It should be noted that in some of these studies, animals were 
    exposed to fibres that were not respirable for the rodent; in
    others, animals were exposed principally to dusts, the fibrous
    content of which was unspecified.  For carbon fibres, only one of
    the studies described here involved exposure to fibres that were
    respirable for the rat (Warheit  et al., in press/a).

    FIGURE 2

    FIGURE 3

         In a study designed to investigate the histopathological
    effects of carbon fibres on the lungs of guinea-pigs, production of
    a particulate aerosol from a chopped PAN-based carbon fibre ("RAE
    type 2") proved difficult, although with similar apparatus
    concentrations of 6000 respirable fibres/ml of asbestos (type 
    unspecified) had been maintained for long periods (Holt & Horne, 
    1978).  The maximum concentration of carbon particles "smaller than
    5 µm" produced by the apparatus was 370/ml, with 99%  being
    nonfibrous.  Of these, 2.9 fibres/ml ("black fibres") were of 
    respirable size in the dust cloud.  These were mainly less than
    10 µm in length and about 1 µm in diameter.  A total of
    15 specific-pathogen-free (SPF) guinea-pigs were exposed to the 
    aerosol described above.  At various time intervals up to 104 h 
    post-exposure, groups of animals were sacrificed.  Histopathological
    examination of the lungs of exposed animals revealed  macrophages
    filled with nonfibrous carbon particles, together with a few carbon
    fibres which were generally extracellular.  There  were no
    ferruginous bodies with a carbon core and no pathological effects
    (Holt & Horne, 1978).

         In a follow-up study in which the test atmosphere was similar 
    to that in the investigation described above, groups of SPF
    guinea-pigs (2-9 per group) inhaled carbon dust for 100 h and were
    killed at various intervals up to 2 years post-exposure.  The
    respirable fraction of the inhaled dust was mainly nonfibrous and
    there were very few fibrous particles in sections of the lung. 
    There was some indication, however, that extracellular submicron
    particles present in the tissue were washed out during histological
    processing.  An occasional ferruginous body was observed.  Submicron
    carbon dust reached the alveoli; phagocytosis of particles began
    immediately but proceeded slowly over many months and dust-filled 
    macrophages were still evident after 2 years (Holt, 1982).

         As described in section, a study was conducted in which 
    male Crl:CDBR rats were exposed (nose only) to aerosols of pitch- or
    PAN-based rat-respirable carbon fibres at target concentrations of
    50 or 100 mg/m3 (47 and 62 fibrils/ml) for periods ranging from 1
    to 5 days (6 h/day) and evaluated at 0, 24 and 72 h, 10 days, and 1
    and 3 months post-exposure.  Pigment-laden alveolar macrophages were
    observed primarily at the junctions of the terminal bronchioles
    (Warheit  et al., in press/a).

    5.2.2  Aramid fibres

         Smaller fibrils can peel from the surface of para-aramid fibres 
    as ribbons and complex branching.  Because these fine fibrils have 
    unusual shapes and tend to be statically charged, they frequently 
    agglomerate.  It is necessary to use specially prepared samples, 
    enriched in respirable fibrils with high-pressure air mills, to

    generate significant airborne para-aramid fibril concentrations 
    (i.e. > 100 fibres/ml) in inhalation studies on experimental 

         In a short-term (2 week) inhalation study on rats (as outlined 
    in section, Lee  et al. (1983) reported that, after
    6 months, inhaled Kevlar fibrils accumulated mainly at the
    bifurcations of the alveolar ducts and adjoining alveoli, with only
    a few fibrils being deposited in peripheral alveoli of the acinus.

         The pattern of deposition and persistence of aramid fibrils in 
    a two-year study was similar to that stated above (Lee  et al.,
    1988).  Kevlar fibrils, which are much more curled than chrysotile
    fibres, were retained mostly in the respiratory bronchioles and
    alveolar duct region, especially in the ridges of alveolar duct
    bifurcations.  One year after termination of a 12-month exposure to
    400  fibrils/ml, the lengths of fibres in lung tissue appeared to be 
    reduced.  At the three highest dose levels in the study (25, 100 and 
    400 fibrils/ml), there was a minute amount of dust accumulation in
    the alveolar macrophages and in tracheobronchial lymph nodes
    resulting from the transmigration of intrapulmonary Kevlar fibrils. 
    Most particles in the alveolar macrophages were less than 1 µm long.

         In a study by Warheit  et al. (1992), Crl:CD rats were exposed 
    (nose only) to ultrafine Kevlar fibrils (6 h/day) for 3 or 5 days at 
    concentrations ranging from 600 to 1300 fibrils/ml (gravimetric 
    concentrations ranging from  2 to 13 mg/m3) and evaluated at 0, 
    24, 72 and 96 h, 1 week, and 1, 3 and/or 6 months.  Kevlar fibres 
    were found to be deposited at alveolar duct bifurcations located 
    nearest the bronchiolar-alveolar junctions.  The median lengths and
    diameters of ultrafine Kevlar samples in the air and in the lungs
    were virtually identical immediately following exposure.   There was
    no morphological evidence that fibrils had translocated to
    epithelial or interstitial compartments, in contrast to patterns 
    observed with chrysotile asbestos.  Fibre clearance studies 
    demonstrated a transient increase in the numbers of retained 
    fibrils at 1 week post-exposure, with rapid clearance of fibres 
    thereafter.  The transient increase in the number of fibres could 
    have been due to transverse cleaving of the fibres, since the 
    average lengths of retained fibres continued to decrease over time.  
    In this respect, a progressive decrease in the mean length (12.5 to 
    7.5 µm) and diameter (0.33 to 0.24 µm) of inhaled fibres was
    measured over a 6-month post-exposure period.  The percentage of
    fibres longer than 15 µm decreased from 30% at time 0 to < 5% at
    6 months post-exposure (Warheit  et al., in press/b).

         Following intratracheal instillation in rats of 25 mg "Kevlar 
    polymer dust" containing a low but undetermined proportion of 
    fibres considered to be in the respirable range (< 1.5 µm in 
    diameter and between 5 and 60 µm in length) in physiological  saline
    for 21 months, large particles were found in the terminal

    bronchioles, and smaller particles with dimensions of < 5 µm were 
    present in alveolar ducts (Reinhardt, 1980).

         In a study in which 5 mg of Kevlar fibres (fibre size 
    distribution and sample preparation methods unspecified) was 
    injected intraperitoneally into Wistar rats, it was reported that 
    fragments were transported through lymphatic pathways and stored in
    lymph nodes where they caused inflammatory reactions (Brinkman &
    Müller, 1989).

    5.2.3  Polyolefins

         Groups of 22 male Fischer-344 rats were exposed nose-only
    (6 h/day, 5 days/week) for 90 days to filtered air or to 15, 30 or 
    60 mg/m3 polypropylene fibre (99.9% purity; 12.1, 20.1 and
    45.8  fibres/ml, respectively, size-selected to have an average
    diameter of 1.6 µm (46% < 1 µm) and an average length of 20.4 µm). 
    There was a strong association between the administered
    concentration, the time of exposure and the lung fibre burden. 
    Although the length and diameter did not change during the study,
    the authors hypothesized that the segmentation of these fibres on
    SEM stubs may have resulted from chemical alteration or partial
    dissolution of sections of the fibres within the lungs, which made
    these sections dissolve further during final processing for SEM
    analysis.  This segmentation increased with the administered
    concentration and period of exposure, as well as with the period of
    recovery after termination of exposure at 90 days  (Hesterberg
     et al., 1992).

    5.3  In vitro solubility studies

         In an investigation in which the solubility of various natural 
    and synthetic fibres in physiological Gamble's solution (at 37 °C 
    or more for 1 h to 20 weeks and 1 h to 2 weeks for closed and open
    system conditions, respectively; pH not specified) was determined by
    atomic absorption spectrometry, carbon and aramid fibres (source and
    fibre-size distribution unspecified) were found to be "practically
    insoluble".  There was also no evidence of alteration of the surface
    during examination by SEM with energy dispersive spectrometry
    (Larsen, 1989).

         In a study by Law  et al. (1990), test materials including
    three  polymeric organic fibre compositions (polypropylene,
    polyethylene and polycarbonate) were compared for solubility in
    physiological Gamble's solution (pH 7.6).  The test materials were
    subjected to leaching for 180 days in a system that provided a
    continuous flow through sample holders containing the test fibres. 
    After this period, the fibres were examined by electron microscopy
    for changes in surface area, total specimen weight and surface 
    characteristics.  There was virtually no dissolution and no 
    significant change in surface area.  There were only slight weight

    gains, ranging from 0.08 to 0.5%, and no visible surface changes, 
    in contrast to results obtained for several man-made mineral fibres 


    6.1  Experimental animals

    6.1.1  Introduction

         There are several factors that should be considered when
    evaluating experimental data on the biological effects of synthetic
    organic fibres. (Several of these factors were discussed in relation
    to man-made mineral fibres by WHO, 1988). Most importantly,
    synthetic organic fibres should not be considered as a single
    entity, except in a very general way. There are substantial
    differences in the physical and chemical properties (e.g., fibre
    lengths and chemical composition) of the fibres, and it is expected
    that these would be reflected in their biological responses.
    Finally, the fibres used for specific research protocols may be
    altered to determine the biological effects in experimental animals.
    In this case, they may not represent the hazard potential in humans
    exposed to the commercial or degradation products during the
    manufacture, processing, use and disposal.

         Notwithstanding the above comments, there are certain
    characteristics of synthetic organic fibres that are important
    determinants of effects on biological systems. The most important of
    these appear to be fibre size (length, diameter, aspect ratio,
    shape), biopersistencea and durabilitya, chemical composition,
    surface area and chemistry, electrostatic charge and number or mass
    of fibres (dose).


    a  The term  biopersistence refers to the ability of a fibre to
       stay in the biological environment where it was introduced. The
       term is of particular use in inhalation and intratracheal
       instillation studies, because a large percentage of the fibres
       that reach the lung are removed by pulmonary clearance and
       relatively few are retained  (persist).  The length of time
       that fibres persist in the tissue is also a function of their
        durability, which is directly related to their chemical
       composition and physical characteristics. The term  solubility,
       as used here, relates to the behaviour of fibres in various
       fluids. In general, the term  solubility is more appropriate
       for use in  in vitro than  in vivo studies, because the
       degradation of fibres in tissues is not only a function of
       their solubility. While the concept of biopersistence is
       important, quantitative procedures for evaluation of this
       parameter have not been established.

         Other considerations relate to the extrapolation of
    experimental findings for hazard assessment in man. It appears that
    if a given fibre comes into contact with a given tissue in animals
    producing a response, a similar biological response (qualitative)
    might be expected in humans under the same conditions of exposure.
    There is no evidence that the biological reaction to fibres differs
    between experimental animals and humans. However, there may be
    quantitative differences between species, some being more sensitive
    than others.

         There has been a great deal of debate concerning the relevance 
    of various routes of exposure in experimental animal studies to 
    hazard assessment in man (McClellan  et al., 1992).  The advantages 
    and disadvantages of each of these routes are discussed in the 
    following sections.  It was the consensus of the Task Group that 
    the results of studies by all routes of administration should be 
    considered in evaluating the weight of evidence in hazard 

         Each route cannot be discussed in detail here, but some general 
    observations can be made.  Positive results in an inhalation study 
    on animals have important implications for hazard assessment in 
    man.  Strong scientifically based arguments would need to be made 
    against the relevance of such a finding to man.  Conversely, the 
    lack of a response in an inhalation study does not necessarily mean 
    that the material is not hazardous for humans.  Rats, being 
    obligate nose-breathers, have a greater filtering capacity than 
    humans.  However, if it were demonstrated that the "target tissue" 
    was adequately exposed and that a biologically important response 
    was not noted, then such a result would be of value for hazard 
    assessment in humans.

         As discussed in the Environmental Health Criteria for Man-made
    mineral fibres (WHO, 1988), a negative result in studies  using
    non-physiological exposure conditions (e.g., intratracheal 
    instillation, intrapleural injection or implantation,
    intraperitoneal injection) would suggest that a specific fibre may
    not be hazardous for parenchymal lung tissue and/or the mesothelium. 
    In contrast, a positive result in such studies should be confirmed
    by further investigation in inhalation studies for a complete
    assessment of the hazard for humans.

