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 Organization, or the World Health Organization.

Concise International Chemical Assessment Document 59

First draft prepared by Ms Joann A. Wess, Dr Larry D. Olsen, and Dr Marie Haring Sweeney, National Institute for Occupational Safety and Health, Cincinnati, Ohio, USA

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, 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, 2004

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

Asphalt (bitumen).

(Concise international chemical assessment document ; 59)

1.Hydrocarbons - adverse effects 2.Risk assessment 3.Epidemiologic studies

4.Occupational exposure I.International Programme on Chemical Safety II.Series

ISBN 92 4 153059 6        (LC/NLM Classification: QV 633)

ISSN 1020-6167

©World Health Organization 2004

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Technically and linguistically edited by Marla Sheffer, Ottawa, Canada, and printed by Wissenchaftliche Verlagsgesellschaft mbH, Stuttgart, Germany

FOREWORD

Concise International Chemical Assessment Documents (CICADs) are the latest in a family of publications from the International Programme on Chemical Safety (IPCS) — a cooperative programme of the World Health Organization (WHO), the International Labour Organization (ILO), and the United Nations Environment Programme (UNEP). CICADs join the Environmental Health Criteria documents (EHCs) as authoritative documents on the risk assessment of chemicals.

International Chemical Safety Cards on the relevant chemical(s) are attached at the end of the CICAD, to provide the reader with concise information on the protection of human health and on emergency action. They are produced in a separate peer-reviewed procedure at IPCS. They may be complemented by information from IPCS Poison Information Monographs (PIM), similarly produced separately from the CICAD process.

CICADs are concise documents that provide summaries of the relevant scientific information concerning the potential effects of chemicals upon human health and/or the environment. They are usually based on selected national or regional evaluation documents or on existing EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review by internationally selected experts to ensure their completeness, accuracy in the way in which the original data are represented, and the validity of the conclusions drawn.

The primary objective of CICADs is characterization of hazard and dose–response from exposure to a chemical. CICADs are not a summary of all available data on a particular chemical; rather, they include only that information considered critical for characterization of the risk posed by the chemical. The critical studies are, however, presented in sufficient detail to support the conclusions drawn. For additional information, the reader should consult the identified source documents upon which the CICAD has been based.

Risks to human health and the environment will vary considerably depending upon the type and extent of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk characterization are provided in CICADs, whenever possible. These examples cannot be considered as representing all possible exposure situations, but are provided as guidance only. The reader is referred to EHC 170.¹

While every effort is made to ensure that CICADs represent the current status of knowledge, new information is being developed constantly. Unless otherwise stated, CICADs are based on a search of the scientific literature to the date shown in the executive summary. In the event that a reader becomes aware of new information that would change the conclusions drawn in a CICAD, the reader is requested to contact IPCS to inform it of the new information.

Procedures

The flow chart on page 2 shows the procedures followed to produce a CICAD. These procedures are designed to take advantage of the expertise that exists around the world — expertise that is required to produce the high-quality evaluations of toxicological, exposure, and other data that are necessary for assessing risks to human health and/or the environment. The IPCS Risk Assessment Steering Group advises the Coordinator, IPCS, on the selection of chemicals for an IPCS risk assessment based on the following criteria:

• there is the probability of exposure; and/or
• there is significant toxicity/ecotoxicity.

Thus, it is typical of a priority chemical that

• it is of transboundary concern;
• it is of concern to a range of countries (developed, developing, and those with economies in transition) for possible risk management;
• there is significant international trade;
• it has high production volume;
• it has dispersive use.

The Steering Group will also advise IPCS on the appropriate form of the document (i.e., a standard CICAD or a de novo CICAD) and which institution bears the responsibility of the document production, as well as on the type and extent of the international peer review.

The first draft is usually based on an existing national, regional, or international review. When no appropriate source document is available, a CICAD may be produced de novo. Authors of the first draft are usually, but not necessarily, from the institution that developed the original review. A standard outline has been developed to encourage consistency in form. The first draft undergoes primary review by IPCS to ensure that it meets the specified criteria for CICADs.

The second stage involves international peer review by scientists known for their particular expertise and by scientists selected from an international roster compiled by IPCS through recommendations from IPCS national Contact Points and from IPCS Participating Institutions. Adequate time is allowed for the selected experts to undertake a thorough review. Authors are required to take reviewers’ comments into account and revise their draft, if necessary. The resulting second draft is submitted to a Final Review Board together with the reviewers’ comments. At any stage in the international review process, a consultative group may be necessary to address specific areas of the science. When a CICAD is prepared de novo, a consultative group is normally convened.

