Chrysotile asbestos (EHC 203, 1998)

UNITED NATIONS ENVIRONMENT PROGRAMME
INTERNATIONAL LABOUR ORGANISATION
WORLD HEALTH ORGANIZATION

INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

ENVIRONMENTAL HEALTH CRITERIA 203

CHRYSOTILE ASBESTOS

This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.

First draft prepared by Dr G. Gibbs, Canada (Chapter 2), Mr B.J. Pigg,
USA (Chapter 3), Professor W.J. Nicholson, USA (Chapter 4),
Dr A. Morgan, UK and Professor M. Lippmann, USA (Chapter 5),
Dr J.M.G. Davis, UK and Professor B.T. Mossman, USA (Chapter 6),
Professor J.C. McDonald, UK, Professor P.J. Landrigan, USA and
Professor W.J. Nicholson, USA (Chapter 7), Professor H. Schreier,
Canada (Chapter 8).

Published under the joint sponsorship of the United Nations
Environment Programme, the International Labour Organisation, and the
World Health Organization, and produced within the framework of the
Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization Geneva, 1998

The International Programme on Chemical Safety (IPCS),
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WHO Library Cataloguing in Publication Data

Chrysotile Asbestos.

(Environmental health criteria ; 203)

1.Asbestos, Serpentine - adverse effects
2.Asbestos, Serpentine - toxicity
3.Environmental exposure 4.Occupational exposure
I.International Programme on Chemical Safety II.Series

ISBN 92 4 157203 5 (NLM Classification: WA 754)
ISSN 0250-863X

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR CHRYSOTILE ASBESTOS

PREAMBLE

ABBREVIATIONS

INTRODUCTION

1. SUMMARY

1.1. Identity, physical and chemical properties, sampling and
analysis
1.2. Sources of occupational and environmental exposure
1.3. Occupational and environmental exposure levels
1.4. Uptake, clearance, retention and translocation
1.5. Effects on animals and cells
1.6. Effects on humans
1.7. Environmental fate and effects on biota

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, SAMPLING AND ANALYSIS

2.1. Identity
2.1.1. Chemical composition
2.1.2. Structure
2.1.3. Fibre forms in the ore
2.1.4. Fibre properties
2.1.5. UICC samples
2.1.6. Associated minerals in chrysotile ore
2.2. Physical and chemical properties
2.2.1. Physical properties
2.2.2. Chemical properties
2.3. Sampling and analytical methods
2.3.1. Workplace sampling
2.3.2. Sampling in the general environment
2.3.3. Analytical methods
2.3.3.1 Fibre identification
2.3.3.2 Measurement of airborne fibre
concentrations
2.3.3.3 Lung tissue analysis
2.3.3.4 Gravimetric analysis
2.4. Conversion factors
2.4.1. Conversion from airborne particle to
fibre concentrations
2.4.2. Conversion from total mass to fibre
number concentrations

3. SOURCES OF OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE

3.1. Natural occurrence
3.2. Anthropogenic sources
3.2.1. Production
3.2.2. Manufacture of products
3.2.3. Use of products

4. OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE LEVELS

4.1. Occupational exposure
4.1.1. Mining and milling
4.1.2. Textile production
4.1.3. Asbestos-cement
4.1.4. Friction products
4.1.5. Exposure of building maintenance personnel
4.1.6. Various industries
4.2. Non-occupational exposure
4.2.1. Ambient air
4.2.2. Indoor air

5. UPTAKE, CLEARANCE, RETENTION AND TRANSLOCATION

5.1. Inhalation
5.1.1. General principles
5.1.2. Fibre deposition
5.1.3. Fibre clearance and retention
5.1.3.1 Fibre clearance and retention in humans
5.1.3.2 Fibre clearance and retention in
laboratory animals
5.1.4. Fibre translocation
5.1.4.1 Fibre translocation in humans
5.1.4.2 Fibre translocation in animal models
5.1.5. Mechanisms of fibre clearance
5.2. Ingestion

6. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS

6.1. Introduction
6.2. Effects on laboratory mammals
6.2.1. Summary of previous studies
6.2.2. Recent long-term inhalation studies
6.2.3. Intratracheal and intrabronchial injection studies
6.2.4. Intraperitoneal and intrapleural injection studies
6.2.5. Ingestion studies
6.3. Studies on cells
6.3.1. Genotoxicity and interactions with DNA
6.3.2. Cell proliferation
6.3.3. Inflammation
6.3.4. Cell death and cytotoxicity
6.3.5. Liberation of growth factors and other response of
cells of the immune system

7. EFFECTS ON HUMANS

7.1. Occupational exposure

7.1.1. Pneumoconiosis and other non-malignant respiratory
effects
7.1.2. Lung cancer and mesothelioma
7.1.2.1 Critical occupational cohort studies -
chrysotile
7.1.2.2 Comparisons of lung cancer
exposure-response - critical studies
7.1.2.3 Other relevant studies
7.1.3. Other malignant diseases
7.1.3.1 Critical occupational cohort studies
involving chrysotile
7.1.3.2 Other relevant studies
7.2. Non-occupational exposure

8. ENVIRONMENTAL FATE AND EFFECTS ON BIOTA

8.1. Environmental transport and distribution
8.1.1. Chrysotile fibres in water
8.1.2. Chrysotile fibres in soil
8.2. Effects on biota
8.2.1. Impact on plants
8.2.2. Impact on terrestrial life-forms
8.2.3. Impact on aquatic biota

9. EVALUATION OF HEALTH RISKS OF EXPOSURE TO CHRYSOTILE ASBESTOS

9.1. Introduction
9.2. Exposure
9.2.1. Occupational exposure
9.2.1.1 Production
9.2.1.2 Use
9.2.2. General population exposure
9.3. Health effects
9.3.1. Occupational exposure
9.3.1.1 Fibrosis
9.3.1.2 Lung cancer
9.3.1.3 Mesothelioma
9.3.2. General environment
9.4. Effects on the environment

10. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH

11. FURTHER RESEARCH

REFERENCES

RÉSUMÉ

RESUMEN

NOTE TO READERS OF THE CRITERIA MONOGRAPHS

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This publication was made possible by grant number 5 U01 ES02617-
15 from the National Institute of Environmental Health Sciences,
National Institutes of Health, USA, and by financial support from the
European Commission.

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PREAMBLE

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WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR CHRYSOTILE
ASBESTOS

Members

Professor J.M. Dement, Duke Occupational Health Services, Duke
University, Durham, NC, USA (Vice-Chairperson)^a

Professor J.Q. Huang, Shanghai Medical University, Shanghai,
China

Professor M.S. Huuskonen, Institute of Occupational Health,
Helsinki, Finland^b

Professor G. Kimizuka, Department of Pathobiology, School of
Nursing, Chiba University, Chiba, Japan

Professor A. Langer, Environmental Sciences Laboratories,
Brooklyn College of the City University of New York, Brooklyn,
New York, USA (Co-Rapporteur)

Ms M.E. Meek, Priority Substances Section, Environmental Health
Directorate, Health Protection Branch, Health Canada, Ottawa,
Ontario, Canada (Chairperson)^c

Ms M. Meldrum, Health and Safety Executive, Toxicology Unit,
Bootle, United Kingdom (Co-Rapporteur)

Dr H. Muhle, Fraunhofer Institute for Toxicology and Aerosol
Research, Hanover, Germany

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

Professor J. Peto, Section of Epidemiology, Institute of Cancer
Research, Royal Cancer Hospital, Sutton, Surrey, United Kingdom

Dr L. Stayner, Risk Analysis and Document Development Branch,
Education and Information Division, National Institute for
Occupational Safety and Health, Morgantown, West Virginia, USA

^a Professor J.M. Dement chaired the meeting sessions when
discussions on Chapters 9, 10 and 11 were held. These sessions were
held in camera without the presence of observers. He also chaired
the final session when the whole document was adopted.
^b Not present at the last session
^c Not present at the discussions on Chapter 10.

Dr V. Vu, Health and Environmental Review Division, US
Environmental Protection Agency, Washington, D.C., USA

Observers

Mr D. Bouige, Asbestos International Association (AIA), Paris,
France^a

Dr G.W. Gibbs, Committee on Fibres, International Commission on
Occupational Health, Spruce Grove, Alberta, Canada^b

Secretariat

Dr Paolo Boffetta, Unit of Environmental Cancer Epidemiology,
International Agency for Research on Cancer, Lyon, France

Dr I. Fedotov, Occupational Safety and Health Branch, International
Labour Office, Geneva, Switzerland

Mr Salem Milad, International Registry of Potentially Toxic
Chemicals, United Nations Environment Programme, Geneva,
Switzerland

Professor F. Valic, IPCS Scientific Adviser, Andrija œtampar
School of Public Health, Zagreb University, Zagreb, Croatia
(Responsible Officer and Secretary of Meeting)

Resource persons

Professor J. Corbett McDonald, Department of Occupational and
Environmental Medicine, National Heart and Lung Institute,
London, United Kingdom^b

Professor W.J. Nicholson, Department of Community Medicine,
Mount Sinai School of Medicine, New York, NY, USA

^a Present only during first two days of the meeting (i.e. before the
discussions on Chapters 9, 10 and 11 were held)
^b Not present during the discussions on Chapters 9, 10 and 11, which
were held in camera

IPCS TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR CHRYSOTILE
ASBESTOS

A Task Group on Environmental Health Criteria for Chrysotile
Asbestos met at WHO Headquarters, Geneva, Switzerland, from 1 to 6
July 1996. Dr M. Mercier, Director IPCS, opened the Meeting and
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 third draft of the monograph, made an
evaluation of the risks for human health and the environment from
exposure to chrysotile asbestos, and made recommendations for health
protection and further research.