         However, the Task Group believes that the use of
    instillation/injection studies may not necessarily be appropriate
    for certain synthetic organic fibres.  This recommendation is based
    on the lack of, or weak response to, para-aramid fibres seen in
    three intraperitoneal studies (Pott  et al., 1987, 1989; Davis,
    1987), whereas a definite neoplastic response was seen in a chronic 
    inhalation study (Lee  et al., 1988) (see also Appendix 1).  The 
    authors of these intraperitoneal studies speculated that the lack of 
    response to para-aramid fibres may be due to the ability of this

    type of fibre to agglomerate and thereby reduce the actual number 
    of single fibres.  These results suggested to the Task Group that a 
    negative result in an instillation/injection study with selected 
    synthetic organic fibres should not always be considered to 
    indicate an absence of hazard in humans by inhalation exposure.

          In vivo dissolution and translocation studies play an
    important  part in our understanding of the behaviour of fibres in
    the lung.  Such studies (Bernstein  et al., in press) of
    comparative biopersistence can play a part in hazard identification.

         In several studies, the effects of prostheses composed of 
    various synthetic organic fibres on tissues have been examined 
    (Neugebauer  et al., 1981; Tayton  et al., 1982; Makisato  et al.,
    1984; Parsons  et al., 1985; Henderson  et al., 1987).  These
    studies have not been reviewed here since they are not relevant to
    an assessment of the effects of inhaled synthetic organic fibres.

    6.1.2  Carbon/graphite fibres

         There are no adequate studies available in which the fibrogenic 
    or carcinogenic potential of carbon/graphite fibres have been 
    examined.  Inhalation

         In a subchronic inhalation study, a group of 60 male
    Sprague-Dawley rats was exposed to pulverized carbon fibres
    (20 mg/m3; bulk product PAN-based Celion fibres with mean diameter
    of 7 µm and 20-60 µm in length; aerosol not characterized), 6 h/day, 
    5 days/week for up to 60 weeks, and a similarly sized control group
    was exposed to air alone (Owen  et al., 1986).  There was a slight
    decrease in the rate of body weight gain in the first 4 weeks of
    exposure.  In the post-exposure period, however, the average body
    weight of the exposed rats was slightly but not significantly less
    than that of the control animals.  Although there were variable 
    changes in the average airway resistance for inspiration, and the 
    respiratory rate and minute volume of the exposed animals were 
    significantly less than those of the controls, these changes were
    not considered by the authors to be related to exposure to carbon 

         In animals exposed to carbon fibres, there was a low-grade, 
    diffuse increase in alveolar macrophages containing fibrous 
    particles in the lungs but no pulmonary fibrosis or inflammatory 
    reaction.  By 32 weeks post-exposure, there were only occasional 
    alveolar macrophages containing fibres or particles scattered 
    throughout the lungs (Owen  et al., 1986).  The Task Group noted 
    that the lack of observed effects in the lungs may be attributable 
    to the fact that the administered fibres were not in the respirable 
    size range for the rat.

         Groups of unspecified numbers of male Crl:CDBR rats were 
    exposed (nose only) to aerosols of pitch- or PAN-based respirable 
    carbon fibres at target concentrations of 50 or 100 mg/m3 for 
    periods ranging from 1 to 5 days (6 h/day) and evaluated at 0, 24 
    and 72 h, 10 days, and 1 and 3 months (Warheit  et al., in
    press/a).  A 5-day exposure to respirable pitch-based carbon fibres
    (47 or 106 mg/m3; 47 and 62 fibres/ml, probably > 5 µm in
    length;  MMADs of 1.3 µm and 1.6 µm, respectively) produced
    dose-dependent, transient inflammatory responses in the lungs of 
    exposed rats, manifested by increased levels of neutrophils and 
    concomitant significant increases in lactate dehydrogenase, protein 
    or alkaline phosphatase in bronchoalveolar fluids at early
    post-exposure time periods.  These changes were reversible within 
    10 days after exposure. There were no significant differences in 
    the morphology or  in vitro phagocytic capacities of macrophages 
    recovered from rats between the sham-exposed control group and 
    those exposed to pitch-based carbon fibres. Results from cell 
    labelling studies in rats exposed to pitch-based carbon fibres for 
    5 days demonstrated an increased turnover of lung parenchymal cells
    at 10 days or 1 month after exposure, which did not correlate with
    the measures of inflammation in bronchoalveolar fluids.  No
    increases in turnover of terminal bronchiolar cells were measured 
    at any time post-exposure.  Pigment-laden alveolar macrophages and
    minimal type II epithelial cell hyperplasia were observed primarily
    at the junctions of the terminal bronchioles and alveolar ducts.  In
    an additional group of rats used as negative controls, exposed for
    6 h to PAN-based carbon fibres (diameter outside the respirable
    range; MMAD > 4.4 µm), there were no cellular, cytotoxic or
    alveolar/capillary membrane permeability changes at any time
    post-exposure.  Intratracheal administration

         Following characterization of dusts from machined composites 
    containing five graphite fibres and one fibrous glass, as described 
    in section (Boatman  et al., 1988), groups of five male 
    pathogen-free Charles River rats received a single intratracheal 
    injection of 5 mg of human respirable fractions of the dusts in 
    sterile, pyrogen-free phosphate-buffered saline (Luchtel  et al.,
    1989).  The graphite composites included in the study were a 
    proprietary material designated as graphite-p, two graphite 
    materials manufactured from PAN, one pitch-based graphite composite
    and graphite-PAN/Kevlar.  The machining operations included spindle
    shaping (3450 or 10 000 rpm), hand routing (23 000 rpm) and use of a
    saber saw.  The mean particle diameter of tested fractions was
    2.7 µm, with 74% being less than 3.0 µm in diameter (99.9% < 10 µm
    in aerodynamic diameter).  For comparison, similarly sized groups of
    animals were exposed to phosphate-buffered saline, aluminium oxide
    (negative control) and quartz (positive control).  No information on
    the fibre content of the dust samples was presented.

         In a study by Martin  et al. (1989), one of the lungs of each
    rat  was examined histopathologically one month following injection 
    and the other was lavaged to recover airway cells and fluid.  For 
    the six composite-epoxy materials, there was a continuum of lung 
    response (from an increase in alveolar macrophages to fibrosis) 
    that fell between that observed in the positive (quartz) and 
    negative (non-fibrous aluminium oxide) control groups.  None of the
    composite dusts induced effects that were as severe as those
    observed with quartz.  However, four of the dusts (fibrous glass 
    composite, the two graphite materials manufactured from PAN and the
    pitch-based graphite composite) produced responses (discussed below)
    that were more severe than those to aluminium oxide.  Among the
    composite materials, the reactions for one of the PAN-based graphite
    and fibrous glass composites were the most severe.  The pitch-based
    graphite and one of the PAN-based composites caused the greatest
    increase in total cells in lung lavage fluid.  However, total cells
    recovered in animals treated with any of the composite samples did
    not differ significantly from those  for animals treated with either
    NaCl or aluminium oxide.  In contrast, the quartz-treated animals
    had ten times more lavaged cells than the NaCl-treated animals and
    five times more than any of the animals treated with composite
    samples.  There were also highly significant differences in the
    types of cells present in the lavage fluid among the various groups,
    with the same pitch-based graphite and one of the PAN-based
    composites producing the greatest increase in both percentage and
    total number of lavaged neutrophils among the composites tested. 
    The percentage and the absolute number of neutrophils were greater
    in the quartz-treated animals than in any other group.

         The authors noted that bolus administration by intratracheal 
    instillation may overwhelm lung defence mechanisms, but felt that 
    these results raised the possibility that some types of composite 
    dusts may be fibrogenic in humans (Luchtel  et al., 1989).  Intraperitoneal administration

         Groups of twelve albino Wistar rats (six male and six female) 
    were injected intraperitoneally with "carbon dust" (50 mg/kg body
    weight) suspended in physiological saline at a concentration of
    10 mg/ml.  Particle diameters determined by electron microscopy
    ranged from 0.2 to 15 µm.  The particle concentration in the
    injected solution measured by haemocytometer (resolution power,
    1 µm) was 3.75 x 106 (reported as x 10-6) per ml suspension 
    containing 0.5 mg of dust. At 1 and 3 months after administration, 
    there were no treatment-related lesions in tissues examined 
    histopathologically (i.e. omentum, spleen, liver and pancreas), 
    whereas in U.I.C.C.  Rhodesian chrysotile-exposed animals
    (12.5  mg/kg body weight at 2.5 mg/ml in physiological saline) there 
    were characteristic fibrotic nodules in the peritoneum (Styles & 
    Wilson, 1973).  The Task Group noted that in the published account

    of this study it was not clear whether the material was fibrous,
    particulate or both.  Dermal administration

         In a study reported by Depass (1982), four types of carbon 
    fibres (continuous pitch-based filament, pitch-based carbon fibre 
    mat, polyacrylonitrile continuous fibres and oxidized PAN-based 
    fibres) were ground and suspended in benzene (25 µl of a 10% (w/v)
    suspension) and applied to the clipped skin of the back of 40 male
    C3H/HeJ mice three times weekly until death.  No statistically
    significant increases in skin tumours were observed in any of the
    exposed groups compared to controls receiving vehicle alone.  The
    Task Group considered this study inadequate for an evaluation due to
    the lack of reporting data regarding the nature of the materials
    including the particle size and morphology of the test materials.

    6.1.3  Aramid fibres  Inhalation

         A summary of the design of the inhalation studies with aramid 
    fibrils is presented in Table 6.

         Reinhardt (1980) reported a summary of the results of an acute
    and a 2-week inhalation study in rats exposed to a mixture of
    paraaramid dust containing 2% elongated particles with an aspect
    ratio of at least 3:1 (fibre). Details provided in the published
    account of these studies were insufficient for evaluation. In the
    short-term study, rats (number and strain unspecified) were exposed
    to Kevlar dust (130 mg/m3) 4 h/day, 5 days/week, for 2 weeks.
    Control rats were exposed to filtered air concurrently. Following
    the final exposure, half of both the test and control group animals
    were sacrificed and 21 tissues examined histopatho logically. The
    remaining rats were sacrificed and examined similarly following a
    14-day recovery period. During exposure, test animals were slightly
    less active and gained less weight than controls, effects which were
    not observed during the recovery period. In rats examined after the
    tenth exposure, there were numerous macrophages in the lung tissue.
    At the end of the recovery period, these macrophages decreased in
    number and formed discrete clusters, which the author interpreted as
    a "non-specific response to foreign particles in the lung"
    (Reinhardt, 1980). The Task Group noted that available information
    presented in the published account of this study was insufficient to
    determine the fraction of the aerosol which was respirable for the

        Table 6.  Inhalation studies on aramid fibres


    Fibre type            Concentrations of fibrils            Size          Exposure type     Histopathological   Number and       Reference
                                                        distributionc,d      and duration      evaluation          strain of
                                                                                               (sacrifices)        animal
                            Mass            Number
                           (mg/m3)         (per ml)

    Para-aramid dust         150        low proportion                      whole body;        not specified     not specified      Reinhardt
    with 2% elongated                      < 1.5 µm                         acute, 4 h         (14 day)          (rat)              (1980)
    (fibres)                 130         not specified    not specified     4 h/day, 5 days/   0 and 14 days     not specified
                                                                            week, 2 weeks      post-exposure     (rat)

    Kevlar fibrils            0                0          (length: 53%      whole body;        0,                5 male rats/       Lee et al.
                                                          > 20 µm)          6 h/day, 5 days/   2 weeks           group Crl:CD       (1983)
                         0.1 ± 0.06           1.3         85% < 5 µm MMAD   week, 2 weeks      3 months          Sprague-Dawley
                         0.52 ± 0.14          26          87% < 5 µm MMAD                      6 months          (Charles River)
                          3.0 ± 0.4           280         94% < 5 µm MMAD                      (post-exposure)
                         18.2 ± 2.8     not determined    49% < 5 µm MMAD
                         17.6a ± 4.4    not determined    13% < 5 µm MMAD

    Table 6 (contd).