The CICAD Final Review Board has several important functions:

• to ensure that each CICAD has been subjected to an appropriate and thorough peer review;
• to verify that the peer reviewers’ comments have been addressed appropriately;
• to provide guidance to those responsible for the preparation of CICADs on how to resolve any remaining issues if, in the opinion of the Board, the author has not adequately addressed all comments of the reviewers; and
• to approve CICADs as international assessments.

Board members serve in their personal capacity, not as representatives of any organization, government, or industry. They are selected because of their expertise in human and environmental toxicology or because of their experience in the regulation of chemicals. Boards are chosen according to the range of expertise required for a meeting and the need for balanced geographic representation.

Board members, authors, reviewers, consultants, and advisers who participate in the preparation of a CICAD are required to declare any real or potential conflict of interest in relation to the subjects under discussion at any stage of the process. Representatives of nongovernmental organizations may be invited to observe the proceedings of the Final Review Board. Observers may participate in Board discussions only at the invitation of the Chairperson, and they may not participate in the final decision-making process.

This CICAD on asphalt (bitumen) was based upon a review prepared by the US National Institute for Occupational Safety and Health (NIOSH, 2000). Additional data were identified through an updated literature search to February 2003. Information on the peer review of the source document is presented in Appendix 1. Information on the peer review of this CICAD is presented in Appendix 2. This CICAD was approved as an international assessment at a meeting of the Final Review Board, held in Varna, Bulgaria, on 8–11 September 2003. Participants at the Final Review Board meeting are listed in Appendix 3. The International Chemical Safety Card on asphalt (ICSC 0162), produced by the International Programme on Chemical Safety (IPCS, 2002), has also been reproduced in this document.

Asphalt (CAS No. 8052-42-4), more commonly referred to as bitumen in Europe, is a dark brown to black, cement-like semisolid or solid or viscous liquid produced by the non-destructive distillation of crude oil during petroleum refining. Oxidized asphalt (CAS No. 64742-93-4), also called air-blown or air-refined asphalt, is asphalt (CAS No. 8052-42-4) that has been treated by blowing air through it at elevated temperatures to produce physical properties required for the industrial use of the final product. Performance specifications (e.g., paving asphalts and roofing asphalts), not chemical composition, direct asphalt production. The exact chemical composition of asphalt is dependent on the chemical complexity of the original crude petroleum and the manufacturing process. Crude petroleum consists mainly of aliphatic compounds, cyclic alkanes, aromatic hydrocarbons, polycyclic aromatic compounds (PACs), and metals (e.g., iron, nickel, and vanadium). The proportions of these chemicals can vary greatly because of significant differences in crude petroleum from oil field to oil field or even at different locations in the same oil field. While the manufacturing process may change the physical properties of asphalt dramatically, the chemical nature of the asphalt does not change unless thermal cracking occurs. Although no two asphalts are chemically identical and chemical analysis cannot be used to define the exact chemical structure or chemical composition of asphalt, elemental analyses indicate that most asphalts contain 79–88 weight per cent (wt%) carbon, 7–13 wt% hydrogen, traces to 8 wt% sulfur, 2–8 wt% oxygen, and traces to 3 wt% nitrogen.

When asphalts are heated, vapours are released; as these vapours cool, they condense. As such, these vapours are enriched in the more volatile components present in the asphalt and would be expected to be chemically and potentially toxicologically distinct from the parent material. Asphalt fumes are the cloud of small particles created by condensation from the gaseous state after volatilization of asphalt. However, because the components in the vapour do not condense all at once, workers are exposed not only to asphalt fumes but also to vapours. The physical nature of the fumes and vapours has not been well characterized. Nevertheless, a chemical analysis of oxidized roofing asphalt and non-oxidized paving asphalt fumes identified many of the same chemical classes. In addition, differences in the way in which asphalts are handled during paving and roofing operations probably influence the composition of asphalt fumes and vapours. Since the compositions of asphalts and asphalt fumes and vapours vary depending on temperature, manufacturing process, presence of additives and modifiers, and work practices, it should be no surprise to learn that laboratory-generated asphalt fumes that mimic asphalt fumes in the environment are difficult to produce. Researchers have concluded that temperature, rate of stirring, and pulling versus pushing the collection air all affect the chemical composition of the fumes.