The first drafts were prepared by Dr G. Gibbs, Canada
(Chapter 2), Mr B.J. Pigg, USA (Chapter 3), Professor W.J. Nicholson,
USA (Chapter 4), Dr A. Morgan, UK and Professor M. Lippmann, USA
(Chapter 5), Dr J.M.G. Davis, UK and Professor B.T. Mossman, USA
(Chapter 6), Professor J.C. McDonald, UK, Professor P.J. Landrigan,
USA and Professor W.J. Nicholson, USA (Chapter 7), Professor H.
Schreier, Canada (Chapter 8).

In the light of international comments, the second draft was
prepared under the coordination of Professor F. Valiœ, Croatia.
Chapter 8 was modified by a group of experts in risk assessment
(Professors J. Hughes, USA, J. Peto, UK, and J. Siemiatycki, Canada).

Professor F. Valiœ was responsible for the overall scientific
content of the monograph and for the organization of the meeting, and
Dr P.G. Jenkins, IPCS Central Unit, for the technical editing of the
monograph.

The efforts of all who helped in the preparation and finalization
of the monograph are gratefully acknowledged.

ABBREVIATIONS

ACM asbestos-containing material
AOS activated oxygen species
ATEM analytical transmission electron microscopy
BAL bronchoalveolar lavage
BP benzo (a)pyrene
CI confidence interval
EDXA energy-dispersive X-ray analyser
f fibre
FGF fibroblast growth factor
LDH lactate dehydrogenase
mpcf millions of particles per cubic foot
mpcm millions of particles per cubic metre
NHMI N-nitrosoheptamethyleneimine
OR odds ratio
p particle
PCOM phase contrast optical microscopy
PDGF platelet-derived growth factor
PMR proportional mortality ratio
RR relative risk
SAED selected area electron diffraction
SEM scanning electron microscopy
SMR standardized mortality ratio
TEM transmission electron microscopy
TPA 12-O-tetradecanoylphorbol-13-acetate
TWA time-weighted average
UICC Union Internationale Contre le Cancer (reference
asbestos samples)

INTRODUCTION

As early as 1986 the International Programme on Chemical Safety
(IPCS) published the Environmental Health Criteria (EHC 53) on the
health effects of natural mineral fibres with particular emphasis on
asbestos (IPCS, 1986). During the next 7 years, efforts were focused
on possible reduction of environmental asbestos exposure (IPCS, 1989;
WHO/OCH, 1989), including the evaluation of a number of possible
substitute fibres such as man-made mineral fibres (IPCS, 1988), and
selected organic synthetic fibres (IPCS, 1993).

In 1992, four WHO Member States invited the Director-General of
WHO to request the IPCS to update that part of EHC 53 concerning the
health effects of chrysotile asbestos. The Director-General accepted
the request and instructed the IPCS to develop an EHC specifically for
chrysotile asbestos taking into consideration that (a) the
International Labour Organisation had recommended the discontinuation
of the use of crocidolite asbestos; (b) amosite asbestos was, for all
practical purposes, no longer exploited; and (c) there was still
wide-spread production and use of chrysotile asbestos in the world.

A number of reputable scientists (selected solely on the basis of
their contributions to the open scientific literature) were approached
with the request to develop individual scientific chapters for the
first draft. Chapters 5, 6 and 7 were drafted by two or three authors
independently. On the basis of these texts a coherent draft was
prepared by the IPCS.

The drafts of chapters 5, 6 and 7 were sent for preliminary
review to a limited number of recognized experts proposed by IPCS
participating institutions. The full draft of the document was
submitted to the standard IPCS worldwide evaluation procedure by
circulating it for comments to more than 140 IPCS Contact Points.

Taking into account all the relevant comments, a second draft was
developed by the IPCS. Chapter 7, drafted independently by three
authors, was modified by a working group of experts and focuses on
lung cancer and mesothelioma risks in populations exposed almost
exclusively to chrysotile. The discussion in this chapter has been
restricted primarily to direct observation from epidemiological
studies.

The third draft was submitted for evaluation, modification and
finalization to a Task Group of experts appointed by WHO. None of the
primary authors was appointed to be a member of the Task Group.

1. SUMMARY

1.1 Identity, physical and chemical properties, sampling and analysis

Chrysotile is a fibrous hydrated magnesium silicate mineral that
has been used in many commercial products. It is widely used in global
commerce today. Its physical and chemical properties as a mineral are
observed to vary among the exploited geological deposits. The minerals
that accompany the fibre in ores are many, and among these may be some
varieties of fibrous amphibole. Tremolite is thought to be especially
important in this respect; its form and concentration range greatly.

Analysis of chrysotile in the workplace currently entails the use
of light and electron microscopes. Various instruments and devices
have been previously used to monitor environments for the presence and
concentration of both total dust and fibres. The membrane filter
technique and phase contrast optical microscopy are commonly used
today for workplace assay (expressed as fibres per ml air); and the
transmission electron microscopy is also employed. Environmental
assays require the use of transmission electron microscopy. Tissue
burden studies have been employed to improve information regarding
exposures. Depending on the degree of attention to detail in these
studies, inferences regarding mechanisms and etiology have been drawn.

Gravimetric and thermal precipitator and midget impinger
techniques were previously used for workplace characterization, and
these dust (not fibre) values are the only early exposure indices
available for gauging exposure-response relationships. There have been
many attempts to convert these values to fibres per volume of air, but
these conversions have had very limited success. Conversion factors
have been found to be industry-specific and even operation-specific;
universal conversion factors carry high variances.

1.2 Sources of occupational and environmental exposure

Low concentrations of chrysotile are found throughout the crustal
environment (air, water, ice caps and soil). Both natural and human
activities contribute to fibre aerosolization and distribution.
Anthropogenic sources include dusts from occupational activities,
which cover ore recovery and processing, manufacturing, application,
usage and, ultimately disposal.

Production occurs in 25 countries, and there are seven major
producers. Annual world production of asbestos peaked at over 5
million tonnes in the mid-1970s but has since declined to a current
level of about 3 million tonnes. Manufacturing of chrysotile products
is undertaken in more than 100 countries, and Japan is the leading
consumer country. The current main activities resulting in potential
chrysotile exposure are: (a) mining and milling; (b) processing into
products (friction materials, cement pipes and sheets, gaskets and
seals, paper and textiles); (c) construction, repair and demolition;
(d) transportation and disposal. The asbestos-cement industry is by
far the largest user of chrysotile fibres, accounting for about 85% of
all use.

Fibres are released during processing, installation and disposal
of asbestos-containing products, as well as through normal wear of
products in some instances. Manipulation of friable products may be an
important source of chrysotile emission.

1.3 Occupational and environmental exposure levels

Based on data mainly from North America, Europe and Japan, in
most production sectors workplace exposures in the early 1930s were
very high. Levels dropped considerably to the late 1970s and have
declined substantially to present day values. In the mining and
milling industry in Quebec, the average fibre concentrations in air
often exceeded 20 fibres/ml (f/ml) in the 1970s, while they are now
generally well below 1 f/ml. In the production of asbestos-cement in
Japan, typical mean concentrations were 2.5-9.5 f/ml in 1970s, while
mean concentrations of 0.05-0.45 f/ml were reported in 1992. In
asbestos textile manufacture in Japan, mean concentrations were
between 2.6 and 12.8 f/ml in the period between 1970 and 1975, and
0.1-0.2 f/ml in the period between 1984 and 1986. Trends have been
similar in the production of friction materials: based on data
available from the same country, mean concentrations of 10-35 f/ml
were measured in the period between 1970 and 1975, while levels
0.2-5.5 f/ml were reported in the period between 1984 and 1986. In a
plant in the United Kingdom in which a large mortality study was
conducted, concentrations were generally above 20 f/ml in the period
before 1931 and generally below 1 f/ml during 1970-1979.

Few data on concentrations of fibres associated with the
installation and use of chrysotile-containing products are available,
although this is easily the most likely place for workers to be
exposed. In the maintenance of vehicles, peak concentrations of up to
16 f/ml were reported in the 1970s, while practically all measured
levels after 1987 were less than 0.2 f/ml. Time-weighted average
exposures during passenger vehicle repair in the 1980s were generally
less than 0.05 f/ml. However, with no controls, blowing off debris
from drums resulted in short-term high concentrations of dust.

There is potential for exposure of maintenance personnel to mixed
asbestos fibre types due to large quantities of friable asbestos in
place. In buildings with control plans, personal exposure of building
maintenance personnel in the USA, expressed as 8-h time-weighted
averages, was between 0.002 and 0.02 f/ml. These values are of the
same order of magnitude as typical exposures during telecommunication
switchwork (0.009 f/ml) and above-ceiling work (0.037 f/ml), although
higher concentrations were reported in utility space work (0.5 f/ml).
Concentrations may be considerably higher where no control plans have
been introduced. In one case, short-term episodic concentrations were
1.6 f/ml during sweeping and 15.5 f/ml during dusting of library books
in a building with a very friable chrysotile-containing surface
formulation. Most other 8-h time-weighted averages are about two
orders of magnitude less.

Based on surveys conducted before 1986, fibre concentrations
(fibres > 5 µm in length) in outdoor air, measured in Austria,
Canada, Germany, South Africa and the USA, ranged between 0.0001 and
about 0.01 f/ml, levels in most samples being less than 0.001 f/ml.
Means or medians were between 0.00005 and 0.02 f/ml, based on more
recent determinations in Canada, Italy, Japan, the Slovak Republic,
Switzerland, United Kingdom and USA.

Fibre concentrations in public buildings, even those with friable
asbestos-containing materials, are within the range of those measured
in ambient air. Concentrations (fibres > 5 µm in length) in buildings
in Germany and Canada reported before 1986 were generally less than
0.002 f/ml. In more recent surveys in Belgium, Canada, the Slovak
Republic, United Kingdom and USA, mean values were between 0.00005 and
0.0045 f/ml. Only 0.67% of chrysotile fibres were longer than 5 µm.