    Fibre type            Concentrations of fibrils            Size          Exposure type     Histopathological   Number and       Reference
                                                        distributionc,d      and duration      evaluation          strain of
                                                                                               (sacrifices)        animal
                            Mass            Number
                           (mg/m3)         (per ml)

    Kevlar fibrils            0                0          more than 70%     whole body;        3 months          100 male and 100   Lee et al.
                         0.08 ± 0.04      2.4 ± 0.80      by mass, < 5 µm   6 h/day, 5 days/   6 months          female per group   (1988)
                         0.32 ± 0.08      25.5 ± 9.9      MMAD              week, 2 years      12 months;        Crl:CD(SD)BR
                         0.63 ± 0.14       100 ± 37                                            final sacrifice   (Charles River)
                         2.23 ± 0.46      411b ± 109                                           at 24 months (no
                                                                                               recovery period
                                                                                               after 24 months)

    Kevlar fibrils        2.0-13.3         600-1300       3.2-4.7 µm MMAD   nose only;         0                 4 males per        Warheit
                                                          CMC = 10 µm       3 or 5 days        24 h              group Crl:CDBR     et al.
                                                          CMD = 0.3 µm                         72 h              (Charles River)    (1992)
                                                                                               96 h
                                                                                               1 week
                                                                                               1, 3, 6 months

    a   Commercial Kevlar sample
    b   Exposure was for 1 year only due to toxic response. Surviving animals were held for an additional 1 year without exposure.
    c   It should be noted that the rat can inhale into the distal airways fibres with diameters less than 1 to 1.5 µm.
    d   MMAD = mass median aerodynamic diameter

         In a short-term inhalation study, groups of male Sprague-Dawley
    rats (number in each group unspecified) were exposed to
    0, 0.1 mg/m3 (1.3 fibrils/ml), 0.5 mg/m3 (26 fibrils/ml),
    3.0 mg/m3 (280 fibrils/ml) or 18 mg/m3 (number concentration not
    deter mined) of ultrafine Kevlar pulp fibrils prepared specially by
    a high-pressure air impingement device (60-70% < 1 µm in diameter
    and between 10 and 30 µm long) for 6 h/day, 5 days/week, for 2
    weeks. Another group was exposed to 18 mg/m3 commercial Kevlar
    fibres (2.5 mg/m3 of respirable dust) for the same period. Five
    rats in each group were killed at the end of exposure and at several
    periods up to 6 months after exposure. In animals exposed to 0.1 or
    0.5 mg/m3 of the ultrafine fibres there was a macrophage response
    in the alveolar ducts and adjoining alveoli, which was almost
    completely reversible within 6 months after exposure. In rats
    exposed to 3 mg/m3 ultrafine fibres or 18 mg/m3 of the
    commercial product, there was occasional patchy thickening of
    alveolar ducts with dust and inflammatory cells (but no collagen)
    6 months after exposure. In the group exposed to 18 mg/m3
    ultrafine Kevlar, there were granulomatous lesions with dust cells
    in the respiratory bronchioles, alveolar ducts and adjoining alveoli
    after two weeks of exposure. One month following exposure, there was
    patchy fibrotic thickening in the alveolar duct regions and adjacent
    alveoli, as well as dust cells. The fibrotic lesions were markedly
    reduced in cellularity, size and numbers from 3 to 6 months after
    exposure but contained networks of reticulum fibres with some
    collagen fibres (Lee  et al., 1983). (It should be noted that there
    is some discrepancy between the results presented in the published
    account of this study and a summary of this study included in the
    report of the longer-term bioassay (Lee  et al., 1988 described

         Groups of 24 male Crl:CDBR rats were exposed (nose only) to
    ultrafine Kevlar fibres (fibrils) 6 h/day for 3 or 5 days at
    concentrations ranging from 600 to 1300 fibres/ml (gravimetric
    concentrations ranging from 2 to 13 mg/m3) and subgroups of four
    rats were subsequently evaluated at 0, 24, 72 and 96 h, 1 week, and
    1, 3 and/or 6 months post-exposure (Warheit  et al., 1992; in
    press/b). The authors suggested that at higher gravimetric
    concentrations, there was probably an agglomeration of fibres in the
    aerosols. Five-day exposures elicited a transient granulocytic
    inflammatory response with an influx of neutrophils into alveolar
    regions and concomitant increases in bronchio-alveolar lavage fluid
    levels of alkaline phosphatase, lactate dehydrogenase and protein,
    which returned to control levels at time intervals of between 1 week
    and 1 month post-exposure. Macrophage function (as determined by
    surface morphology and  in vitro phagocytic and chemotactic
    capacities) in Kevlar-exposed alveolar macrophages was not
    significantly different from that of sham controls at any time
    interval. Increased pulmonary cell labelling was noted in terminal
    bronchiolar cells immediately after exposure but values returned to
    control levels one week later. Histopathological examination of the

    lungs of Kevlar-exposed animals revealed only minor effects,
    characterized by the presence of fibre-containing alveolar
    macrophages situated primarily at the junctions of terminal
    bronchioles and alveolar ducts.

         In the only chronic inhalation study conducted for any of the
    organic fibres considered in this document, groups of 100 male and
    female Crl:CD(SD)BR weanling rats were exposed to ultrafine Kevlar
    fibrils at concentrations of 0, 2.4, 25.5, and 100 fibrils/ml (0,
    0.08, 0.31 and 0.63 mg/m3, respectively), 5 days/week for two
    years (Lee  et al., 1988). An additional group of 100 animals was
    exposed according to the same schedule to 400 fibrils/ml
    (2.23 mg/m3), and, owing to toxicity, exposure was terminated at
    12 months and the animals were followed for an additional year.
    There were interim sacrifices of 10 males and 10 females per group
    at 3, 6 or 12 months. Fibrils were separated from the Kevlar pulp
    matrix by high-pressure air impingement as described above for the
    study conducted by Lee  et al. (1983). A summary of the effects on
    the lung is presented in Table 7. At a concentration of
    2.5 fibrils/ml, the alveolar architecture of the lungs was normal
    with a few dust-laden macrophages in the alveolar airspaces
    (alveolar macrophage response was considered by the authors to be
    the no-observed-adverse-effect level). At 25 and 100 fibrils/ml,
    there was a dose-related increase in lung weight, a dust cell
    response, slight type II pneumocyte hyperplasia, alveolar
    bronchiolarization and a "negligible" amount of collagenized
    fibrosis in the alveolar duct region. In addition, at 100
    fibrils/ml, "cystic keratinizing squamous cell carcinomas" (CKSCC;
    tumours not observed spontaneously in this strain or in man) were
    found in four female rats (6%) but not in any male animals. Female
    rats also had more prominent foamy alveolar macrophages, cholesterol
    granulomas and alveolar bronchiolar ization, and this was related to
    the development of CKSCC. At 400 fibrils/ml, 29 male and 14 female
    rats died due to obliterative bronchiolitis resulting from dense
    accumulation of inhaled Kevlar fibrils in the ridges of alveolar
    duct bifurcations during exposure for one year to 400 fibrils/ml. In
    the case of animals surviving one year post-exposure, the lung dust
    content, average fibre length and the pulmonary lesions in surviving
    rats were markedly reduced, but there were slight centriacinar
    emphysema and minimal fibrosis in the alveolar duct region. One male
    (1/36; 3%) and six female (6/56; 11%) rats in this experimental
    group developed CKSCCs.

         The CKSCCs developed between 18 and 24 months of age. The
    investigators reported that microscopically, it was extremely
    difficult to distinguish between squamous metaplasia and CKSCC since
    the lung tumours were well differentiated and there was no evidence
    of either tumour metastasis or invasion to adjacent tissue. The
    tumours were, therefore, considered by the authors to be benign
    neoplastic lesions which were classified as CKSCC due to the fact
    that there is no benign type of squamous cell lung tumour widely

    accepted in humans. The investigators also suggested that these
    changes should perhaps be considered as either metaplastic or
    dysplastic rather than as neoplastic lesions. In instillation
    studies in rats with silica, however, CKSCCs have been described as
    precursors of squamous cell carcinomas, which develop if the life
    span is sufficiently long (Pott  et al., in press). The Task Group
    members noted that there is considerable debate concerning the
    biological potential of these lesions (CKSCC) and their relevance to
    humansa. It was the view of the Task Group that the CKSCCs were
    related to exposure to aramid and that these lesions are part of the
    neoplastic spectrum. However, the Task Group also felt that the high
    dose (400 fibres/ml) exceeded the maximum tolerated dose. Finally,
    the Task Group noted that the study was terminated at 24 months; a
    longer observation time might have yielded a higher incidence of
    these tumours.  Intratracheal administration

         In the same brief account referred to in section, 
    Reinhardt (1980) reported the protocol and results of a study in 
    which 2.5 mg of shredded Nomex paper in physiological saline was 
    instilled into the trachea of rats (number, strain, instillation 
    schedule and nature of control group not specified).  Fibre sizes 
    were reported to vary from 2 to 100 µm in length and 2-30 µm in 
    diameter. The lungs of groups of rats were examined histologically 
    at 2 and 7 days, 3 and 6 months, and at 1 and 2 years.  There were 
    no adverse effects other than initial transitory acute inflammation 
    followed by foreign body granuloma formation.  These mild tissue 
    reactions became less obvious post-exposure and the lungs were 
    essentially normal without formation of collagenized fibrosis two 
    years after exposure.


    a  After the meeting of the IPCS Task Group an international panel
       of 13 pathologists evaluated these cystic lesions. The summary
       of their evaluation and the names of the participants are in
       Appendix 1.

        Table 7.  Summary of effects on the lungs of chronic inhalation of aramid fibrilsa


    Exposure          Mortality      Lung fibrosis            Keratinized squamous    Cystic keratinizing     Adenomas
    concentration                                             cell metaplasia         squamous cell
    (fibrils/ml)                                                                      carcinoma (CKSCC)b

    0                     0                 -                         -                       -               1/69 (1%) male
                                                                                                              0/68 (0%) female

    2.5                   0                 -                         -                       -               1/69 (1%) male
                                                                                                              0/64 (0%) female

    25                    0          dose-related increase            -                       -               1/67 (2%) male
                                     in severity and                                                          0/65 (0%) female

    100                   0                                           -               0/68 male               1/68 (2%) male
                                                                                      4/69 (6%) female        3/69 (4%) female

    400              29/65 male                               0/36 male               1/36 (3%) male          2/36 (6%) male
                    14/70 female                              6/69 (9%) female        6/56 (11%) female       2/56 (4%) female

    a  From: Lee et al. (1988)
    b  See also Appendix 1

         In a similar briefly documented study, 25 mg Kevlar in 
    physiological saline was instilled intratracheally into rats (number
    and strain not specified) and control rats were administered saline
    alone.  Animals were sacrificed 2, 7, and 49 days and 3, 6, 12 and
    21 months after treatment and the respiratory tract was examined
    histopathologically.  Following instillation, particles could be
    detected in lung tissue.  An initial, non-specific inflammatory
    response subsided within about a week and foreign body granulomas
    with minimal collagen were seen at later sacrifices.  Tissue
    responses decreased with increasing time post-exposure (Reinhardt,
    1980).  Intraperitoneal administration

         In studies by Pott  et al. (1987), 5-week-old female Wistar
    rats were administered 10 mg Kevlar fibres prepared by ultrasonic
    treatment in three weekly intraperitoneal injections of 2, 4 and
    4 mg. At the end of the study (surviving animals sacrificed
    2.5 years after treatment), 4 out of 31 animals (12.9%) had tumours
    (sarcoma, mesothelioma or carcinoma of the abdominal cavity).  In an
    additional study in which there was an attempt to obtain finer
    fibres and better suspension by drying, milling and ultrasonic
    treatment, 20 mg Kevlar (50% < 3.4 µm in length and 50% < 0.47 µm
    in width) in saline was injected intraperitoneally into 8-week-old
    Wistar rats (5 injections of 4 mg weekly). At 28 months after
    injection, the percentage of tumour-bearing animals was 5.8
    (preliminary results; 34 animals sacrificed and 18 survivors). The
    authors commented that it was difficult to produce a homogeneous
    suspension of Kevlar fibres and that, as a result, these fibres were
    more likely to be present in clumps in the peritoneal cavity than
    were other dusts. In these studies, tumour incidences after
    injection of 0.25 to 0.5 mg actinolite, chrysotile, crocidolite or
    erionite were between 50 and 80%, and, of 204 rats injected
    intraperitoneally with saline alone, 5 (2.5%) had malignant tumours
    in the abdominal cavity. The Task Group noted that there were some
    discrepancies between the reported results of this investigation and
    the later study by Pott  et al. (1989). Moreover, it was unclear
    whether these references refer to the same or different studies.