The major types of asphalt products are paving asphalts and roofing asphalts. Asphalt is also used in asphalt-based paints as protective coatings to prevent corrosion of metals; in lining irrigation canals, water reservoirs, dams, and sea defence works; in adhesives in electrical laminates; and as a base for synthetic turf. In the USA, approximately 300 000 workers are employed at hot-mix asphalt facilities and paving sites; an estimated 50 000 workers are employed in asphalt roofing operations; and about 1500–2000 workers are employed in approximately 100 roofing manufacturing plants. In Western Europe, there are approximately 4000 asphalt mixing plants employing 5–10 individuals per plant. Approximately 100 000 members of paving crews apply these asphalt mixes to road surfaces across Western Europe.

Although a variety of sample collection and analytical methods are available for evaluating asphalt fume exposures, most of them are non-specific and cannot be used to characterize total asphalt fume exposure. Also, readily accessible body fluids and/or physiological functions have been sampled or monitored for biomarkers of exposure to asphalt fumes. Biomarkers specific to asphalt fume exposures have not yet been identified.

Limited data are available on the concentration of asphalt in environmental media. Characterization of concentrations of asphalt fractions in air samples and plant samples collected at various distances from a highway indicated that these concentrations were <4 × 10^–3 mg/m³ and <4 mg/g dry plant material, respectively. An assessment of the effects of runoff from asphalt pavement on streams in California, USA, indicated that concentrations of all polycyclic aromatic hydrocarbon (PAH) analytes in all stream and road runoff samples were below the detection limit of 0.5 µg/litre. Although detectable levels of heavy metals were present in stream and runoff water, the authors concluded that no significant upstream versus downstream differences existed in the concentration of any heavy metal across all streams. Metal concentrations were elevated in runoff water from road surfaces relative to upstream samples. These elevated concentrations could be due to sources other than asphalt (e.g., vehicle emissions, crankcase oil drippings, etc.).

While asphalt fume concentrations associated with health effects have not been well characterized, symptoms of eye, nose, or throat irritation are reported by workers during open-air paving. In the occupational setting, results of recent studies indicate that, in general, most time-weighted average (TWA) air concentrations for total particulates (TP) and benzene-soluble particulates (BSP) ranged from 0.041 to 4.1 mg/m³ and from 0.05 to 1.26 mg/m³, respectively. Average personal exposures, calculated as full-shift TWAs, were generally below 1.0 mg/m³ for TP and 0.3 mg/m³ for BSP.

Asphalt fumes and vapours may be absorbed following inhalation and dermal exposure. Because asphalt is a complex mixture, its pharmacokinetic behaviour will vary depending upon the properties of the individual constituents. Therefore, it is inappropriate to make generalizations regarding the extent of absorption, distribution, and metabolism of asphalt.

Results of several in vitro studies indicate that while field-generated paving asphalt fume condensates were not mutagenic and did not induce DNA adduct formation, paving fume condensates generated in the laboratory were mutagenic and did induce DNA adduct formation. In contrast, one study reported that the particulate fractions of asphalt fumes collected in the personal breathing zone (PBZ) of workers during paving operations were mutagenic in the Ames Salmonella assay. Moreover, intratracheal exposure of rats to field-generated asphalt paving fumes caused a statistically significant increase in the level and activity of CYP1A1 (a major PAH-inducible isozyme of cytochrome P450) in the lung and increased micronuclei formation in bone marrow erythrocytes. Only laboratory-generated roofing asphalt fumes have been tested in genotoxicity studies. These fumes have been shown to be mutagenic, to cause increased micronuclei formation, and to inhibit intercellular communication in Chinese hamster lung fibroblasts (V79 cells) and in human epidermal keratinocytes. Equivocal results have been reported for asphalt-based paints. While in one study none of the asphalt-based paints examined demonstrated mutagenic activity, in another study other asphalt-based paints induced DNA adduct formation in adult and fetal human skin samples. Results of carcinogenicity studies indicated that laboratory-generated roofing asphalt fume condensates caused tumours when applied dermally to mice and that some asphalt-based paints contained chemicals capable of initiating tumours in mice. No animal studies have examined the carcinogenic potential of either field- or laboratory-generated paving asphalt fume condensates.