1.4 Uptake, clearance, retention and translocation

The deposition of inhaled chrysotile asbestos is dependent upon
the aerodynamic diameter, the length and the morphology of the fibre.
Most airborne chrysotile fibres are considered respirable because
their fibre diameters are less than 3 µm, equal to an aerodynamic
diameter of about 10 µm. In laboratory rats, chrysotile fibres are
deposited primarily at alveolar duct bifurcations.

In the nasopharyngeal and tracheobronchial regions, chrysotile
fibres are cleared via mucocilliary clearance. At the alveolar duct
bifurcations the fibres are taken up by epithelial cells. Fibre length
is an important determinant of alveolar clearance of chrysotile
fibres. There is extensive evidence from animal studies that short
fibres (less than 5 µm long) are cleared more rapidly than long fibres
(longer than 5 µm). The mechanisms of the relatively more rapid
clearance of chrysotile fibres compared to those of amphiboles are not
completely known. It has been hypothesized that short chrysotile
fibres are cleared through phagocytosis by alveolar macrophages, while
long chrysotile fibres are cleared mainly by breakage and/or
dissolution. To what extent chrysotile fibres are translocated to the
interstitium, pleural tissue and other extrathoracic tissues is not
fully understood.

Analyses of human lungs of workers exposed to chrysotile asbestos
indicate much greater retention of tremolite, an amphibole asbestos
commonly associated with commercial chrysotile in small proportions,
than of chrysotile. The more rapid removal of chrysotile fibres from
the human lung is further supported by findings from animal studies
showing that chrysotile is more rapidly cleared from the lung than are
amphiboles including crocidolite and amosite.

Available data from studies in humans and animals are
insufficient to evaluate the possible uptake, distribution and
excretion of chrysotile fibres from ingestion. Available evidence
indicates that, if penetration of chrysotile fibres across the gut
wall does occur, it is extremely limited. One study indicated an

increased level of chrysotile fibres in the urine of workers
occupationally exposed to chrysotile.

1.5 Effects on animals and cells

Various experimental samples of chrysotile fibres have been shown
in numerous long-term inhalation studies to cause fibrogenic and
carcinogenic effects in laboratory rats. These effects include
interstitial fibrosis and cancer of the lung and pleura. In most
cases, there appears to be an association between fibrosis and tumours
in the rat lung. Fibrogenic and carcinogenic effects have also been
found in long-term animal studies (mainly in rats) using other modes
of administration (e.g., intratracheal instillation and intrapleural
or intraperitoneal injection).

Exposure/dose-response relationships for chrysotile-induced
pulmonary fibrosis, lung cancer and mesothelioma have not been
adequately investigated in long-term animal inhalation studies.
Inhalation studies conducted to date, mainly using a single exposure
concentration, show fibrogenic and carcinogenic responses at airborne
fibre concentrations ranging from 100 to a few thousand fibres/ml.
When data from various studies are combined, there appears to be a
relationship between airborne fibre concentrations and lung cancer
incidence. This type of analysis, however, may not be scientifically
sound as different experimental conditions were used in available
studies.

In non-inhalation experiments (intrapleural and intraperitoneal
injection studies), dose-response relationships for mesothelioma have
been demonstrated for chrysotile fibres. Data from these types of
studies, however, may not be suitable for the evaluations of human
risk from inhalation exposure to fibres.

Tremolite asbestos, a minor component mineral of commercial
chrysotile, has also been shown to be carcinogenic and fibrogenic in a
single inhalation experiment and an intraperitoneal injection study in
rats. Exposure/dose-response data are not available to allow direct
comparison of the cancer potency of tremolite and chrysotile.

The ability of fibres to induce fibrogenic and carcinogenic
effects appears to be dependent on their individual characteristics,
including fibre dimension and durability (i.e. biopersistence in
target tissues), which are determined in part by the physico-chemical
properties. It has been well documented in experimental studies that
short fibres (shorter than 5 µm) are less biologically active than
long fibres (longer than 5 µm). It is still uncertain, however,
whether short fibres have any significant biological activity.
Furthermore, it is not known how long a fibre needs to remain in the
lung in order to induce preneoplastic effects, since the appearance of
asbestos-related cancer generally occurs later in the animal's life.

The mechanisms by which chrysotile and other fibres cause
fibrogenic and carcinogenic effects are not completely understood.
Possible mechanisms of fibrogenic effects of fibres include chronic
inflammation process mediated by production of growth factors (e.g.,
TNF-alpha) and reactive oxygen species. With regard to fibre-induced
carcinogenicity, several hypotheses have been proposed. These include:
DNA damage by reactive oxygen species induced by fibres; direct DNA
damage by physical interactions between fibres and target cells;
enhancement of cell proliferation by fibres; fibre-provoked chronic
inflammatory reactions leading to prolonged release of lysozymal
enzymes, reactive oxygen species, cytokines and growth factors; and
action by fibres as co-carcinogens or carriers of chemical carcinogens
to the target tissues. It is likely, however, that all these
mechanisms contribute to the carcinogenicity of chrysotile fibres, as
such effects have been observed in various in vitro systems of human
and mammalian cells.

Overall, the available toxicological data provide clear evidence
that chrysotile fibres can cause fibrogenic and carcinogenic hazard to
humans. The data, however, are not adequate for providing quantitative
estimates of the risk to humans. This is because there are inadequate
exposure-response data from inhalation studies, and there are
uncertainties concerning the sensitivities of the animal studies for
predicting human risk.

Chrysotile fibres have been tested in several oral
carcinogenicity studies. Carcinogenic effects have not been reported
in available studies.

1.6 Effects on humans

Commercial grades of chrysotile have been associated with an
increased risk of pneumoconiosis, lung cancer and mesothelioma in
numerous epidemiological studies of exposed workers.

The non-malignant diseases associated with exposure to chrysotile
comprise a somewhat complex mixture of clinical and pathological
syndromes not readily definable for epidemiological study. The prime
concern has been asbestosis, generally implying a disease associated
with diffuse interstitial pulmonary fibrosis accompanied by varying
degrees of pleural involvement.

Studies of workers exposed to chrysotile in different sectors
have broadly demonstrated exposure-response or exposure-effect
relationships for chrysotile-induced asbestosis, in so far as
increasing levels of exposure have produced increases in the incidence
and severity of disease. However, there are difficulties in defining
this relationship, due to factors such as uncertainties in diagnosis
and the possibility of disease progression on cessation of exposure.

Furthermore, some variation in risk estimates are evident among
the available studies. The reasons for the variations are not entirely
clear, but may relate to uncertainties in exposure estimates, airborne
fibre size distributions in the various industry sectors and
statistical models. Asbestotic changes are common following prolonged
exposures of 5 to 20 f/ml.

The overall relative risks for lung cancer are generally not
elevated in the studies of workers in asbestos-cement production and
in some of the cohorts of asbestos-cement production workers. The
exposure-response relationship between chrysotile and lung cancer risk
appears to be 10-30 times higher in studies of textile workers than in
studies of workers in mining and milling industries. The relative
risks of lung cancer in the textile manufacturing sector in relation
to estimated cumulative exposure are, therefore, some 10-30 times
greater than those observed in chrysotile mining. The reasons for this
variation in risk are not clear, so several hypotheses, including
variations in fibre size distribution, have been proposed.

Estimation of the risk of mesothelioma is complicated in
epidemiological studies by factors such as the rarity of the disease,
the lack of mortality rates in the populations used as reference, and
problems in diagnosis and reporting. In many cases, therefore, risks
have not been calculated, and cruder indicators have been used, such
as absolute numbers of cases and deaths, and ratios of mesothelioma
over lung cancers or total deaths.

Based on data reviewed in this monograph, the largest number of
mesotheliomas has occurred in the chrysotile mining and milling
sector. All the observed 38 cases were pleural with the exception of
one of low diagnostic probability, which was pleuro-peritoneal. None
occurred in workers exposed for less than 2 years. There was a clear
dose-response relationship, with crude rates of mesotheliomas
(cases/ 1000 person-years) ranging from 0.15 for those with cumulative
exposure less than 3530 million particles per m³ (mpcm)-years
(< 100 million particles per cubic foot (mpcf)-years) to 0.97 for
those with exposures of more than 10 590 mpcm-years (> 300
mpcf-years).

Proportions of deaths attributable to mesotheliomas in cohort
studies in the various mining and production sectors range from 0 to
0.8%. Caution should be exercised in interpreting these proportions as
studies do not provide comparable data stratifying deaths by exposure
intensity, duration of exposure or time since first exposure.

There is evidence that fibrous tremolite causes mesothelioma in
humans. Since commercial chrysotile may contain fibrous tremolite, it
has been hypothesized that the latter may contribute to the induction
of mesotheliomas in some populations exposed primarily to chrysotile.
The extent to which the observed excesses of mesothelioma might be
attributed to the fibrous tremolite content has not been resolved.

The epidemiological evidence that chrysotile exposure is
associated with an increased risk for cancer sites other than the lung
or pleura is inconclusive. There is limited information on this issue
for chrysotile per se, although there is some inconsistent evidence
for an association between asbestos exposure (all forms) and
laryngeal, kidney and gastrointestinal tract cancers. A significant
excess of stomach cancer has been observed in a study of Quebec
chrysotile miners and millers, but possible confounding by diet,
infections or other risk factors has not been addressed.

It should be recognized that although the epidemiological studies
of chrysotile-exposed workers have been primarily limited to the
mining and milling, and manufacturing sector, there is evidence, based
on the historical pattern of disease associated with exposure to mixed
fibre types in western countries, that risks are likely to be greater
among workers in construction and possibly other user industries.

1.7 Environmental fate and effects on biota

Serpentine outcroppings occur world-wide. Mineral components,
including chrysotile, are eroded through crustal processes and are
transported to become a component of the water cycle, sediment
population and soil profile. Chrysotile presence and concentrations
have been measured in water, air and other units of the crust.