         In a subsequent report by Pott  et al. (1989), in which both
    the  fibre size distribution (90% of the fibre diameters in the 
    administered material were < 0.76 µm, and 50% of fibre lengths 
    were > 4.9 µm and 90% were < 12 µm) and number of fibres were 
    characterized, there was no significant increase in peritoneal 
    tumours.  There were tumours in 3 out of 53 (5.7%) female Wistar 
    rats compared to 2 out of 102 (2%) in the controls at 130 weeks 
    following intraperitoneal administration of 4 weekly doses of 5 mg 
    of milled bulk Kevlar (total dose, 20 mg).  The number of Kevlar 
    fibres administered was 1260 x 106.  In contrast, following 
    administration of a much smaller total dose of UICC chrysotile

    (0.25 mg, 202 x 106 fibres), tumour incidence was 68% (Pott  et
     al., 1989).

         Based on an examination of two animals from the study of Pott
     et al. (1989), Brinkman & Müller (1989) described the following
    stages of events following intraperitoneal injection of 5 mg of
    Kevlar fibres (fibre size distribution or sample preparation methods
    not specified) suspended in 1 ml physiological saline injected into
    8-week-old Wistar rats at weekly intervals for 4 weeks. At 28 months
    after the first injection, the rats were sacrificed and the greater
    omentum with pancreas and adhering lymph nodes were removed and
    examined histologically by light and scanning electron microscopy.
    In an initial stage, there were multinucleated giant cells with
    phagocytosis of the Kevlar fibres and an inflammatory reaction. In a
    second stage, granulomas with central necrosis developed indicating
    the cytotoxic nature of the fibres. A third stage was characterized
    by "mesenchymal activation with capsular structures of collagenous
    fibres as well as a slight submesothelial fibrosis". Finally, the
    reactive granulomatous changes in the greater omentum of the rats
    were accompanied by proliferative mesothelial changes which, in one
    of the two animals examined, led to mesothelioma. The authors
    commented that the reaction to Kevlar in the intraperitoneal test
    resembled the well-studied reaction to similar injections of glass
    or asbestos fibres. It was also noted that, as in the case of
    mineral fibres, fragments of Kevlar fibres were transported through
    lymphatic pathways and stored in lymph nodes where they caused
    inflammatory reactions.

         In a study in which 25 mg of Kevlar (fibre size distribution
    unspecified) were administered intraperitoneally to Sprague-Dawley
    rats (20 of each sex), there were no peritoneal mesotheliomas at
    termination (104 weeks) (Maltoni & Minardi, 1989). In an additional
    study by the same investigators, there were no peritoneal
    mesotheliomas at 76 weeks in similarly sized groups of rats of the
    same strain following intraperitoneal administration of 1, 5 or 10
    mg Kevlar fibres (fibre size distribution unspecified) (Maltoni &
    Minardi, 1989).

         Doses of either 0.25, 2.5 or 25 mg of Kevlar pulp vigorously
    disaggregated by a turreted tissue homogenizer (96% with diameters
    < 1 µm; 56% with diameters < 0.25 µm) were administered by single
    intraperitoneal injection in phosphate buffered saline to three
    groups of 3-month-old male AF/Han strain rats comprising 48, 32 and
    32 animals, respectively (Davis, 1987). An additional group of 12
    animals was injected with 25 mg of disaggregated Kevlar pulp and
    killed at intervals of between 1 week and 9 months after injection
    to examine the early histopathological reaction. A group of 48
    untreated rats was maintained as a control. The authors commented
    that it was not possible to report the fibre length distribution or
    fibre number concentration since it was often not possible to
    determine whether disaggregated fibrils were still attached or

    simply tangled with the larger fibres in the material prepared for
    injection. Although the number of separate free fibrils greatly
    exceeded that of the larger fibres present, the latter most probably
    made up the bulk (by mass) of the injected material. Consequently,
    the number of fibrils per unit mass injected that were within the
    size range normally considered to be most potent in the induction of
    mesothelioma was much lower than for most asbestos or man-made
    mineral fibre preparations examined to date. (The dust generation
    technique used in the studies of Lee  et al. (1983, 1988) produced
    a much finer preparation than was used in the current study).

         There were no significant differences in survival between the 
    exposed and control groups.  The cellular reaction to injected 
    Kevlar was considered to be vigorous with development of large 
    cellular granulomas.  Although not a significant increase, 2 out of 
    32 animals in the highest dose group (25 mg) developed peritoneal 
    mesotheliomas and it was concluded that the Kevlar preparation 
    possessed a low but definite carcinogenic potential (Davis, 1987).

    6.1.4  Polyolefin fibres

         There are no adequate studies in which the fibrogenic or 
    carcinogenic potential of polyolefin fibres have been examined.  Inhalation

         In a study by Hesterberg  et al. (1992), groups of 22 male 
    Fischer-344 rats were exposed nose-only (6 h/day, 5 days/week for 90
    days) to filtered air or to 15, 30 or 60 mg/m3 polypropylene 
    fibre (99.9% purity; 12.1, 20.1 and 45.8 fibres/ml, respectively, 
    size-selected to have an average diameter of 1.6 µm (46% < 1 µm) 
    and an average length of 20.4 µm).  No abnormal clinical signs were
    observed in any exposure group.  There were no statistically
    significant changes in body or lung weight or excess mortality as
    compared to the control.  Necropsy and histopathological
    investigations were performed on subgroups of 6 to 10 rats randomly
    selected from each group immediately after 30 and 90 days of
    exposure and 30 days after the 90-day exposure was terminated.  At
    all time points in the study there were dose- and duration-dependent
    changes in the lungs characterized by increased cellularity and
    early bronchiolitis but no deposition of collagen.  These cellular
    changes appeared to be reversible at the lower dose levels 30 days
    post-exposure.  There was a strong association between the
    administered concentration, the time of exposure and the lung fibre
    burden.  Intratracheal administration

         In a study (summary report) by M.B. Research Laboratories 
    (1980), single doses (unspecified) of ozonized (i.e. charge 
    neutralized) "polyethylene SHFF", ozonized "polypropylene  SHFF" or

    "HHF polypropylene" (source and fibre size  distribution
    unspecified) were administered by intratracheal insufflation in
    Tween 60 to groups of 40 male Long-Evans rats.  No effects on the
    lung were reported, but the Task Group considered that the data
    presented in this report were insufficient for evaluation.  Intraperitoneal administration

         Groups of 12 Wistar rats (Alderley Park strain; 6 male and 6 
    female) were injected intraperitoneally with a single dose of
    50  mg/kg (5 ml; 10 mg/ml) of either polyethylene ("Alkathene") or 
    polypropylene dusts in physiological saline (Styles & Wilson, 1973). 
    Particle diameters determined by electron microscopy were 3 to
    75  µm (polyethylene) and 4 to 50 µm (polypropylene).  Particle 
    concentrations in the injected solution measured by haemocyto meter
    (resolution power, 1 µm) in 1 ml suspensions containing 0.5 mg of
    dust were 2.38 x 106 (reported as x 10-6) for polyethylene and
    1.94 x 106 (also reported as x 10-6) for polypropylene.  At 1
    and 3 months, animals were sacrificed and the omentum, spleen, liver
    and pancreas examined histopathologically.  No treatment-related
    lesions were observed, whereas in U.I.C.C. Rhodesian
    chrysotile-exposed animals (12.5 mg/kg body weight at 2.5 mg/ml in
    physiological saline) there were characteristic fibrotic nodules
    (Styles & Wilson, 1973).  It was not clear to the Task Group 
    whether this material was fibrous, particulate or both.

         No significant increase in peritoneal tumours was observed in 
    one intraperitoneal study, reported by Pott  et al. (1987, 1989), 
    where 10 mg of polypropylene fibres (50% < 7.4 µm in length and 
    50% < 1.1 µm in diameter) in saline was injected intraperitoneally 
    into 8-week-old Wistar rats once a week for 5 weeks (total, 50 mg). 
    At 28 months after injection, 4% (2 out of 51) of the animals had
    tumours (sarcoma, mesothelioma or carcinoma of the  abdominal
    cavity) (Pott  et al., 1989).  In other studies, tumour  incidences
    after injection of 0.25 to 0.5 mg actinolite, chrysotile, 
    crocidolite or erionite were between 50 and 80%; 2 out of 102 rats 
    injected intraperitoneally with saline alone had malignant tumours 
    in the abdominal cavity (Pott  et al., 1987, 1989).  The Task Group 
    noted that there were some discrepancies between the reported 
    results of this investigation and the later study by Pott  et al.
    (1989).  Moreover, it was not clear whether these references 
    referred to the same or different studies.

    6.2  In vitro studies

          In vitro short-term studies, e.g., cytotoxicity,
    cytogenicity, and  cell transformation studies, contribute to an
    understanding of the  mechanisms of action of fibres.  The results
    of such studies are  useful in the overall assessment of fibre
    toxicity, but should not be  used alone for hazard assessment.

    However, it should be noted  that there are no known negative
    controls for  in vitro studies with  fibrous materials.

    6.2.1  Carbon fibres

         Martin  et al. (1989) evaluated the  in vitro effects of a
    series of five graphite fibre composite materials machined by
    various operations (as characterized by Boatman  et al. (1988) and
    described in section in rabbit alveolar macrophages by
    trypan blue exclusion, release of 51Cr from prelabelled
    macrophages and phagocytosis as measured by light microscopy. The
    Task Group noted that it was not clear in the report whether this
    material was fibrous, particulate or both. Approximately 74% and
    99.9% of the particles in each sample were less than 3.0 µm and
    10 µm in aerodynamic diameter, respectively. For comparison, a
    fibreglass composite material, aluminium oxide (negative control)
    and alpha quartz (positive control) were also tested. Following
    administration of 500 µg/ml, two of the samples ("graphite-PAN with
    epoxy and aromatic amine curing agent with geometric mean particle
    diameter of 1.8 µm" and "graphite-pitch with epoxy and aromatic
    amine curing agent with geometric mean particle diameter of 1.6 µm")
    were consistently the most cytotoxic producing the greatest release
    of 51Cr from labelled alveolar macrophages and the greatest
    reduction in viability based on trypan blue exclusion. The
    cytotoxicity of "fibreglass with epoxy and amine curing agent with
    geometric mean particle diameter of 2.6 µm" and "graphite-PAN with
    epoxy and amine curing agent with geometric mean particle diameter
    of 1.1 µm" was similar to that of the negative control (aluminium
    oxide). The cytotoxicity of "graphite-p with nonepoxy
    polyetherketone thermoplastic with geometric mean particle diameter
    of 1.6 µm" and "graphite-PAN/Kevlar with epoxy and amine curing
    agent with geometric mean particle diameter of 1.9 µm" was

         In studies conducted by Styles & Wilson (1973), "carbon dust" 
    was not considered cytotoxic in rat alveolar and peritoneal 
    macrophages.  Less than 2% of peritoneal macrophages and 5% of 
    alveolar macrophages were killed following phagocytosis of carbon 
    dust with particle diameters determined by electron microscopy to 
    range from 2 to 15 µm.  Particle concentrations measured by 
    haemocytometer (resolution power, 1 µm) in 1-ml suspensions 
    containing 0.5 mg of dust were 3.75 x 106 (reported as x 10-6). 
    Cell cultures were incubated for 2 h with an unspecified volume of a
    stock suspension containing 150 mg of dust per ml in (BSS); samples
    were taken at 0, 1, and 2 h after the addition of the dust (Styles &
    Wilson, 1973).

         "Acetone reconstituted benzene extracts" of two carbon fibre
    types (pitch-based and PAN-based carbon fibres) were tested in a
    series of genotoxicity assays (US EPA, 1988).  The test materials
    were not mutagenic but were weakly clastogenic.  The Task Group

    considered that these studies are of little or no value for an
    evaluation since no information was presented concerning the nature
    of the test substances.