Acute effects of exposure to asphalt among workers in the various sectors of the asphalt industry (hot-mix plants, terminals, roofing application, paving, roofing manufacturing) include symptoms of irritation of the serous membranes of the conjunctivae (eye irritation) and the mucous membranes of the upper respiratory tract (nasal and throat irritation) and coughing. These health effects appear to be mild in severity and transient in nature. Additional symptoms include skin irritation, pruritus, rashes, nausea, stomach pain, decreased appetite, headaches, and fatigue, as reported by workers involved in paving operations, insulation of cables, and the manufacture of fluorescent light fixtures. Results from recent studies indicated that some workers involved in paving operations experienced lower respiratory tract symptoms (e.g., coughing, wheezing, and shortness of breath) and pulmonary function changes; bronchitis has also been reported. The lowest TP exposure that caused respiratory tract problems was 0.02 mg/m³. However, data from the available studies are insufficient to determine the relationship between asphalt fume exposures and the above reported health effects.

Burns may also occur when hot asphalt is handled. Burned areas usually include the head and neck, arms, hands, and legs.

The largest study to examine the health effects of occupational exposure to asphalt included a cohort of 29 820 workers from eight different countries engaged in road paving, asphalt mixing, roofing, waterproofing, or other specified jobs where exposure to asphalt fumes was possible. Overall mortality for the entire cohort (exposed and non-exposed workers) was below expected (standardized mortality ratio [SMR] = 0.92). For job classifications involving bitumen or asphalt exposure, overall mortality was not elevated (SMR = 0.96); mortality from lung cancer was increased among bitumen workers when compared with ground and building construction workers (SMR = 1.17, 95% confidence interval [CI] = 1.04–1.30). Overall mortality from head and neck cancer was elevated for bitumen workers only (SMR = 1.27, 95% CI = 1.02–1.56). Mortality from other malignant neoplasms was not increased. Further analysis suggested a slight increase in lung cancer mortality among road pavers after adjusting for coal tar pitch and allowing for a 15-year lag (SMR = 1.23, 95% CI = 1.02–1.48).

The investigators (Boffetta et al., 2003b) assessed two different metrics for exposure: average and cumulative exposure. For lung cancer, a positive association was observed for lagged average level of exposure, but not for lagged cumulative exposure. Corresponding indices of unlagged average and cumulative exposure showed a positive dose–response with lung cancer risk based on 63 deaths; relative risks [RRs] were 1.43 (95% CI = 0.87–2.33), 1.77 (0.99–3.19), and 3.53 (1.58–7.89) for 2.2–4.6, 4.7–9.6, and 9.7+ mg/m³ years of cumulative exposure and 2.77 (95% CI = 1.69–4.53), 2.43 (1.38–4.29), and 3.16 (1.83–5.47) for 1.03–1.23, 1.24–1.36, and 1.37+ mg/m³ average exposure (P-value of test for trend, 0.01 for both variables). The investigators concluded that the exposure–response analyses suggest an association between lung cancer mortality and indices of average level of exposure to bitumen fumes; however, they could not rule out that confounding played some role in this association.

A meta-analysis of 20 epidemiological studies failed to find overall evidence for a lung cancer risk among pavers and highway maintenance workers exposed to asphalt (RR = 0.87, 95% CI = 0.76–1.08). However, the analysis demonstrated an overall statistically significant excess of lung cancer among roofers (RR = 1.78, 95% CI = 1.5–2.1). Because, in the past, roofers have been exposed to coal tar and asbestos, which are known human carcinogens, it is uncertain to what extent these findings may be attributable to asphalt exposures.

The same meta-analysis reported increases in risk of bladder cancer (RR = 1.22, 95% CI = 0.95–1.53), stomach cancer (RR = 1.28, 95% CI = 1.03–1.59), and leukaemia (RR = 1.41, 95% CI = 1.05–1.85) in workers generally classified as asphalt workers, but not roofers. Interpretation of the findings of these 20 studies is limited by a lack of consistency among studies and the potential for confounding by other substances. Furthermore, many of these findings are from studies organized by broad job classifications that are prone to errors in defining asphalt exposures.

The extremely limited nature of the available data to serve as a basis for estimation of exposure of the general population should be borne in mind when attempting to determine exposure of the general population to asphalt, asphalt fumes and vapours, and asphalt-based paints. The concentrations of asphalt fractions — polar aromatics (polars), naphthene aromatics (aromatics), and saturates — measured in air samples collected 2.0–83.6 m from the highway were 0.54–3.96 × 10^–3 mg/m³ air, 1.77–9.50 × 10^–4 mg/m³ air, and 0.21–1.23 × 10^–4 mg/m³ air, respectively. These values are extremely low in comparison with occupational exposures determined in the various sectors of the asphalt industry; personal exposures to TP and BSP ranged from 0.041 to 4.1 mg/m³ and from 0.05 to 1.26 mg/m³, respectively. However, the chemical composition of the air samples collected along the highway and at the worksites may differ. In addition to respiratory absorption, dermal absorption may also occur and play a pivotal role in asphalt exposure.