Chrysotile and its associated serpentine minerals chemically
degrade at the surface. This produces profound changes in soil pH and
introduces a variety of trace metals into the environment. This has in
turn produced measurable effects on plant growth, soil biota
(including microbes and insects), fish and invertebrates. Some data
indicate that grazing animals (sheep and cattle) undergo changes in
blood chemistry following ingestion of grasses grown on serpentine
outcrops.

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, SAMPLING AND
ANALYSIS

2.1 Identity

2.1.1 Chemical composition

Chrysotile, referred to as white asbestos, is a naturally
occurring fibrous hydrated magnesium silicate belonging to the
serpentine group of minerals. The chemical composition, crystal
structure and polytypic forms of the serpentine minerals have been
described by Langer & Nolan (1994).

The composition of chrysotile is close to the ideal unit cell
formula (Mg₃Si₂O₅(OH)₄); substitution by other elements in the
crystal structure is possible. According to Skinner et al. (1988)
substitution possibilities are:

(Mg_3-x-y R_x⁺² R_y⁺³)(Si_2-y R_y⁺³)O₅ (OH)₄,

where R²⁺ = Fe²⁺, Mn²⁺ or Ni²⁺ and R³⁺=Al³⁺ or Fe³⁺.

Results of a typical chemical analysis are shown in Table 1 of
Environmental Health Criteria 53 (IPCS, 1986).

Trace amounts of some other elements, such as Na, Ca and K, are
probably due to the presence of other minerals admixed in the ore (see
section 2.1.6).

2.1.2 Structure

Chrysotile is a sheet silicate with a basic building block of
(Si₂O₅)_n in which three of the oxygen atoms in each tetrahedron
base are shared with adjacent tetrahedra in the same layer. The apical
oxygens of the tetrahedra in the silica sheet become a component
member of the overlying brucite layer (Mg(OH)₂) (Speil & Leineweber,
1969). As the dimensions of the cations in the silica and brucite
sheets are different, strain is produced, which is accommodated by the
formation of a scroll structure. Yada (1967) produced transmission
electron micrographs that permitted visualization of this
morphological feature. The curvature occurs with the brucite layer on
the outer surface. The resulting capillaries are common to most
specimens although solid cores have been found.

When more than one structure occurs, they are called polytypes:
orthochrysotile (orthorhombic structure), clinochrysotile (monoclinic
structure) and parachrysotile (cylindrical or polygonal Povlen-type
structures) (Wicks, 1979). Most chrysotile is a mixture of the ortho-
and clino-polytypes in various proportions (Speil & Leineweber, 1969).

2.1.3 Fibre forms in the ore

Chrysotile can occur in the host rock as "cross-fibre" (fibre
axes at right angles to the seam or vein), "slip-fibre" (fibre axes
parallel to the seam) or massive fibre (in which there is no
recognizable fibre orientation, as in the New Idria deposit in USA).

2.1.4 Fibre properties

Depending on the relative flexibility, fibres may be "harsh" or
"soft". Chrysotile fibres generally occur with properties between
these end-types (Badollet, 1948). While amphibole fibres are generally
harsh, most chrysotile fibres are soft, although fibres displaying
intermediate properties also occur. Harshness has been reported to be
related to the water content of the fibre, i.e. the higher the water
content the "softer" the fibre (Woodroofe, 1956), relative contents of
clino- and ortho-chrysotile, and the presence of fine mineral
intergrowth ( Speil & Leineweber, 1969).

Harsh chrysotile fibres tend to be straighter and less flexible
than the soft fibres. Inhalation of respirable straight fibres is
reported to be associated with greater penetration to the terminal
bronchioles than in the case of "curly" fibres (Timbrell, 1965, 1970).

The fibres can be classified into crude chrysotile (hand-selected
fibres in essentially native or unfiberized form) and milled fibres
(after mechanical treatment of the ore). Fibre grades used for
different products vary from country to country. The Canadian system
has been described by Cossette & Delvaux (1979). The Canadian grading
system is widely used internationally.

At the turn of this century, the fibres of major commercial
importance were several centimetres long. With time, as new
applications developed, shorter fibres became important. This change
is likely to have altered the nature of exposure in some
circumstances.

2.1.5 UICC samples

Two UICC (Union Internationale Contre le Cancer) standard
reference samples of chrysotile asbestos were available for use in
experimental work. One was from Zimbabwe (Chrysotile A) and the other
was a composite sample of fibres from Canadian mines in the eastern
townships of Quebec (Chrysotile B). The physico-chemical properties of
these samples are well characterized and details of their composition
and properties have been reported (Timbrell et al., 1968; Rendall,
1970). These mixtures were artificial and did not reflect any one
commercially available fibre.

2.1.6 Associated minerals in chrysotile ore

The mineral dusts to which miners or millers might be exposed are
determined by the minerals associated with each of the chrysotile ore
deposits. These depend on the composition of the original rock types
and on the materials added or removed during geological events,
surface weathering processes, etc. The spacial relationships among
these components within ore bodies vary significantly from deposit to
deposit.

Iron is ubiquitous in chrysotile deposits derived from ultramafic
rocks. In some of these, magnetite occurs in intimate association with
the fibres (e.g., in Quebec). In other deposits types, e.g., in
carbonate rocks, the iron content is low (e.g., in Arizona). Brucite,
or nemalite (the fibrous form of brucite), is found in some deposits.
Micas, feldspars, altered feldspars, talc and carbonate minerals may
be present. Langer & Nolan (1994) listed minerals likely to be
associated with ultramafic rocks in which chrysotile is found, and
Gibbs (1971a) listed more than 70 minerals occurring in the Quebec
chrysotile mining region. Minerals such as magnetite, calcite and
zeolites may also occur in a fibrous form.

Amphiboles may also be encountered, some in fibrous form. These
latter minerals have been found in studies of lung tissues of exposed
workers. Tremolite, ferro-tremolite, actinolite, anthophyllite and
other amphibole minerals have been described. Their occurrence in ore
bodies is both heterogeneous in distribution and variable in
concentration. Addison & Davies (1990) found tremolite in 28 out of 81
ore samples (34.6%) at concentrations (when detected) from 0.01 to
about 0.6%. The average concentration was about 0.09%. The form of the
amphibole, whether asbestos or massive, was not given. This
information may be crucial in considering the mineral type as an agent
of disease, especially for mesothelioma.

Trace metals have been described in association with fibres,
particularly chromium, cobalt, nickel, iron and manganese (Cralley et
al., 1967; Gibbs, 1971a; Morgan & Cralley, 1973; Oberdörster et al.,
1980). Concentrations in mills in the late 1960s were several times
higher than those measured at textile plants at that time (Gibbs,
1971a).

Naturally occurring chrysotile has been shown to contain trace
quantities of organic compounds, predominantly straight-chain alkanes
(Gibbs, 1971b). Processed fibres may also contain organic compounds
including polycyclic aromatic hydrocarbons (Gibbs, 1971a; Gibbs & Hui,
1971). Concentrations of polycyclic aromatic hydrocarbons in the air
of chrysotile mills were found to be lower than levels in urban areas
(Gibbs, 1971a). Fibres can also be contaminated by alkanes and by
antioxidants from storage in polyethylene bags (Commins & Gibbs, 1969;
Gibbs & Hui, 1971).

Radon concentrations in the Quebec chrysotile mines were reported
to be below 0.3 Standard Working Level (Gibbs, 1971a). This has been
rejected as an agent of disease among miners, especially for lung
cancer.

2.2 Physical and chemical properties

The mineralogy and properties of chrysotile have been summarized
by Wicks (1979), Pooley (1987), and Langer & Nolan (1994).

2.2.1 Physical properties

The physical properties of chrysotile, as they affect human
health, have been described in Langer & Nolan (1986, 1994) and IPCS
(1986).

Harshness has been discussed in section 2.1.4.

Heating of chrysotile fibre at 700°C for an hour converts it to
an amorphous, anhydrous magnesium silicate material (Speil &
Leineweber, 1969). Intensive dry grinding also destroys the structure
of chrysotile. Analysis of wear debris from brake linings made with
asbestos has shown that virtually all of the chrysotile fibre is
converted to amorphous material, in association with the mineral
forsterite (a recrystallization product). The conversion is explained
by localized temperatures above 1000°C at the point of contact between
the brake lining and the drum (Lynch, 1968; Rowson, 1978; Williams &
Muhlbaier, 1982). The fibres found in the brake wear debris are
predominantly (99%) less than 0.4 µm in length (Rohl et al., 1977;
Williams & Muhlbaier, 1982). Rodelsperger et al. (1986) found less
than 1% of fibres longer than 5 µm.

Size and shape are the most important characteristics for
defining the respirability of fibres. For workplace regulatory
purposes a fibre has been defined most frequently as having an aspect
ratio (ratio of fibre length to fibre diameter) of at least 3:1.
Regulatory definitions usually impose a length of 5 µm or greater for
workplace assay.

Chrysotile bundles may be split longitudinally to form thinner
fibres. The ultimate fibre is called a fibril. Yada (1967), by means
of high resolution transmission electron microscopy, showed that basic
spiral elements of chrysotile consist of 5 silica-magnesia units with
approximately 10 silica-magnesia units forming the 0.007 µm wall of a
single fibril. The diameter of the ultimate fibril is about 0.03 µm.

The fibres of significance in health risk evaluation are those
that can be inhaled. Timbrell (1970, 1973) showed that chrysotile
fibres less than about 3.5 µm in diameter can enter the conducting
airways of the lung. The radius of curvature of the chrysotile fibre
may play a role in the ability of a fibre to penetrate to distant
sites along the conducting airways.

As it is possible to have long narrow fibres and short narrow
fibres, descriptions of fibrous aerosols by "mean or median diameter",
or "mean or median length" do not provide sufficient information.
Comparisons of fibrous aerosols to which subjects are exposed may
therefore be limited. The measurements of dimensions are
time-consuming and complete data sets are scant.