    6.2.2  Aramid fibres

         Aqueous solutions containing 25, 50, 100 and 250 µg/ml  Kevlar,
    extracted from commercial grade Kevlar by dispersion  and settling
    in distilled water (90% < 5 µm in length and 0.25 µm in diameter;
    average length and diameter, 2.72 and 0.138 µm, respectively), were
    cytotoxic to pulmonary alveolar macrophages obtained from adult male
    Long-Evans black hooded rats, based on determination of leakage of
    cytoplasmic lactic dehydrogenase, lysosomal enzymes,
    beta-galactosidase, and ATP content (incubation time, 18 h).  The
    cytotoxic response in freshly harvested and cultured cells was
    considered to be similar to or greater than that for UICC B Canadian
    chrysotile (Dunnigan  et al., 1984).  The Task Group noted that due
    to their short length, these fibres would not be included in fibre
    counts in the occupational setting, according to WHO criteria (WHO,

    6.2.3  Polyolefin fibres

         In studies conducted by Styles & Wilson (1973), polyethylene 
    ("Alkathene") and polypropylene were considered, on the basis of 
    cytotoxicity in rat alveolar and peritoneal macrophages, to be 
    among the least toxic of various dusts.  Less than 2% of peritoneal 
    macrophages and 5% of alveolar macrophages were killed following
    phagocytosis of polyethylene and polypropylene with particle
    diameters determined by electron microscopy to range from 3 to
    75 µm and 4 to 50 µm, respectively.  The Task Group noted that it
    was not clear in the report whether this material was fibrous,
    particulate or both.  Particle concentrations measured by
    haemocytometer (resolution limit, 1 µm) in 1 ml suspensions 
    containing 0.5 mg of dust were 2.38 x 106 (reported as x 10-6)
    and 1.94 x 106 (reported as x 10-6).  Cell cultures were
    incubated for 2 h with an unspecified volume of a stock suspension
    containing 150 mg of dust per ml in BSS; samples were taken at 0, 1,
    and 2 h after the addition of the dust (Styles & Wilson, 1973).

         Extracts from three different types of polyethylene granules 
    (each with and without additives) and three polyethylene films 
    (high content of additives) were not mutagenic in  Salmonella 
     typhimurium strains TA98, TA100 and TA1537 (Fevolden & Moller,
    1978).  Information included in the published account of this study
    (limited to an abstract) was insufficient for evaluation.


         The data available on the health effects of synthetic organic 
    fibres in humans are extremely limited.  Information is currently 
    limited to case reports and small cross-sectional morbidity studies 
    of workers, without standardized methods and appropriate control 
    groups.  The negative results of some of these studies are most 
    likely a function of their limited power to detect an effect, since 
    only relatively small groups of workers with relatively low and 
    short exposures have been examined to date.  On the other hand, 
    positive results reported from studies with possibly higher 
    exposures are poorly documented and observed effects may be due, in
    part, to other substances present in the occupational environment.

    7.1  Carbon/graphite fibres

         In a cross-sectional study of 88 out of 110 workers in a
    PAN-based continuous filament carbon fibre production facility,
    there were no adverse respiratory effects, as assessed by
    radiographic and spirometric examination (for determination of FEV1
    and FVC) and the replies to questionnaires on respiratory symptoms. 
    Total dust concentrations were 0.08 to 0.39 mg/m3, with 40% being
    in the respirable range.  Only 31 of the workers examined, however, 
    had been employed for more than 5 years in the facility in which 
    carbon fibre production began in 1972 (Jones  et al., 1982).  The 
    Task Group considered that the short duration of exposure in this 
    study was insufficient for a reliable assessment of the potential 
    health effects of these fibres.

         Troitskaya (1988) reported that in a population of 327 
    examined workers in a PAN-based carbon fibre production facility,
    67.9% had pharyngitis or rhinopharyngitis, 34% reported bronchitis
    and 39.6% had dermal effects.  In addition to carbon fibres, workers
    were exposed to other substances such as ammonia, acrylonitrile and
    "carbon oxides".  It was not possible for the Task Group to assess
    the validity of these results based on data provided in the
    published account of the study.

         Cases of dermatitis in two workers were reported during a 
    clean-up operation at the site of an aircraft crash where carbon 
    fibres were detected at concentrations of up to 7 fibres/ml.  
    Additional details on the monitoring methods used in this study are
    presented in section 4.2 (Formisano, 1989).

    7.2  Aramid fibres

         There was no change in diffusing capacity in workers (n = 167) 
    involved in polyester fibre processing, for which exposures to 
    para-aramid fibres and sulfur dioxide were "low", when  compared to
    those in a non-exposed control group (n = 142) (Pal   et al.,
    1990).  The Task Group noted that no firm conclusions could be drawn

    on the basis of this study due to the lack of an  appropriate
    control group.

         Reinhardt (1980) briefly reported the results of patch testing 
    of panels of human volunteers to assess skin irritancy and 
    sensitization.  In these studies, which involved more than 100 
    individuals, there was no skin sensitization but some minimal skin 
    irritation following dermal contact with Kevlar or Nomex fabrics 
    (Reinhardt, 1980).  It was reported that because these fibres, 
    especially Kevlar, are stiff, there is a potential for causing 
    abrasive skin irritation under restrictive contact.


    8.1  Exposure

         Many factors determine the exposure levels and airborne fibre 
    characteristics for the synthetic organic fibres considered in this 
    document. Most important among these factors are: (1) the nominal 
    diameter of the parent fibre and distribution about this nominal 
    diameter; and (2) the tendency of some fibre types to form smaller 
    diameter fibres or to liberate smaller diameter fibrils during 
    processing. Fibres of concern for deposition in the bronchoalveolar 
    region in humans are those less than approximately 3 µm in 
    diameter.  Limited data are available concerning the fraction of 
    fibres smaller than 3 µm in diameter for most synthetic organic 
    fibrous products, although nominal fibre diameters are reported  to
    be generally greater than 5 µm.  Para-aramid fibres have fine-curled
    fibrils of less than 1 µm in diameter which can break off during
    processing.  Some polyolefin fibres are produced which also have
    nominal diameters of 0.1-2 µm.  Although carbon and graphite fibres
    normally have nominal diameters greater than 5 µm, some data suggest
    that diameters are reduced during incineration or other burning such
    as might occur after an aircraft crash.

         Only limited data are available concerning occupational 
    exposures to synthetic organic fibres and virtually no data are 
    available with respect to environmental fate, distribution and 
    general population exposures.  Sampling and analytical methods used
    for measuring synthetic organic fibre exposures are those normally
    used for asbestos and man-made mineral fibres.  Although these
    methods may be suitable, little method validation has taken place
    for synthetic organic fibres.

         Occupational exposure data summarized in section 4.1 generally 
    demonstrate low-level fibre exposures in fibre production 
    facilities.  Exposure levels in carbon fibre production are reported 
    to be generally less than 0.1 fibres/ml, although levels of 
    approximately 0.3 fibres/ml have been recorded. Exposure levels in
    facilities producing para-aramid fibres typically are less than
    0.1 fibres/ml, although levels of over 2 fibres/ml have been 
    measured during subsequent processing.  Spinning and weaving of 
    para-aramid staple yarns produces higher exposures (average 0.18 to
    0.55 fibres/ml).  Exposure levels in polypropylene fibre production
    and use are generally less than 0.1 fibres/ml, although  levels of
    0.5 fibres/ml have been reported.  Virtually no data exist for
    secondary fibre uses and applications.  Several exposure studies 
    have reported levels in mg/m3.  While these data may be useful in 
    overall evaluation of total dust exposures in the workplace,
    gravimetric determinations of airborne dust levels are of very
    limited  value with respect to assessment of organic fibre

         While airborne synthetic organic fibre concentrations for fibre 
    types considered in this document are generally lower than
    0.5 fibres/ml in the occupational environment, primarily in
    production, the possibility for higher exposures in different 
    applications and uses exists, particularly for those operations that 
    vigorously disturb fibres and the fibre matrix.  Additionally, 
    applications may involve exposures to other hazardous substances in
    the workplace.

    8.2  Health effects

         The potential adverse effects for humans from inhalation 
    exposure to synthetic organic fibres are the development of 
    malignant and non-malignant respiratory diseases.  Concern for 
    these effects is based on the health evidence derived from exposure
    to other respirable and durable fibrous materials.  Other effects of
    concern include contact dermatitis and skin irritation from dermal

         There is limited information on the health effects of synthetic 
    organic fibres in humans.  The information that does exist is 
    considered inadequate for assessment because of numerous 
    shortcomings including the inability to evaluate chronic effects 
    due to the relative short period (20 years) that the fibres under 
    review have been in production and use.  However, there is some 
    suggestive evidence that exposure to carbon and aramid fibres can 
    cause contact dermatitis and skin irritation.

         Toxicological data on carbon fibres and polyolefins fibres are 
    also limited.  The available animal data on acute and short-term 
    inhalation exposures to carbon and polypropylene fibres indicate 
    minimal respiratory system toxicity.  Information on the chronic 
    and carcinogenic effects of these fibres via inhalation is 
    unavailable.  However, the lack of appropriate animal data does not
    reduce the concern for potential health effects associated with 
    long-term exposures to these fibres.

         The available animal studies on exposure to respirable
    para-aramid fibrils indicate that acute and short-term inhalation 
    exposures at  concentrations as high as 1300 fibres/ml induce 
    minimal pulmonary toxicity in rats.  However, the results of the 
    only chronic inhalation study indicate that respirable para-aramid 
    fibres caused lung fibrosis (> 25 fibres/ml) and lung neoplasms
    (> 100 fibres/ml) in the rat.  On the basis of limited available
    data, a potential for fibrogenic and carcinogenic effects may exist
    from exposure to these synthetic organic fibres in the occupational 
    environment.  The potential health risk associated with exposure to
    these synthetic organic fibres in the general environment is unknown
    at this time, but is likely to be very low.


         The data reviewed in this report support the conclusion that 
    respirable, durable organic fibres are of potential health concern.  
    The following actions are suggested for protection of human health.

    1.   To the maximum extent possible, the organic fibres that are
         produced should be  non-inspirable or at least
          non-respirable.  Respirable fibres should not be produced by
         splitting or abrading during subsequent processing, use and

    2.   If small-diameter respirable fibres are necessary for specific
         products or applications, these fibres should not be
          biopersistent or exhibit other toxic effects.

    3.   All fibres that are  respirable and  biopersistent must
         undergo testing for toxicity and carcinogenicity.  Exposures to
         these fibres should be controlled to the same degree as that
         required for asbestos until data supporting a lesser degree of
         control become available.  The available data suggest that
         para-aramid fibres fall within this category.  Furthermore,
         other respirable organic fibres should be considered to fall
         within this category until data indicating a lesser degree of
         hazard become available.

    4.   Populations potentially exposed to respirable organic fibres
         should have their exposure monitored in order to evaluate
         exposure levels and the possible need for additional control

    5.   Populations identified as being those most exposed to
         respirable organic fibres should be enrolled in preventive
         medicine programmes that focus on the respiratory system. These
         data should be reviewed periodically for any early signs of
         adverse health effects.


    10.1  Sampling and analytical methods

         Sampling and analytical methods for synthetic organic fibres 
    have generally been adapted from methods that have been used for 
    asbestos.  These include the use of membrane filter samples with 
    analyses by phase contrast optical microscopy or limited use of 
    scanning and transmission electron microscopy.  Further validation 
    of these methods for synthetic organic fibres is necessary, with 
    particular attention to: (1) possible effects of high electrostatic 
    charges of organic fibres on sampling and analysis; (2) effects of 
    sample preparation on the integrity of fibres; and (3) combined 
    effects of fibre size and refractive index on visibility of organic 
    fibres under phase contrast microscopy.  These parameters could 
    introduce a negative bias in air sample results.

         In addition, methods for sampling and analysis of synthetic 
    organic fibres in biological tissues need further development and 

    10.2  Exposure measurement and characterization

         Much more information is needed relative to levels and 
    characteristics of exposure in plants producing and using synthetic 
    organic fibres.  Complete size distributions, with special attention 
    to those fibres less than approximately 3 µm in diameter, are 
    needed for synthetic organic fibres.  While some data are available 
    for the fibre-producing industries, very little information 
    concerning respirable fibre exposure is available in industries 
    using or applying these fibres.

         Information concerning environmental releases of synthetic 
    organic fibres or fibre concentrations in environmental media is 
    also very limited.  Data need to be collected concerning the 
    environmental fate and distribution of these materials and 
    resultant non-occupational exposures.