Results of most distributions reported are incomplete. Unless
specific steps have been taken to evaluate very long fibres,
transmission electron microscopy (TEM) will understate the number of
long fibres (>20 µm). Because the proportion of very long fibres is
low, random scanning rarely encounters them. Scanning electron
microscopy (SEM) usually requires coating of the specimen. Most
preparation techniques obscure single chrysotile fibrils. In addition,
if chemical analysis of individual fibres is not made, other fibres
may be erroneously reported as chrysotile.

It has been noted that the vast majority of airborne chrysotile
fibres are short, the percentage of fibres more than 5 µm long in
mining and milling being about 1.3 and 4.1%, respectively (Gibbs &
Hwang, 1980), while data show that up to 24% of fibres may be longer
than 5 µm in certain textile spinning operations (Gibbs, 1994).
Virtually all airborne fibres have a diameter of less than 3 µm and
are thus respirable.

The cross-section of a chrysotile fibril is approximately
circular (see figure in Yada, 1967). This is important in calculating
the mass of individual fibres. Generally, the surface area depends on
the degree of fibre openness. The New Idria (Coalinga) material has a
surface area of about 78 m²/g and an average fibril diameter of
0.0275 µm, while the Canadian 7R has a surface area of about 50 m²/g
and an average fibril diameter of 0.0375 µm (Speil & Leineweber,
1969). It has been suggested that surface area plays a role in
imparting biological potential.

Timbrell (1975) reported the magnetic properties of fibres.
Chrysotile showed no preferred orientation in magnetic fields.

It has been observed that industrial processing of fibres from
different sources may affect total airborne dust concentrations.

2.2.2 Chemical properties

Chrysotile exhibits significant solubility in aqueous neutral or
acidic environments (Langer & Pooley, 1973; Jaurand et al., 1977;
Spurny, 1982). In contact with dilute acids or aqueous medium at pH
less than 10, magnesium leaches from the outer brucite layer (Nagy &
Bates, 1952; Atkinson, 1973; Morgan & Cralley, 1973). Magnesium loss
has also been demonstrated in vivo. The surface area of leached
chrysotile is greatly increased (Badollet & Gannt, 1965). The
solubility of the outer brucite layer of chrysotile in body fluids
greatly affects bioaccumulation in lung tissues. The role of chemical
properties in the biological behaviour of chrysotile has been recently
discussed (Langer & Nolan, 1986, 1994).

The adsorption of polar organic agents on the surface of
chrysotile is reported to be higher than that of less polar or non-
polar agents (Speil & Leineweber, 1969; Gorski & Stettler, 1974). The
binding of carcinogens such as benzo (a)pyrene, nitrosonornicotine
and N-acetyl-2-aminofluorene to chrysotile has been studied by
Harvey et al. (1984). Adsorption of components of cigarette smoke onto
the surface of chrysotile fibres has been suggested to play a role in
the etiology of lung cancer in fibre-exposed cigarette smokers. The
fibre may act as a vehicle which transports polycyclic aromatic
hydrocarbons across membranes of the target cells (Gerde & Scholander,
1989).

2.3 Sampling and analytical methods

The collection of samples from air, water, biological specimens,
soils or sediments must follow an appropriate sampling strategy. A
review of methods for sampling asbestos fibres has been published
(IPCS, 1986).

The most commonly used analytical methods involve phase-contrast
optical microscopy (PCOM) (in the workplace) and transmission electron
microscopy (TEM) (in the general environment). PCOM is
resolution-limited and non-specific for fibre characterization. TEM
overcomes both limitations (Dement & Wallingford, 1990).

2.3.1 Workplace sampling

The most widely used method for the last 20 years has been the
membrane filter method. Several attempts have been made to standardize
the method (CEC, 1983; ILO, 1984; AIA, 1988; NIOSH, 1989a; ISO, 1993).
A recommended method for the determination of airborne fibre
concentration by PCOM (membrane filter method) has been published
(WHO, 1997).

A known volume of air is drawn through a membrane filter on which
the number of fibres is determined using a phase contrast microscope
(see section 2.3.3.2). Special attention should be given to flow
rates, sampling time, face velocity through the filter, and where,
when and how to sample. Preference should be given to assessing
individual exposure by personal sampling. The sampling strategy should
be selected to yield the best estimate of an 8-h time-weighted average
concentration. Excursions may be evaluated for regulatory purposes. If
the purpose of the measurement is evaluation of control measures,
other methods may also be used.

2.3.2 Sampling in the general environment

Methods for sampling ambient air depend on the method of
analysis, but generally involve filtering airborne particles from
relatively large volumes of air using membrane filters. Strategies and
sampling methods have been described by Rood (1991) and reviewed in
detail in the Health Effects Institute study of asbestos in public
buildings (HEI, 1991).

For analysis of water, sample specimens are collected and
filtered through polycarbonate filters. If there is much organic
debris, this must be removed to improve particle detection. The fibres
must be re-prepared before analysis. The instrumental method is the
same as that used for air samples.

2.3.3 Analytical methods

Analyses are performed to identify the fibre or fibres present
and to determine their concentrations.

2.3.3.1 Fibre identification

Several methods have been developed to identify chrysotile
asbestos using dispersion staining methods and polarization microscopy
(Julian & McCrone, 1970; McCrone, 1978; Churchyard & Copeland, 1988;
NIOSH, 1989a). NIOSH (1989b) described the procedure specifically for
the analysis of asbestos bulk samples.

The limit of visibility of fibres, depending on the microscope
and light source used, is in the range 0.2-0.3 µm. With most high
quality research microscopes, chrysotile fibres of 0.22 µm are
generally reported as being observable. The experience and expertise
of the microscopist and the quality of the laboratory set-up both
influence the outcome.

Fibres with diameters less than about 0.22 µm cannot be seen with
a light optical microscope. When fibres with diameters less than this
value need to be analysed, TEM is used. This method is generally
applied to the identification and characterization of fibres in water
and in ambient air (Chatfield, 1979, 1987; Rood, 1991; ISO, 1991; HEI,
1991). The most reliable method of identifying chrysotile fibres is
the combination of morphology, chemistry and electron diffraction
(Skikne et al., 1971; Langer & Pooley, 1973). Several methods for the
determination of amphibole fibres in chrysotile have been described
(Addison & Davies, 1990).

Analytical methods using scanning electron microscopy (SEM) have
also been developed (AIA, 1984; WHO, 1985; ISO, 1992).

2.3.3.2 Measurement of airborne fibre concentrations

a) Workplace

In the PCOM method, the membrane filter is dissolved or collapsed
using a solvent with a refractive index which matches the refractive
index of the filter medium, rendering it invisible. Fibres entrained
on the filter are made readily visible.

The number of fibres of specified length and diameter in a known
area of the filter is counted at magnifications of 400 to 500. A
graticule has been designed for this purpose. Development of the
HSE/NPL slide (LeGuen et al., 1984), which permits laboratories to
standardize the limit of visibility of their microscopes and
microscopists, has improved the potential for interlaboratory
agreement in counts.

Improvements in the mounting techniques and counting strategy has
resulted in higher fibre counts than those found using the same
techniques in the early 1970s (HSE, 1979; Gibbs, 1994). This change
was estimated in the United Kingdom to cause a two-fold increase in
the reported fibre concentrations (HSE, 1979).

Instrumentation for automatic counting has been developed (e.g.,
Kenny, 1984) but has failed to receive wide international recognition.

b) Ambient air

The diameter of most chrysotile fibres found in the
non-occupational environment is below the resolution of the light
optical microscope (Rooker et al., 1982).

The most reliable method for determining the concentration of
chrysotile fibres in ambient air is TEM. Most currently available
transmission electron microscopes have a resolution of about 0.2 nm;
in combination with an energy-dispersive X-ray analyser (EDXA), TEM
can chemically characterize fibres down to a diameter of 0.01 µm. The
disadvantage of TEM is the small area that can be scanned when
employing very high magnifications. This makes analysis of the long
fibres (>5 µm) more limited in accuracy (Coin et al., 1992). A review
of the use of TEM and a comparison of direct and indirect methods of
filter preparation have been published recently (HEI, 1991).

SEM has been used in the measurement of chrysotile. Most SEMs
have a resolution intermediate between that of TEM and PCOM.

2.3.3.3 Lung tissue analysis

Several methods have been described (Langer & Pooley, 1973;
Gaudichet et al., 1980; Rogers et al., 1991a,b). All methods use
ashing or digestion of tissues, TEM, SAED and EDXA. International
standardization of these methods has not as yet been carried out. For
this reason comparison of results from different laboratories is often
difficult to make.

2.3.3.4 Gravimetric analysis

Gravimetric methods have been applied in some countries for the
evaluation of workplace conditions and emissions (Rickards, 1973;
Middleton, 1982). Relatively large samples of dust are needed and the
methods do not distinguish between the fibres and non-fibrous dusts
nor among mineral components of each group. In view of this and the

current belief that counts of fibres better define the health risk,
gravimetric methods are limited in application. However, it must also
be recognized that bulk dust assay is a useful index for control
evaluation and should be used if membrane filter techniques are
unavailable.

2.4 Conversion factors

The concentrations of airborne chrysotile fibres in the workplace
are expressed as the number of fibres per millilitre (f/ml) of air,
fibres per litre (f/litre) of air or fibres per cubic metre (f/m³) of
air, or in milligrams per cubic metre (mg/m³) of air. Concentrations
are expressed as number of fibres per cubic metre or nanograms per
cubic metre (ng/m³) in the general environment.

The number of fibres per millilitre, obtained by the method of
membrane filtration and PCOM, is currently used by regulatory agencies
in most countries for the workplace. It is for this reason that the
conversion of results obtained by different methods into membrane
filter equivalents has been performed. Critiques of such conversions
have been published (Walton, 1982; Valiœ, 1993; Gibbs, 1994).

2.4.1 Conversion from airborne particle to fibre concentrations

In almost all epidemiological studies in which health effects
have been related to exposure, concentration measurements were made
using methods quite different from the membrane filter technique. The
early instruments employed were the thermal precipitator in the United
Kingdom, and the midget impinger in North America. Gravimetric
measurements have also been used.

Attempts to convert the midget impinger count to an equivalent
membrane filter fibre count have shown that no single conversion
factor applies. Large variations in the ratios of midget impinger to
membrane filter counts occur in different industries, between jobs
within a single industry, or at a single plant site (Ayer et al.,
1965; Gibbs & Lachance, 1974). Similar conversion problems were
encountered in other countries where attempts were made to convert
konimeter or thermal precipitator results to membrane filter
equivalents (DuToit & Gilfillan, 1979; DuToit et al., 1983; Valiœ &
Cigula, 1992).

Side-by-side study of conversion factors has shown the
correlation between particle and fibre counts to be limited. Both
industry and operation-specific correlations have been made but are
only site-specific. Although some comparisons made for epidemiological
studies have yielded valuable data, no universal factor has ever been
found. High variance exists. Temporal change in dust conditions in
plants may have also affected conversion factors (Dagbert, 1976). The
range of conversion ratios between work sites has been large (Doll &
Peto, 1985). For purposes of exposure-response studies, conversions
based on industry- and operation-specific data have proven valuable in
some instances.

2.4.2 Conversion from total mass to fibre number concentrations

The conversions from total mass concentrations of dust determined
gravimetrically into the fibre number concentrations may also be
generally subject to great errors (Pott, 1978; IPCS, 1986). However,
in some specific industries a good correlation has been achieved (Fei
& Huang, 1989; Huang, 1990).

When measurements of airborne fibre concentrations are made using
transmission electron microscopy, determination of fibre lengths and
diameters are necessary. If chrysotile is split into fibrils,
approximate mass can be calculated by determining the fibre dimensions
and using fibre density in the calculation.

3. SOURCES OF OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE

3.1 Natural occurrence

Chrysotile is present in most serpentine rock formations. As a
result, chrysotile originating from serpentine rock is often found in
air and water due to natural weathering (Nicholson & Pundsack, 1973;
Neuberger et al., 1996).

Workable deposits are present in over 40 countries. Twenty-five
of these currently produce chrysotile. Canada, South Africa, Russia
and Zimbabwe have 90% of the established world reserves (Shride,
1973).

Chrysotile is emitted from both natural and industrial sources.
No measurements concerning the extent of release of airborne fibres
through natural weathering processes are available. A study of the
mineral content of the Greenland ice cap showed that airborne
chrysotile existed long before it was used commercially on a large
scale. Ice core dating showed the presence of chrysotile as early as
1750 (Bowes et al., 1977).

Chrysotile is introduced into water by the weathering of
chrysotile-containing rocks and ores, in addition to the effects of
industrial effluents and atmospheric pollution (Canada Environmental
Health Directorate, 1979). The largest concentrations of asbestos in
drinking-water generally occur from erosion of asbestos deposits
(Polissar, 1993; Neuberger et al., 1996). Millette JR ed. (1983) has
attributed chrysotile in water supplies to erosion from natural
sources in areas such as San Francisco, Sherbrooke and Seattle.
Millette et al. (1980) have shown that in the USA asbestos in
drinking-water is primarily chrysotile.

3.2 Anthropogenic sources

Chrysotile was at one time used in many applications, which
included both friable and non-friable products (Shride, 1973).
Currently, the human activities resulting in potential chrysotile
exposure can be divided into broad categories: (a) mining and milling,
(b) processing of asbestos into products (such as friction materials,
cement pipe and sheet, gaskets and seals, paper and textiles), (c)
construction and repair activities, and (d) transportation and,
especially, disposal of chrysotile-containing waste products.

Chrysotile is by far the predominant asbestos fibre consumed
today, e.g., in the USA 98.5% asbestos consumption in 1992 was
chrysotile (Pigg, 1994).

3.2.1 Production

Although there are 25 countries currently producing chrysotile,
seven countries account for the major part of world production
(Brazil, Canada, China, Kazakhstan, Russia, South Africa and Zimbabwe)
(US Department of Interior, 1993).

World production of asbestos increased 50% between 1964 and 1973
when it reached 5 million tonnes (US Department of Interior, 1991),
but production has generally declined since the mid-1970s to its
current level of 3.1 million tonnes. Table 1 shows the yearly
production levels by countries between 1988 and l992.

Table 2 shows the decline in major asbestos uses in the USA
during the period 1977-1991 (US Department of Interior, 1986, 1991).

Chrysotile ore is usually mined in open-pit operations. Possible
sources of emissions are drilling, blasting, loading broken rock and
transporting ore to the primary crusher or waste sites. Subsequently,
the ore is crushed and emissions may result during unloading, primary
crushing, screening, secondary crushing, conveying and stockpiling. A
drying step follows, involving conveying the ore to the dryer
building, screening, drying, tertiary crushing, conveying ore to dry
rock storage building and dry rock storage. The next step is the
milling of the ore. In well-controlled mills, this is largely confined
in the mill building, and presents low emissions because the mill air
is collected and ducted through control devices (US EPA, 1986). In
poorly controlled mills the emissions may be high.

3.2.2 Manufacture of products

Chrysotile use today mainly involves products where it is
incorporated into matrices. The asbestos-cement industry is by far the
largest user of asbestos fibres world-wide, accounting for some 85% of
all use. Asbestos-cement production facilities exist in more than 100
countries and produce 27 to 30 million tonnes annually (Pigg, 1994).
Asbestos-cement products contain 10-15% of asbestos, mostly
chrysotile, although limited amounts of crocidolite have been used in
large diameter, high-pressure pipes.

There are five major asbestos-cement products: (a) corrugated
sheets; (b) flat sheets and building boards; (c) slates; (d) moulded
goods, including low-pressure pipes; and (e) high-pressure water pipes
(Pigg, 1994).

Possible emission sources are: (a) feeding of asbestos fibres
into the mix; (b) blending the mix; and (c) cutting or machining
end-products. Emissions may vary according to the dust control
measures and technology.

Although declining in the North American and Western European
markets, asbestos-cement product manufacturing continues to grow in
South America, South-East Asia, the eastern Mediterranean region and
eastern Europe (Pigg, 1994). Japan, Thailand, Malaysia, Korea and
Taiwan imported 430 000 tonnes, well over 30% of world-wide imports in
1989 (Industrial Minerals, 1990). It has been reported that "asbestos
use" (the generic term used by the author) in Japan has reached
proportions which indicate that it leads the world in consumption of
fibres (Frank, 1995).

Table 1. World production, of asbestos (tonnes)^a (from: US Department of Interior, 1993)

Country^b 1988 1989 1990 1991 1992

Argentina 2328 225 300^e 250^e 50

Bosnia & Herzegovina^c -- -- -- -- 1000

Brazil 227 653 206 195 232 332^r 233 100^r 233 000

Bulgaria 300 300 500^r 500^e,r 500

Canada 710 357 701 227 685 627 689 000^r 585 000

China^e 150 000^r 181 000^r 221 000^r 230 000 240 000

Columbia^{e, d} 7600 7900 8000 8000 8000

Cyprus 14 585 -- -- --- --

Egypt 166 312 369 450^r 450

Greece 71 114 73 300^r 65 993^r 5500^e,r --

India 31 123 36 502 26 053^r 24 094^r 25 000

Iran^e 3410^r,g 3300 2800^r 3000^r 3000

Italy 94 549 44 348 3862 3000^e,r 1500

Japan^e 5000 5000 5000 5000 5000

Kazakhstan^f -- -- -- -- 300 000

Korea 2428 2361 1534 1500^e 1600

Russia -- -- -- -- 1 400 000

Serbia & Montenegro^c -- -- -- -- 1700

South Africa 145 678 156 594 145 791 148 525^r 123 951^g

Swaziland 22 804 27 291 35 938 13 888^r 35 000

Turkey 50^e -- -- -- --

Former-USSR^e 2 600 000 2 600 000 2 400 000 2 000 000 --

Table 1. (continued)

Country^b 1988 1989 1990 1991 1992

USA
(sold or used by producers) 18 233 17 427 W 20 061 15 573

Former-Yugoslavia 17 030 9111 6578 5500^e --

Zimbabwe 186 581 187 006^r 160 861^r 141 697^r 140 000

Total 4 310 989^r 4 259 399 4 002 538^r 3 533 065^r 3 120 524

^a Marketable fibre production. Table includes data available until 19 April 1993
^b In addition to the countries listed, Afghanistan, Czechoslovakia, North Korea and Romania also
produce asbestos, but output is not officially reported, and available general information is
inadequate for the formulation of reliable estimates of output levels.
^c Formerly part of Yugoslavia; data were not reported separately until 1992.
^d Estimated fibre production (in tonnes), based on reported crude production, was as follows:
1988: 152 896; 1989:-158 149; 1990: 159 600; 1991: 160 332; 1992: 160 000 (estimated).
^e Estimated
^f Formerly part of the USSR; data were not reported separately until 1992.
^g Reported figure.
^r Revised
^W Withheld to avoid disclosing proprietary data; excluded from "total"

Table 2. Demand for asbestos in the USA
(Thousand tonnes) (US Department of Interior, 1986, 1991)

1977 1984 1991

Asbestos-cement pipe 115 37 4
Asbestos-cement sheet 27 12 2
Coating and compounds 36 22 1
Flooring products 150 46 -
Friction products 57 48 10
Installation: electrical 4 1 -
Installation: thermal 17 2 -
Packing and gaskets 28 13 3
Paper products 7 2 -
Plastics 8 1 -
Roofing products 70 7 15
Textiles 10 2 -
Other 143 33 1

Total^a 672 226 34

^a The totals given are not the exact sums of the values for
individual products, owing to independent rounding.

Other asbestos products consume smaller quantities of chrysotile
asbestos. Friction products, gaskets and asbestos paper are among
them. Production of shipboard and building insulation, roofing and,
particularly, flooring felts and other flooring materials, such as
vinyl asbestos tiles, has declined considerably, some of them having
disappeared completely from the market place. Friable asbestos
materials in building construction have been phased out in many
countries due to international recommendations.