    10.3  Human epidemiology

         No reliable data exist concerning chronic effects of synthetic 
    organic fibre exposures.  Multi-centre studies are needed in order 
    to develop adequately sized cohorts for epidemiological studies.  
    Both cross-sectional and longitudinal studies of respiratory 
    morbidity, cancer mortality, and cancer incidence are needed.

    10.4  Toxicology studies

         With the exception of para-aramid fibres and fibrils, no 
    adequate toxicity data are available for other synthetic organic 
    fibres.  These data are badly needed.  Emphasis must be placed on

    chronic inhalation studies using respirable fibres of these 
    materials.  These studies must use fibre sizes which are respirable 
    in the animal species being used and at levels which are at or near 
    the maximum tolerated doses.  They should include studies of tissue
    burden in order to confirm the expected tissue doses.  A much better
    understanding of those characteristics of synthetic organic fibres
    (e.g., particle charge, agglomeration) that could effect deposition
    is needed.

         More data are needed concerning the biopersistence of 
    synthetic organic fibres.  The critical period of lung residence 
    necessary for development of adverse health effects remains to be 
    determined.  Better test materials for measuring biopersistence are 


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    Tayton K, Phillips G, & Ralis Z (1982) Long term effects of carbon
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    Troitskaya NA (1988) [Hygienic evaluation of working conditions in
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    Verwijst LPF (undated) Measuring exposure to fibres at the workplace
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    Volk HF (1979) Carbon (carbon and artificial graphite). In: Grayson
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    Warheit DB, Kellar KA, & Hartsky MA (1992) Pulmonary cellular
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    aramid fibrils: evidence for biodegradability of inhaled fibrils.
    Toxicol Appl Pharmacol, 116: 225-239.

    Warheit DB, Hansen JF, Carakostas MC, & Hartsky MA (in press/a)
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    effects. Ann Occup Hyg.

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    Biopersistence of inhaled organic and inorganic fibers in the lungs
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                 (5-6 October 1992)

         An international panel of 13 pathologists met to evaluate the
    cystic lesion observed in the lungs of rats in chronic inhalation
    studies with Kevlar1 aramid fibrils (Lee  et al., 1988) and
    titanium dioxide particles (Lee  et al., 1985). Slides
    representative of the entire spectrum of cystic keratinizing lesions
    observed in these studies were sent to each of these pathologists
    prior to the meeting.

         The pathologists agreed that the most appropriate morphologic
    diagnosis is Proliferative Keratin Cyst (PKC). The lesion is a cyst
    lined by a well-differentiated stratified squamous epithelium and
    with a central keratin mass. Growth appears to have occurred by
    keratin accumulation and by peripheral extension into the alveolar
    spaces. The lesion is sharply demarcated except in those areas in
    which there has been extension into adjacent alveoli. The squamous
    epithelium has few mitotic figures and dysplasia is absent.

         All participants agreed that the lesion is not a malignant
    neoplasm. The majority was of the opinion that the lesion is not
    neoplastic. A minority considered that the lesion is probably a
    benign tumour. The participants had not seen a similar lung lesion
    in humans.

         Morphologic features of the cystic lesion that were used to
    exclude malignancy were the lack of invasion of the pleura, blood
    vessels or the mediastinum as well as the absence of dysplasia and
    the paucity of mitotic figures.


    1  Kevlar is registered trademark of the DuPont Company for its
       para-aramid fibre.

    (5-6 October 1992)


    Dr M. Brockmann, Institut für Pathologie, Universitätsklinik 
       "Bergmann's Heil", Bochum, Germany

    Dr W.W. Carlton, Department of Veterinary Pathobiology, School of
       Veterinary Medicine, Purdue University, West Lafayette, Indiana,

    Dr J.M.G. Davis, Institute of Occupational Medicine, Edinburgh,
       United Kingdom

    Dr V.J. Feron, T.N.O. Toxicology and Nutrition Institute, Zeist,

    Dr M. Kuschner, Pathology Department, SUNY at Stonybrook Health
       Science Center, Stonybrook, New York, USA

    Dr K.P. Lee, Haskell Laboratory, E.I. du Pont de Nemours & Company,
       Haskell Laboratory for Toxicology and Industrial Medicine,
       Newark, Delaware, USA

    Dr L.S. Levy, Institute of Occupational Health, University of 
       Birmingham, Birmingham, United Kingdom  (Chairman)

    Dr E. McCaughey, Canadian Reference Center for Cancer Pathology,
       Ottawa Civic Hospital, Ottawa, Ontario, Canada

    Dr P. Nettesheim, Laboratory of Pulmonary Pathobiology, National
       Institute of Environmental Health Sciences, Research Triangle
       Park, North Carolina, USA

    Dr K. Nikula, Inhalation Toxicology Research Institute, 
       Albuquerque, New Mexico, USA

    Dr R. Renne, Batelle Pacific Northwest, Richland, Washington, USA

    Dr M. Schultz, Institut für Pathologie, Bezirkskrankenhaus
       Magdeburg, Magdeburg, Germany

    Dr V.S. Turusov, Cancer Research Center, Russian Academy of Medical
       Sciences, Moscow, Russia

    Dr J.C. Wagner, Preston, Weymouth, Dorset, United Kingdom


    Dr C.L.J. Braun, Akzo NV, Arnhem, Netherlands

    Dr R.C. Brown, MRC Toxicology Unit, Medical Research Council
       Laboratories, Carshalton, Surrey, United Kingdom

    Dr S.R. Frame, E.I. du Pont de Nemours & Company, Haskell 
       Laboratory for Toxicology and Industrial Medicine, Newark, 
       Delaware, USA

    Dr N.F. Johnson, Inhalation Toxicology Research Institute,
       Albuquerque, New Mexico, USA

    Dr G.L. Kennedy, Jr, E.I. du Pont de Nemours & Company, Haskell
       Laboratory for Toxicology and Industrial Medicine,  Newark,
       Delaware, USA

    Dr E.A. Merriman, E.I. du Pont de Nemours & Company, Wilmington,
       Delaware, USA

    Dr C.F. Reinhardt, E.I. du Pont de Nemours & Company, Haskell
       Laboratory for Toxicology and Industrial Medicine, Newark,
       Delaware, USA

    Dr J.W. Rothuizen, Rothuizen Consulting, En Thiéré, Genolier, 

    Dr O. von Susani, Du Pont International SA, Grand-Saconnex, Geneva,

    Dr D.B. Warheit, E.I. du Pont de Nemours & Company, Haskell
       Laboratory for Toxicology and Industrial Medicine, Newark,
       Delaware, USA


    1.  Identité, propriétés physiques et chimiques

         Les fibres de carbone/graphite sont des filaments de carbone
    produits par traitement à haute température de l'une ou l'autre des
    trois matières premières suivantes: rayonne (cellulose régénérée),
    brais de goudron, de houille ou de pétrole, or encore polyacrylo
    nitrile (PAN). Le diamètre nominal des fibres de carbone varie de 5
    à 15 µm. Les fibres de carbone sont flexibles; elles conduisent
    l'électricité et la chaleur et les variétés à haute performance sont
    dotées d'un module de Young élevé (coefficient d'élasticité qui
    mesure la souplesse ou la rigidité d'un matériau) et d'une forte
    résistance à la traction. Elles sont résistantes à la corrosion,
    légères, réfringentes et chimiquement inertes (sauf à l'oxydation). 
    En outre, elles sont très stables à la traction, possèdent un faible
    coefficient de dilatation thermique et une faible densité et sont
    très résistantes à l'abrasion et à l'usure.

         Les fibres aramides sont préparées par réaction de diamines
    aromatiques sur des dichlorures d'acides aromatiques. On les produit
    sous la forme de filaments continus, de fibres discontinues et de
    pulpe. Il existe deux types principaux de fibres aramides, le para-
    et le méta-aramide, qui ont toutes deux un diamètre nominal de 12 à
    15 µm. Les fibres para-aramides sont parfois munies de fibrilles
    finement recourbées et enchevêtrées qui se situent dans la gamme des
    particules respirables (< 1 µm de diamètre) sur la surface de la
    fibre centrale. Ces fibrilles peuvent se détacher par abrasion lors
    de la fabrication ou de l'utilisation des fibres et être suspendus
    dans l'atmosphère. En général, les fibres aramides présentent une
    résistance à la traction moyenne à très élevée, une résistance à
    l'allongement moyenne et un module de Young moyen à très élevé.
    Elles résistent à la chaleur, aux produits chimiques et à

         Les fibres polyoléfiniques sont constituées de polymères à
    longue chaîne composés d'au moins 85% en poids d'unités d'éthylène,
    de propylène et d'autres oléfines; le polyéthylène et le
    polypropylène sont utilisés dans le commerce. A part pour certains
    types comme les microfibres, le diamètre nominal de la plupart des
    fibres de polyoléfine est suffisamment important pour que très peu
    d'entre elles se situent dans la limite des particules respirables.

         Les polyoléfines sont très hydrophobes et chimiquement inertes.
    Leur résistance à la traction est beaucoup plus faible que celle des
    fibres de carbone ou des fibres d'aramide et elles sont relativement
    inflammables, avec un point de fusion situé entre 100 et 200 °C.

         Les méthodes qui ont été mises au point pour le comptage des
    fibres minérales sont utilisées en hygiène industrielle pour la
    surveillance des fibres organiques synthétiques. Toutefois la valeur
    de ces méthodes n'a pas été vérifiée dans ce cas. Des facteurs
    tenant aux propriétés électrostatiques, à la solubilité dans la
    préparation des échantillons et à l'indice de réfraction peuvent
    poser des problèmes lorsqu'on a recours à ces méthodes.

    2.  Sources d'exposition humaine et environnementale

         On estime que la production mondiale de fibres de carbone et de
    graphite a dépassé 4000 tonnes en 1984. En ce qui concerne les
    fibres aramides, elle était supérieure à 30 000 tonnes en 1989 et la
    production de fibres polyoléfiniques dépassait les 182 000 tonnes
    pour les seuls Etats-Unis d'Amérique. Les fibres de carbone et
    d'aramide sont utilisées principalement pour la fabrication de
    matériaux composites dont les industries aérospatiales, militaires
    et autres ont besoin pour améliorer la résistance, la rigidité, la
    durabilité et la conductivité électrique ou la tenue à la chaleur de
    certains éléments. Les fibres polyoléfiniques sont surtout utilisées
    dans l'industrie textile.

         Des cas d'exposition aux fibres organiques synthétiques sur les
    lieux de travail ont été rapportés. Ces fibres peuvent pénétrer dans
    l'environnement lors de la production, de la transformation ou de la
    combustion des matériaux composites ou lors de leur mise au rebut.
    On ne dispose que de très peu de données sur les rejets effectifs de
    ces produits dans l'environnement.

         Les données dont on dispose sur le transport, la distribution
    et la transformation des fibres organiques dans l'environnement se
    limitent à l'identification des produits résultant de
    l'incinération, par les services municipaux, des déchets de
    matériaux composites à base de fibres de carbone et des produits de
    pyrolyse des fibres de carbone et des fibres aramides. Des
    expériences au cours desquelles on a simulé la combustion dans des
    incinérateurs municipaux ont permis de constater qu'il y avait
    réduction du diamètre et de la longueur des fibres de carbone. La
    décomposition pyrolytique de fibres de carbone et d'aramide produit
    principalement des hydrocarbures aromatiques, du dioxyde et de
    monoxyde de carbone et des cyanures.

    3.  Concentrations dans l'environnement et exposition
        de l'homme

         Des poussières de fibres organiques synthétiques peuvent être 
    libérées sur les lieux de travail lors d'opérations telles que la 
    production, le bobinage, coupe le tissage et la coupe des fibres, de 
    même qu'au cours de l'usinage, de la réalisation et de la 
    manipulation des matériaux composites.

         Dans le cas des fibres de carbone et de graphite, les 
    concentrations en fibres respirables sont généralement inférieures 
    à 0,1 fibres/ml, mais on a mesuré des concentrations pouvant aller 
    jusqu'à 0,3 fibres/ml à proximité des opérations de coupe ou de 
    bobinage.  Des fibres peuvent également être libérées dans 
    l'environnement lors de l'usinage (perçage, sciage, etc) des 
    composites à base de fibres de carbone, encore que la plupart des 
    matériaux respirables ainsi produits soient non fibreux.