Moulded brake linings on disc- and drum-type car brakes are among
the chrysotile products that are still manufactured. Woven brake
linings and clutch facings for heavy vehicle use are made from
high-strength chrysotile yarn and fabric reinforced with wire; this
material is dried and impregnated with resin. In the moulding process,
the fibres are combined with the resin, which is then thermoset. Final
treatment involves curing by baking and grinding to customer
specifications.

3.2.3 Use of products

Many chrysotile-containing products have entered global commerce.
The nature of the product and local work practices determine dust
emissions. Non-friable products and appropriate technological controls
greatly reduce fibre release. Manipulation of friable products without
controls may release high levels of airborne dust. However, some
conditions may produce chrysotile aerosols even with non-friable
products, e.g., the use of high-speed power tools without controls.

Concern about the possible exposure of inhabitants of buildings
with asbestos-containing materials has led to extensive monitoring
(HEI, 1991). In this respect the exposure of custodian and maintenance
staff is still being studied (see Chapter 4).

Manufacturing data are not available from individual countries
concerning specific chrysotile-containing products.

4. OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE LEVELS

Few recent reports of occupational and environmental exposure
levels are available, particularly those that differentiate among the
forms of asbestos. Workplace concentrations were very high when
monitoring first began (in the 1930s). In countries where controls
were implemented, the levels generally reduced considerably with time
and continue to decline. In contrast, there is less difference between
the early results of measurements in both outdoor and indoor
non-occupational environments (1970s) and recent data.

Environmental Health Criteria 53 (IPCS, 1986) reported that 58.5%
of samples had fibre concentrations of < 0.5 f/ml and 80.7% < 1.0
f/ml in textile industries in the United Kingdom over the period
1972-1978. Corresponding measurements in France in 1984 were 65.3% with
< 0.5 f/ml and 85.4% with < 1.0 f/ml. It also reported 86.5% of
samples with < 0.5 f/ml and 95.0% with < 1 f/ml in asbestos-cement
industries in the United Kingdom during the period 1972-1978.
Corresponding measurements in France in 1984 were 93.5% with < 0.5
f/ml and 97.4% with < 1.0 f/ml. In industries manufacturing friction
products, 71.0% of samples had < 0.5 f/ml and 85.5% < 1.0 f/ml in
the United Kingdom during 1972-1978, while the corresponding results
in France in 1984 were 62.8% with < 0.5 f/ml and 85.0% with < 1.0
f/ml. Typical concentrations (fibres > 5 µm in length) in outdoor air
measured in various locations in Austria, Canada, Germany, South
Africa and the USA ranged from < 0.0001 to about 0.01 f/ml,
concentrations in most samples being less than 0.001 f/ml.
Concentrations (fibres > 5 µm in length) measured in various
buildings in Canada and Germany ranged from values below the limit of
detection to 0.01 f/ml. The highest concentrations were found in
buildings with sprayed-on friable asbestos.

4.1 Occupational exposure

This section focuses mainly on exposures found in industries
where only commercial chrysotile was used. Emphasis is placed on data
obtained directly by the membrane filter method, but, in the case of
some older studies, data are conversions from original particle
counts. In the latter case, fibre concentrations are subject to the
limitations discussed in sections 2.4.1 and 2.4.2.

4.1.1 Mining and milling

Several sets of data have been published concerning the exposure
levels of mine and mill workers employed in the production facilities
of Thetford Mines and Asbestos, Quebec, Canada. A substantial body of
exposure data was collected by using midget impingers and enumerating
all dust particles (Gibbs & Lachance, 1972). Table 3 lists mean
concentrations of dust in the mills in millions of particles per m³
(mpcm) and per cubic foot (mpcf) of air during the period 1949 to
1965. The mill with the highest dust concentrations had more than
twice the mean values given in Table 3, and that with the lowest
concentrations had less than one half.

Table 3. Mean dust concentrations in asbestos mills of Quebec, Canada
(from Gibbs & Lachance, 1972)

Concentration 1949 1951 1953 1955 1957 1959 1961 1963 1965

mpcm 2650 1940 1770 1130 1060 570 350 530 180
mpcf 75 55 50 32 30 16 10 15 5

Studies of the relationships between particle counts and fibre
concentrations have shown poor correlation (Gibbs & Lachance, 1974;
Dagbert, 1976). Gibbs & Lachance (1974) stated that no single
conversion factor could be applied to all mines and mills. Assuming a
conversion factor of roughly 106 f/ml for each mpcm (3 f/ml for each
mpcf), it can be calculated that mean fibre concentrations in the
Quebec mills before mid-1955 were well above 150 f/ml (see discussions
in section 2.4).

Nicholson et al. (1979) reported fibre concentrations obtained by
the membrane filter method in five mines and mills of Thetford Mines,
Quebec, Canada during the period October 1973 to October 1975 (Table
4).

In Zimbabwe, Cullen et al. (1991) reported estimates of fibre
levels prior to 1980. After 1980, the measured concentrations were
below 10 f/ml in all facilities. In India, the concentrations measured
in four mills in 1989 by Mukherjee et al. (1992) are presented in
Table 5.

Parsons et al. (1986) reported that the concentrations in
refining and bagging areas in a Newfoundland mill were generally less
than 0.5 f/ml, but concentrations in the screening area ranged up to
13.9 f/ml.

Average concentrations of asbestos fibres (length > 5 µm) in the
Quebec mining industry during the period 1973-1993 are presented in
Fig. 1. The average concentrations in Quebec chrysotile mining towns
are shown in Fig. 2.

4.1.2 Textile production

Nine textile plants in the USA were studied in 1964 and 1965 by
Lynch & Ayer (1966). The results of the membrane filter analysis are
presented in Table 6. The presence of small amounts of amosite or
crocidolite fibres cannot be excluded due to the non-specificity of
the assay instrument (PCOM).

Table 4. Asbestos fibre concentrations^a in five chrysotile mines and mills at
Thetford Mines, Quebec, Canada (from Nicholson et al., 1979)

Location Five mines and mills
1 2 3 4 5

General mill air Number of samples 14 37 5 6 7
mean 35 12 15 18 9
range 14-57 7-27 7-27 12-29 5-12

Bagging asbestos Number of samples 2 6 2 2
mean 16 16 9 16
range 12-20 10-24 4-13 14-17

Quality control Number of samples 2 1 1
mean 22 20 9
range 21-22 - -

Crusher Number of samples 4
mean 26
range 8-47

Dryer Number of samples 2
mean 36
range 27-45

Shops Number of samples 3
mean 10
range 6-15

Non-work location Number of samples 1 2
mean 0.8 1.3
range - 1-1.7

^a The concentration of fibres (> 5 µm) is given in f/ml.

Table 5. Average personal sample fibre concentrations in four
mills in India (from Mukherjee et al., 1992)

Process Fibre concentration (f/ml)
Average Range

Jaw crusher 1.7 1.3-2.1
Pulverizer 8.9 2.3-15.4
Lime mixer 2.6 2.5-2.6
Huller 12.7 8.9-16.4
Primary eccentric screen 12.9 1.8-25.8
Decorticator 8.8 1.3-18.4

Table 6. Mean dust concentrations (f/ml) by plant and operation in nine textile plants in the USA
during the period 1964/1965 (from Lynch & Ayer, 1966)

Operation Fibres^a Textile plants
1 2 3 4 5 6 7 8 9

Fibre preparation A 38.1 12.3 23.3 34.0 - 8.1 7.6 35.5 11.8
B 15.0 10.0 13.3 18.3 - 3.0 4.5 17.0 2.6
Carding A 18.1 13.6 20.6 32.9 - 6.0 17.2 28.2 8.3
B 10.2 9.21 3.3 15.2 - 3.5 8.1 13.4 2.0
Spinning A 9.6 4.1 20.2 29.8 - 5.1 24.8 20.8 7.4
B 6.6 3.2 18.9 15.7 - 3.5 10.8 10.5 1.8
Twisting A 9.3 6.9 15.8 51.4 - 4.8 25.9 16.7 3.1
B 6.4 5.2 7.5 22.4 - 3.3 12.9 7.2 1.1
Winding A 11.7 4.4 9.6 28.6 - 4.5 25.7 7.9 3.6
B 7.5 3.9 8.9 17.5 - 3.2 11.7 2.7 1.3
Weaving A 7.7 7.0 2.9 33.8 4.5 2.9 9.5 8.1 2.9
B 4.8 3.1 2.3 17.8 3.9 2.2 5.7 3.0 1.5

^a A = total fibres, B = fibres longer than 5 µm

The exposure estimates (1930-1975) in an extensively studied
textile plant in South Carolina, USA, in which chrysotile was the
predominant fibre used, are presented in Table 7 (Dement et al.,
1983a).

Table 7. Exposure estimates in a chrysotile textile plant (1930-1975)
(estimated mean exposure to fibres longer than 5 µm in f/ml)^a

Operation Without controls With controls

Fibre preparation 26.2-78.0 5.8-17.2
Carding 10.8-22.1 4.3-9.0
Spinning 4.8-8.2 4.8-6.7
Twisting 24.6-36.0 5.4-7.9
Winding 4.1-20.9 4.1-8.4
Weaving 5.3-30.6 1.4-8.2

^a From: Dement et al. (1983a)

Application of controls in the dusty processes at the South
Carolina plant led to significant reduction of exposure. Currently
available control technology allows much lower levels to be attained.

Table 8 shows a summary of exposure classifications in an English
textile plant in the period 1951-1974 (Peto et al., 1985). The early
particle count data in this report were based on fibre collection with
a thermal precipitator. The conversion factor used, therefore,
reflects only a precipitator-membrane filter relationship. Comments on
the validity of such conversions have been discussed by Walton (1982).