         On a fait état, sur les lieux de travail, de concentrations 
    moyennes en fibrilles de para-aramide aéroportées inférieures à
    0,1 fibrille/ml au cours des opérations concernant les fils continus 
    et à 0,2 fibrilles/ml lors de la coupe en floc et en pulpe.  En 
    filature, on observe des concentrations moyennes en fibrilles 
    aéroportées qui se caractérisent par une valeur inférieure à
    0,5 fibrille/ml, mais on a aussi fait état de concentrations de
    l'ordre de 2,0 fibrilles/ml.  On note aussi des valeurs 
    caractéristiques de l'exposition moyenne inférieures à
    0,1 fibrille/ml pour d'autres applications avec des maxima 
    atteignant 0,3 fibrilles/ml.  On a mis en évidence un risque 
    particulier d'exposition lors du découpage des composites au jet 
    hyperbarique, les concentrations pouvant atteindre  2,91
    fibrilles/ml.  Au cours de découpage au laser de résines époxy 
    renforcées par des fibres d'aramide, des particules d'un diamètre 
    aérodynamique moyen de 0,21 µm on été observées mais l'étude en 
    question n'indique pas la teneur en fibrilles de la poussière.  Au 
    cours de ce type d'opérations, il y également production d'un 
    certain nombre de composés organiques volatils (notamment du 
    benzène, du toluène, du benzonitrile et du styrène) ainsi que des 
    gaz comme le cyanure d'hydrogène, le monoxyde de carbone et le 
    dioxyde d'azote.

         D'après des données limitées établies durant la surveillance de 
    l'air dans un atelier produisant des fibres de polypropylène, les 
    concentrations maximales de fibres aéroportées d'une longueur 
    dépassant les 5 µm, se situent autour de 0,5 fibre/ml, la plupart 
    des valeurs étant inférieures à 0,1 fibre/ml.  L'examen au 
    microscope électronique à balayage a montré que le diamètre des 
    fibres aéroportées allait de 0,25 à 3,5 µm et leur longueur de
    1,7  à 69 µm.  Dans un échantillon unique d'air ambiant prélevé à 
    proximité d'une unité de tissage de fibres de carbone, on a relevé 
    une concentration de 0,0003 fibre/ml.  Les dimensions moyennes  des
    fibres étaient de 706 µm par 3,9 µm.  On a également signalé  la
    présence de fibres de carbone à l'endroit où s'étaient écrasés deux
    aéronefs militaires, présence attribuable à la combustion du
    composite de fibres de carbone utilisé pour la construction de ces
    appareils.  Aucune autre donnée utile sur les concentrations dans 
    l'environnement n'a été fournie.

    4.  Dépôt, élimination, rétention, persistance et redistribution

         On ne dispose que de peu de données sur les fibres organiques
    synthétiques.  Des données relatives aux fibres de para-aramide
    (Kevlar) indiquent que ces fibres se déposent au niveau de la
    bifurcation des canaux alvéolaires.  On pense également que ces
    fibres sont transportées jusqu'aux ganglions lymphatiques

    5.  Effets sur les animaux de laboratoire et les systèmes  in vitro

         On manque de données satisfaisantes résultant d'études
    expérimentales valables sur les divers types de fibres organiques
    synthétiques examinées ici.

         Aucune étude valable n'a été consacrée à l'examen du pouvoir
    fibrogène ou cancérogène des fibres de carbone et de graphite.  
    Parmi les effets observés à la suite de l'exposition, pendant 
    quelques jours, par la voie respiratoire, de rats à des fibres de 
    dimension respirable produites à partir de différents types de 
    brais, on a relevé des réactions inflammatoires, un accroissement 
    du remplacement des cellules parenchymateuses et une hyperplasie 
    minime des cellules alvéolaires (pneumocytes de Type II).  Les 
    données fournies par une étude au cours de laquelle on a procédé à
    des instillations intratrachéennes et à des injections
    intrapéritonéales, sont jugées insuffisantes pour permettre une 
    évaluation, du fait que les matériaux étudiés n'ont pas été 
    caractérisés et que l'étude ne donne pas suffisamment de
    renseignements sur le protocole suivi et les résultats obtenus.  Une
    étude par badigeonnage cutané sur la souris, portant sur quatre 
    types de fibres de carbone en suspension dans le benzène, a été 
    considérée comme insuffisante pour permettre une évaluation de 
    l'activité cancérogène de ces fibres.

         Dans le cas des fibres de para-aramide, l'essentiel des données
    résulte de l'expérimentation sur le Kevlar. Des études de brève
    durée (deux semaines), au cours desquelles des animaux ont été
    exposés à de la poussière de Kevlar par la voie respiratoire, ont
    montré qu'il y avait une réaction dont l'intensité diminuait après
    cessation de l'exposition au niveau des macrophages pulmonaires. 
    Des études de brève durée portant sur des fibrilles ultrafines de
    Kevlar, ont révélé une réaction analogue au niveau de macrophages
    avec des plaques d'épaississement au niveau des canaux alvéolaires.
    Ces deux types de lésion ont également régressé après l'exposition,
    mais trois à six mois plus tard on observait encore une fibrose
    minime résiduelle. L'exposition par la voie respiratoire, pendant
    deux ans, de rats à des fibrilles de Kevlar a produit une fibrose
    pulmonaire liée à cette exposition (à une concentration > 25
    fibres/ml) et a conduit à la formation de tumeurs pulmonaires (11% à
    la concentration de 400 fibres/ml et 6% à la concentration de
    100 fibres/ml chez les femelles; 3% à la concentration de

    400 fibres/ml chez les mâles) d'un type inhabituel (carcinomes
    spino-cellulaires kystiques kératinisants).  Une surmortalité due à
    une toxicité pulmonaire a été observée à la concentration la plus
    élevée, ce qui indique que la dose maximale tolérable avait été
    dépassée. La portée biologique de ces lésions et leur signification
    pour la santé humaine ont été très débattues. Il est possible que
    cette étude, qui s'est achevée au bout de 24 mois, n'ait pas révélé
    la totalité du pouvoir cancérogène des fibrilles.

         L'instillation intertrachéenne d'une dose unique de papier
    désagrégé Nomex (2,5 mg) contenant des fibres d'un diamètre allant
    de 2 à 30 µm a produit une réaction inflammatoire non spécifique.
    Une réaction granulomateuse s'est produite deux ans après
    l'exposition. L'instillation intratrachéenne d'une dose unique de 25
    mg de Kevlar a provoqué une réaction inflammatoire non spécifique
    qui a disparu en l'espace d'environ une semaine.  Ultérieurement, on
    a observé une réaction granulomateuse et une fibrose minime.

         Dans trois études, l'injection intrapéritonéale de fibres de
    Kevlar (jusqu'à 25 mg/kg) a induit une réponse granulomateuse sans
    toutefois que l'incidence des néoplasmes ne présente d'augmentation
    significative. Selon les auteurs de cette étude, l'absence de
    réponse tumorale pourrait s'expliquer par l'agglomération des
    fibrilles de Kevlar dans la cavité péritonéale.

         Il n'existe pas d'études valables au cours desquelles on ait
    examiné le pouvoir cancérogène ou fibrogène des fibres de
    polyoléfines. Une étude au cours de laquelle on a exposé des rats à
    des fibres respirables de polypropylène pendant 40 jours par la voie
    respiratoire (46% des fibres < 1 µm) à des concentrations allant
    jusqu'à 50 fibres/ml, a permis de constater des modifications liées
    à la dose et à la durée de l'exposition et qui se caractérisaient
    par un accroissement de la cellularité et une bronchiolite. On ne
    dispose d'aucune donnée utile sur l'effet de l'instillation
    intratrachéenne. Des études au cours desquelles on a injecté à des
    rats des fibres ou de la poussière de polypropylène dans la cavité
    intrapéritonéale, n'ont pas révélé d'accroissement sensible du
    nombre de tumeurs péritonéales.

         On ne dispose pas de données suffisantes pour pouvoir évaluer
    la toxicité  in vitro et la génotoxicité des fibres organiques
    synthétiques. Dans le cas des aramides, les études montrent que le
    fibrilles de para-aramide fines et courtes présentent des propriétés
    cytotoxiques. Pour ce qui est des fibres polyoléfiniques, il
    semblerait que les fibres de polypropylène présentent une certaine
    cytotoxicité. Les tests de mutagénicité sur des extraits de granulés
    de polyéthylène n'ont donné que des résultats négatifs.

    6.  Effets sur l'homme

         Une étude transversale relative à 88 des 110 employés d'une
    unité de production de fibres de carbone continues à base de
    polyacrylonitrile, n'a révélé aucun effet nocif sur la fonction
    respiratoire comme l'ont montré les examens radiographiques et
    spirométriques et les questionnaires sur les symptômes
    respiratoires. D'autres études moins bien documentées ont fait état
    d'effets indésirables chez des ouvriers travaillant à la production
    de fibres de carbone et de polyamide; les données qui figurent dans
    ces publications sont toutefois insuffisantes pour qu'on puisse se
    prononcer sur la validité des inférences indiquées.

    7.  Résumé de l'évaluation

         On ne dispose que de données limitées sur les niveaux
    d'exposition à la plupart des fibres organiques synthétiques. Les
    données disponibles indiquent en général que sur les lieux de
    travail, l'exposition est faible. Toutefois il subsiste un risque
    d'exposition plus importante lors d'applications et d'utilisations
    futures. On ne dispose pratiquement d'aucune donnée sur la destinée
    et la répartition dans l'environnement de ces fibres ni sur
    l'exposition de la population générale.

         En se basant sur les données toxicologiques limitées fournies
    par l'expérimentation animale, on peut conclure qu'il existe une
    possibilité d'effets nocifs sur la santé en cas d'exposition par la
    voie respiratoire à ces fibres organiques de synthèse sur le lieu de
    travail. On ignore pour l'instant le risque qu'impliquerait
    l'exposition à ces fibres dans l'environnement général, mais il est
    probablement très faible.


    1.  Identidad, propiedades físicas y químicas

         Las fibras de carbono/grafito son formas filamentosas de carbón
    que se obtienen procesando a alta temperatura alguno de los tres
    materiales precursores siguientes: rayón (celulosa regenerada), brea
    (residuo de petróleo o alquitrán) o poliacrilonitrilo (PAN). El
    diámetro nominal de las fibras de carbono oscila entre 5 y 15 µm.
    Las fibras de carbono son flexibles, eléctrica y térmicamente
    conductivas, y sus variedades de alto rendimiento poseen un módulo
    de Young (coeficiente de elasticidad que refleja la mayor o menor
    rigidez del material) alto y una gran resistencia a la tracción. Son
    resistentes a la corrosión, ligeras, refractivas y químicamente
    inertes (excepto a la oxidación), y presentan una gran estabilidad
    frente a las fuerzas de tracción, una baja densidad y expansión
    térmica y una alta resistencia a la abrasión y al desgaste.

         Las fibras de aramida se forman por reacción entre diaminas
    aromáticas y dicloroácidos aromáticos. Son producidas en forma de
    filamentos continuos, hebras y pulpa. Hay dos tipos principales de
    fibras de aramida: para- y meta-aramida, ambas con un diámetro
    nominal de 12-15 µm. Las fibras de para-aramida pueden presentar,
    adheridas a la superficie de su parte central, fibrillas muy rizadas
    y entrelazadas del tamaño de las partículas respirables
    (< 1 µm de diámetro). Estas fibrillas pueden desprenderse y quedar
    suspendidas en el aire en caso de abrasión durante su fabricación o
    empleo. Por lo general, las fibras de aramida presentan una
    resistencia a la tracción entre mediana y muy alta, una elongación
    entre mediana y baja, y un módulo de Young entre moderado y muy
    alto. Son resistentes al calor, a los productos químicos y a la

         Las fibras de poliolefina son polímeros de cadena larga
    compuestos por al menos un 85% (respecto al peso) de etileno,
    propileno u otras unidades de olefina; el polietileno y el
    polipropileno se emplean comercialmente. Salvo algunas excepciones,
    como la microfibra, los diámetros nominales de la mayoría de los
    distintos tipos de fibras de poliolefina son bastante grandes, y no
    abundan los de tamaño respirable.

         Las poliolefinas son extremadamente hidrofóbicas e inertes.  Su
    resistencia a la tracción es notablemente inferior a la del carbono
    o de las fibras de aramida. Son relativamente inflamables, y sus
    temperaturas de fusión oscilan entre 100 y 200 °C.