Kimura (1987) reported geometric mean concentrations of 2.6-12.8
f/ml in the period 1970-1975 and 0.1-0.2 f/ml in the period 1984-1986
in asbestos spinning in Japan.

4.1.3 Asbestos-cement

As mentioned in section 3.2.2, the principal use of chrysotile in
the world today is in asbestos-cement products. In the production of
asbestos-cement pipes, some crocidolite is still used with chrysotile
in certain plants.

Table 9 summarizes the results of the analysis of personal
samples, collected in the late 1970s when reportedly only chrysotile
was used, in an asbestos-cement facility in the USA (Hammad et al.,
1979). In 80% of the samples the concentrations were less than 2 f/ml,
and in about 60% they were less than 0.5 f/ml.

Table 8. Mean concentrations of airborne asbestos fibres in a textile plant^a

Period Very high High Medium Low

1951-1955^b unloading, stacking roving, spinning, carding doubling, rope spinning other areas
28 f/ml l4 f/ml 8 f/ml 4.5 f/ml

1956-1960^b unloading, stacking carding roving, spinning, mixing other areas
28 f/ml 16 f/ml 9 f/ml 4.5 f/ml

1961-1965 unloading, stacking carding carding, roving, other areas
winding, beaming
20 f/ml 15 f/ml 7.5 f/ml 2.5 f/ml

1966-1970 unloading, stacking carding carding, roving, other areas
rope cards
20 f/ml 15 f/ml 7.5 f/ml 2.5 f/ml

1971-1974 none none carding, roving other areas
7.5 f/ml 2.5 f/ml

^a Peto et al. (1985)
^b Results of particle measurements were converted to fibre concentrations using the relationship 35 p/ml = 1 f/ml
Table 9. Chrysotile fibre concentrations (fibres longer than 5 µm)
in selected dust zones of an asbestos-cement production facility^a

Location Number Fibre concentration (f/ml)
of samples range mean

Regrinding 4 0.44-l.2 0.86
Mixing 9 0.51-8.9 2.8
Forming 20 0.12-5.0 0.52
Siding and shingle
finishing 14 0.14-4.9 0.68
Panel finishing 11 0.33-12.0 2.8
Flat and corrugated
finishing 12 0.33-8.0 2.6
Warehouse 5 0.13-2.5 0.63
Maintenance 7 0.20-2.7 0.58

^a From: Hammad et al. (1979)

Exposure estimates in a Canadian plant (Finkelstein, 1983) for
the years 1949, 1969 and 1979 were 40, 20 and 0.2 f/ml, respectively,
for willow operators, 16, 8 and 0.5 f/ml for forming machine
operators, and 8, 4 and 0.3 f/ml for lathe operators. In Japan, Kimura
(1987) reported geometric mean concentrations in bag opening and
mixing of 4.5-9.5 f/ml in 1970-1975 and 0.03-1.6 f/ml in 1984-1986,
whilst in cement cutting and grinding the mean concentrations were
2.5-3.5 f/ml in 1970-1975 and 0.17-0.57 in 1984-1986. Albin et al.
(1990) reported fibre concentrations, based on estimates, in a Swedish
asbestos-cement plant of 1.5-6.3 f/ml during 1956. Later, based on
direct measurements, values were 0.3-5.0 f/ml in 1969 and 0.9-1.7 f/ml
in 1975. Higashi et al. (1994) reported geometric average
concentrations of 0.05-0.45 f/ml measured in area samples and
0.05-0.78 f/ml in personal samples of an asbestos-cement plant.

Few data are available in the open literature on exposures
encountered during installation of asbestos-cement products. It would
be expected that cutting, sanding, drilling or otherwise abrading
asbestos-cement without efficient ventilation controls would give rise
to high exposures (Nicholson, 1978).

Weiner et al. (1994) reported concentrations in a South African
workshop in which chrysotile asbestos-cement sheets were cut into
components for insulation. The sheets were cut manually, sanded and
subsequently assembled. Initial sampling showed personal sample mean
concentrations of 1.9 f/ml for assembling, 5.7 f/ml for sweeping, 8.6
f/ml for drilling and 27.5 f/ml for sanding. After improvements and
clean-up of the work environment, the concentrations were 0.5-1.7
f/ml.

Nicholson (1978) reported concentrations of 0.33-1.47 f/ml in a
room during and after sawing and hammering of an asbestos-cement
panel.

4.1.4 Friction products

Skidmore & Dufficy (1983), based on simulated past conditions
(Table 10), and McDonald et al. (1984) reported data on workplace
exposures during friction product manufacturing.

McDonald et al. (1984) reported that in the 1930s estimated
average dust levels were 35-180 mpcm (1-5 mpcf) in 67% of analysed
locations, while in the 1960s average dust levels were below 7 mpcm
(0.2 mpcf) at 38% of locations and below 18 mpcm (0.5 mpcf) at 67% of
locations in which measurements were obtained.

Table 10. Average concentrations of chrysotile fibres (f/ml) longer > 5 µm from woven
asbestos products during various periods

Pre-1931 1932-1950 1951-1969 1970-1979

Storage/distribution >20 2-5 2-5 0.5-1
Preparation >20 0-20 2-5 1-2
Impregnation/forming >20 2-5 1-2 0.5-1
Grinding >20 5-10 2-5 0.5-1
Drilling, boring >20 2-5 1-2 1-2
Inspection >20 2-5 1-2 0.5-1
Packing >20 1-2 0.5-1 <0.5
Office/laboratory 10-20 <0.5 <0.5 <0.5

* Skidmore & Dufficy (1983)

Kimura (1987) reported geometric mean fibre concentrations of
10.2-35.5 f/ml in 1970-1975, and 0.24-5.5 f/ml in 1984-1986 in
spinning and grinding of friction products in Japan.

A considerable number of reports have included airborne asbestos
concentrations during maintenance and replacement of vehicle brakes.
In the early period, poor or no engineering control measures were
utilized, resulting in high total dust exposure. This was particularly
so during grinding of brakes and compressed air blowing off dust, both
operations of very short duration. Significantly lower levels were
measured when engineering controls were introduced.

An overview of air concentrations measured during maintenance and
replacement of asbestos-containing vehicle brakes is presented in
Table 11.

Table 11. Asbestos air concentrations measured during maintenance and replacement of vehicle brakes

Mean concentration Comment Reference
(f/ml)

3.8^a grinding truck brakes Lorimer et al., 1976
15.9^a blowing off Lorimer et al., 1976
3.8^a grinding Rohl et al., 1976
16.0^a blowing off Rohl et al., 1976
2.5^a dry brushing Rohl et al., 1976
> 1^a 17 of 19 operations Menichini & Marconi, 1982
> 2^a 11 of 19 operations Menichini & Marconi, 1982
0.09^b fibres longer than 5 µm Jahn et al., 1985
6.2^a blowing off, grinding Jahn et al., 1985
0.03^b fibres longer than 5 µm Elliehausen, 1985
0.06^b Ruhe & Lipscomb, 1985
< 0.5 TWA Cheng & O'Kelly, 1986
0.13 maximum Cheng & O'Kelly, 1986
4-5^a fibres longer than 5 µm, blowing off, grinding Rodelsperger et al., 1986
5-10^a fibres longer than 5 µm, blowing off, grinding, trucks Rodelsperger et al., 1986
< 0.05^b Kauppinen & Korhonen, 1987
0.01-0.2^b trucks and buses Kauppinen & Korhonen, 1987
> 1^a blowing off Kauppinen & Korhonen, 1987
< 0.004 Sheehy et al., 1987
< 0.004^b Godbey et al., 1987
0.09-0.12 Van Wagenen, 1987
0.046^b Cooper et al., 1988
0.03^b TWA < 0.002 f/ml Moore, 1988

^a These results are mean personal samples obtained by PCOM; fibres > 5 µm; these represent episodic
releases and not time-weighted averages; operation specific.
^b Mean personal air samples (8-h time-weighted average)

4.1.5 Exposure of building maintenance personnel

The subject of asbestos exposure of maintenance personnel in
buildings has been raised recently and particularly by US OSHA (1994).

Price et al. (1992) estimated the time-weighted averages (TWAs),
of asbestos exposures experienced by maintenance personnel, on the
basis of 1227 air samples. The TWAs, obtained by PCOM, were 0.009 f/ml
for telecommunication switch work, 0.037 f/ml for above-ceiling
maintenance work, and 0.51 f/ml for work in utility spaces. Median
concentrations ranged from 0.01 to 0.02 f/ml.

The Health Effects Institute (1991) evaluated an operation and
maintenance programme in a hospital on the basis of 394 air samples
obtained during 106 on-site activities. The mean asbestos
concentration (PCOM) was about 0.11 f/ml for personal samples and
about 0.012 f/ml for area samples. Eight-hour TWA concentrations
showed that 99% of the personal samples were below 0.2 f/ml, and 95%
were below 0.1 f/ml.

Corn et al. (1994) evaluated exposures of building maintenance
personnel on the basis of about 500 personal samples collected during
maintenance work. However, the building personnel were being monitored
during an asbestos "operations and management" programme, so that
these values may reflect special work practices and environment
conditions. Typical personal exposures are presented in Table 12.

Table 12. Personal asbestos exposures of building maintenance
personnel (fibres longer than 5 µm)^a

Activity Concentration during work 8-h TWA
(f/ml)

Electrical/plumbing work 0-0.035 0.0149
Cable running 0.001-0.228 0.0167
HVAC work 0-0.077 0.0023

^a From: Corn (1994)

Published data for custodial workers, as they exist, reflect
unusual circumstances. Sawyer (1977) studied fibre release from a
friable chrysotile-containing surface formulation during routine
custodial activities performed in the Yale Art and Architecture
Building. The fibre levels, determined by PCOM, ranged from 1.6 f/ml,
obtained during sweeping, to 15.5 f/ml, obtained during dusting of
library books. These values were obtained as short-term episodes. Most
other values, presented as 8-h TWAs, were about two orders of
magnitude lower (HEI, 1991).