         Para la vigilancia, a efectos de higiene industrial, de las
    fibras orgánicas sintéticas se han utilizado métodos desarrollados
    para contar fibras minerales, métodos que, no obstante, no han sido
    validados para esa finalidad. Factores tales como las propiedades

    electrostáticas, la solubilidad en el líquido de montaje y el índice
    refractivo pueden ser fuente de problemas al emplear esos métodos.

    2.  Fuentes de exposición humana y ambiental

         La producción mundial estimada de fibras de carbono y grafito
    superó las 4000 toneladas en 1984. Por lo que se refiere a la
    aramida, se superaron las 30 000 toneladas en 1989, y la producción
    de fibras de poliolefina sobrepasó las 182 000 toneladas (sólo en
    los Estados Unidos de América). Las fibras de carbono y aramida se
    emplean principalmente para fabricar materiales compuestos avanzados
    en las industrias aeroespacial, militar y otras, con el objeto de
    mejorar su resistencia, rigidez, durabilidad, conductividad
    eléctrica o resistencia térmica. Las fibras de poliolefina se
    utilizan normalmente en la industria textil.

         Se han descrito casos de exposición a fibras orgánicas
    sintéticas en el medio ocupacional. Durante la producción,
    elaboración o combustión de materiales compuestos, así como durante
    su evacuación, puede producirse una liberación de fibras orgánicas
    sintéticas en el medio ambiente. Se dispone de muy pocos datos sobre
    la liberación real de esos materiales en el entorno.

         Los datos disponibles sobre el transporte, distribución y
    transformación de fibras orgánicas en el medio ambiente se limitan a
    la identificación de los productos que resultan de la incineración
    municipal de los desechos a partir de compuestos que contienen
    fibras de carbono y de productos de la descomposición pirolítica de
    fibras de carbono y aramidas. Durante la simulación de la
    incineración municipal se redujeron tanto el diámetro como la
    longitud de las fibras de carbono. Entre los principales productos
    de la descomposición pirolítica de fibras de carbono y aramida
    figuran hidrocarburos aromáticos, dióxido de carbono, monóxido de
    carbono, y cianuros.

    3.  Niveles ambientales y exposición humana

         En el lugar de trabajo se liberan fibrillas orgánicas
    sintéticas durante operaciones tales como la producción, bobinado,
    troceado, entrelazado, corte y maquinado de las fibras, así como
    durante la formación y manipulación de materiales compuestos.

         En el caso de las fibras de carbono/grafito, las
    concentraciones de fibra respirable son por lo general inferiores a
    0,1 fibras/ml, pero se han detectado concentraciones de hasta
    0,3 fibras/ml en las proximidades de instalaciones de troceado o
    bobinado. También pueden liberarse fibras durante el maquinado
    (perforación, aserrado, etc.) de compuestos de fibra de carbono, si
    bien la mayor parte del material respirable generado en esos casos
    no es de carácter fibroso.

         Se ha notificado que las concentraciones medias de fibrillas de
    para-aramida en suspensión en el aire del lugar de trabajo son
    inferiores a 0,1 fibrillas/ml trabajando filamentos, y de menos de
    0,2 fibrillas/ml en las instalaciones de corte de vedijas y
    fabricación de pulpa. Durante el procesamiento de la hilaza, la
    concentración media de fibrillas en suspensión en el aire es
    generalmente inferior a 0,5 fibrillas/ml, si bien se han notificado
    niveles de hasta aproximadamente 2,0 fibrillas/ml. Otras
    exposiciones asociadas a usos finales en el lugar de trabajo son
    normalmente inferiores a 0,1 fibrillas/ml como promedio, cifrándose
    las exposiciones máximas en 0,3 fibrillas/ml. Se ha demostrado que
    conlleva un riesgo especial de exposición el corte de materiales
    compuestos por chorro de agua hiperbárico, operación en la que se
    han detectado niveles de hasta 2,91 fibrillas/ml. Se han generado
    partículas de un diámetro aerodinámico medio de 0,21 µm durante el
    corte mediante láser de epoxiplásticos reforzados con fibras de
    aramida, pero no se ha notificado el contenido de fibra del polvo.
    Durante esas operaciones se producen también determinados compuestos
    orgánicos volátiles (en particular benceno, tolueno, benzonitrilo y
    estireno) y otros gases (cianuro de hidrógeno, monóxido de carbono y
    dióxido de nitrógeno).

         Los datos limitados obtenidos en una fábrica de producción de
    fibras de polipropileno muestran que en el caso de las fibras de más
    de 5 µm de longitud su concentración máxima en el aire era de 0,5
    fibras/ml, siendo la mayoría de los valores inferiores a
    0,1 fibras/ml. La microscopía electrónica de barrido mostró que el
    tamaño de las fibras suspendidas en el aire oscilaba entre 0,25 y
    3,5 µm de diámetro y 1,7 y 69 µm de longitud. En una sola muestra
    ambiental recogida cerca de una tejeduría de fibra de carbono, se
    detectó una concentración de 0,0003 fibras/ml. La dimensión de esas
    fibras era como promedio de 706 µm por 3,9 µm. Se ha notificado
    también la liberación de fibra de carbono en el lugar de colisión de
    dos aeronaves militares, de resultas de la combustión del compuesto
    de fibra de carbono empleado en su construcción. No se obtuvo ningún
    otro dato de interés sobre las concentraciones presentes en el medio

    4.  Depósito, eliminación, retención, durabilidad y translocación

         La información obtenida acerca de fibras orgánicas sintéticas
    específicas es escasa. Los datos referentes a las fibras de
    para-aramida inhaladas (Kevlar) indican que éstas se depositan en
    las bifurcaciones de los conductos alveolares. Hay también indicios
    de translocación a los nódulos linfáticos traqueobronquiales.

    5.  Efectos en los animales de experimentación y en los sistemas de
      prueba  in vitro

         En lo que respecta a los tipos de fibra orgánica sintética aquí
    analizados, son muy pocos los datos de buena calidad aportados por
    los estudios experimentales realizados al efecto.

         No hay ningún estudio en que se haya examinado adecuada mente
    el potencial fibrógeno o carcinógeno de las fibras de
    carbono/grafito. En ratas expuestas a la inhalación de corta
    duración (unos días) de fibras de tamaño respirable obtenidas a
    partir de brea se observaron respuestas inflamatorias, un aumento de
    la velocidad de recambio de las células parenquimatosas y una
    hiperplasia mínima de las células alveolares de tipo II. Se
    considera que los resultados de un estudio realizado mediante
    instilación intratraqueal e inyección intraperitoneal no se prestan
    a evaluación, debido a la insuficiente caracterización del material
    de ensayo y a la falta de documentación adecuada sobre el protocolo
    y los resultados. Un estudio realizado con ratones a los que se
    pintó la piel con cuatro tipos de fibra de carbono suspendidos en
    benceno resultó inadecuado para evaluar la actividad carcinógena.

         En el caso de las fibras de paraaramida, la mayor parte de los
    datos proceden de experimentos realizados con Kevlar. Los estudios
    efectuados sobre los efectos de la inhalación de corta duración
    (2 semanas) de polvo de Kevlar han puesto de manifiesto una
    respuesta macrofágica pulmonar cuya gravedad disminuye tras la
    interrupción de la exposición. Los estudios de corta duración
    realizados con fibrillas de Kevlar ultrafinas han revelado una
    respuesta macrofágica parecida y un espesamiento desigual de los
    conductos alveolares. Esos dos tipos de lesiones remitieron también
    tras la exposición, pero a los 3-6 meses persistía todavía un grado
    mínimo de fibrosis. En un estudio realizado con ratas a las que se
    sometió durante dos años a inhalación de fibrillas de Kevlar se
    observó la aparición, relacionada con esa exposición, de fibrosis
    pulmonar (a concentraciones superiores a 25 fibras/ml) y de
    neoplasias pulmonares de un tipo inusitado (carcinoma escamoso
    quístico queratinizante) en un 11% de hembras a concentraciones de
    400 fibras/ml, en un 6% de hembras a concentraciones de 100
    fibras/ml; y en un 3% en los machos a concentraciones de 400
    fibras/ml. El aumento de mortalidad por toxicidad pulmonar se
    observó a la mayor de las concentraciones, lo que sugiere que se
    había sobrepasado la Dosis Máxima Tolerada.  Existe una considerable
    polémica acerca del potencial biológico de esas lesiones y su
    trascendencia para la especie humana. Ese estudio, por haber durado
    sólo 24 meses, tal vez no haya revelado todo el potencial
    carcinógeno de las fibrillas.

         La instilación intratraqueal de una sola dosis de papel Nomex
    troceado (2,5 mg) que contenía fibras de diámetros comprendidos
    entre 2 y 30 µm provocó una respuesta inflamatoria inespecífica; dos
    años después de la exposición tuvo lugar una respuesta

    granulomatosa. La instilación intratraqueal de una sola dosis de 25
    mg de Kevlar provocó una respuesta inflamatoria inespecífica que
    remitió al cabo de una semana aproximadamente; más tarde se observó
    una respuesta granulomatosa y un grado mínimo de fibrosis.

         En tres estudios en que se inyectaron intraperitonealmente
    fibras de Kevlar (hasta 25 mg/kg) se observó una respuesta
    granulomatosa, pero no así un aumento significativo de la incidencia
    de neoplasias. Los autores de esas investigaciones indicaron que si
    no se había observado una respuesta neoplásica era posiblemente
    porque se había producido una aglomeración de las fibrillas de
    Kevlar en la cavidad peritoneal.

         No existen estudios en que se haya examinado adecuadamente el
    potencial fibrógeno o carcinógeno de las fibras de poliolefina.  En
    un experimento realizado con ratas sometidas durante 90 días a
    inhalación de fibras de polipropileno (hasta 50 fibras/ml)
    respirables (46% < 1 µm) se observaron cambios dependientes de la
    dosis y duración de la exposición, consistentes en un aumento de la
    celularidad y bronquiolitis. No se dispone de datos pertinentes
    sobre los efectos de la instilación intratraqueal. En los estudios
    efectuados en ratas mediante inyección intraperitoneal de fibras o
    polvo de polipropileno no se observó ningún aumento significativo de
    la incidencia de tumores peritoneales.

         Los datos de que se dispone para evaluar la toxicidad y
    genotoxicidad  in vitro de las fibras orgánicas sintéticas no son
    adecuadas. En el caso de las aramidas, los estudios han demostrado
    que las fibrillas cortas y finas de para-aramida tienen propiedades
    citotóxicas. Respecto a las fibras de poliolefina, hay algunos
    indicios de que las fibras de polipropileno son citotóxicas. Las
    pruebas de mutagenicidad realizadas con extractos de gránulos de
    polietileno arrojaron resultados negativos.

    6.  Efectos en el ser humano

         En un estudio transversal realizado en 88 de los 110
    trabajadores de una fábrica de producción de fibra de carbono de
    filamento continuo a partir de poliacrilonitrilo, la exploración
    radiográfica y espirométrica y los cuestionarios sobre síntomas
    respiratorios no pusieron de manifiesto ningún efecto respiratorio
    nocivo. En otros estudios no tan bien documentados se notificó el
    hallazgo de efectos adversos en trabajadores dedicados a la
    producción de fibras tanto de carbono como de poliamida; los datos
    publicados sobre estas investigaciones, sin embargo, eran
    insuficientes para determinar la validez de las correlaciones

    7.  Resumen de la evaluación

         La información sobre los niveles de exposición a la mayoría de
    las fibras orgánicas sintéticas es limitada. Los datos disponibles
    indican en general que los niveles de exposición en el entorno
    ocupacional son bajos. Cabe la posibilidad de que en futuras
    aplicaciones y usos se alcancen exposiciones más elevadas. No se
    sabe prácticamente nada sobre lo que ocurre con esos productos en el
    medio ambiente ni sobre su distribución o las exposiciones de la
    población general.

         A tenor de los escasos datos toxicológicos obtenidos con
    animales de laboratorio, cabe concluir que existe la posibilidad de
    que la exposición ocupacional a esas fibras orgánicas sintéticas por
    inhalación tenga efectos adversos en la salud. El riesgo potencial
    para la salud asociado a la exposición a las fibras orgánicas
    sintéticas presentes en el entorno general se desconoce por el
    momento, pero probablemente sea muy bajo.

    See Also:
       Toxicological Abbreviations