The frequency and concentration of potential asphalt exposures may be lower for the general population than for workers. However, in the general population, there are individuals who may be more sensitive to exposures and therefore exhibit more symptoms or other effects. The extent to which these symptoms occur in the general population has not been studied.

In weighing the available data that explore the relationship between exposure to asphalt and asphalt fumes and vapours and adverse health effects, it is important to consider them in the context of the overall limitations of the information. These uncertainties may be caused by the basic chemistry of asphalt, which is a mixture, the small number of in vivo studies, the inclusion of coal tar in roofing and paving asphalts in past decades (and the inclusion in some current formulations), and the mixed results of human studies. However, these limitations or uncertainties should not preclude a judgement regarding human and environmental health. Under various performance specifications, it is likely that asphalt fumes and paints contain carcinogenic substances.

Asphalt and some asphalt products are described below:

• Asphalt (CAS No. 8052-42-4) or bitumen: the residuum produced from the distillation of crude petroleum at "atmospheric and under reduced pressures in the presence or absence of steam" (Puzinauskas & Corbett, 1978). Asphalt is a black or dark brown solid or viscous liquid at room temperature; insoluble in water at 20 °C; partially soluble in aliphatic organic solvents; and soluble in carbon disulfide, chloroform, ether, and acetone (Sax & Lewis, 1987). Outside the USA, asphalt is more commonly referred to as bitumen, and a mixture of bitumen with mineral matter is referred to as asphalt (CONCAWE, 1992). In this document, asphalt is used to refer to the residuum both with and without the addition of mineral matter.
• Oxidized asphalt (CAS No. 64742-93-4) or oxidized bitumen: also known as air-blown or air-refined asphalt; asphalt (CAS No. 8052-42-4) that has been treated by blowing air through it at elevated temperatures to produce physical properties required for the industrial use of the final product. Oxidized asphalts are typically used in roofing operations, pipe coating, undersealing for Portland cement concrete pavements, hydraulic applications (AI, 1990b), and manufacture of paints (Speight, 1992). They are usually classified by their penetration value and softening point (CONCAWE, 1992).
• Natural asphalts or natural bitumens: naturally occurring deposits of asphalt-like material. While these deposits have physical properties that are similar to those of petroleum-derived asphalt, the composition is different (CONCAWE, 1992). Natural asphalt deposits occur in various parts of the world, mainly as a result of mineral oil seepage from the ground. The best known natural deposit is Trinidad’s Pitch Lake; asphalt deposits can also be found in Venezuela, the Dead Sea, Switzerland, and the Athabasca oil sands in northeastern Alberta (IPCS, 1982; Budavari, 1989; Lewis, 1993). These natural asphalts are not discussed in this document.
• Asphalt cement: asphalt that is refined to meet specifications for paving, roofing, industrial, and special purposes (AI, 1990b). Asphalt cements are used mainly as binders (4–10% of the mixture) in hot-mix asphalts and serve to hold the aggregate together (AI, 1990b; Speight, 1992; Roberts et al., 1996). The grade of asphalt cement is measured by either penetration or viscosity.
• Penetration-grade asphalts: asphalts that are further processed by air-blowing, solvent precipitation, or propane deasphalting. A combination of these processes may be used to produce different grades that are classified according to their penetration value (CONCAWE, 1992).
• Cutback asphalt: an asphalt that is liquefied by the addition of diluents (typically petroleum solvents). It is used in both paving and roofing operations, depending on whether a paving or roofing asphalt is liquefied (AI, 1990b; Speight, 1992; Roberts et al., 1996). It is further classified according to the solvent used to liquefy the asphalt cement to produce rapid-, medium-, or slow-curing asphalt. Rapid-curing cutback asphalts are made by adding gasoline or naphtha and are mainly used as surface treatments, seal coats, and tack coats. Medium-curing cutback asphalts are made by the addition of kerosene, and slow-curing cutback asphalts are made by the addition of diesel or other gas oils. Medium- and slow-curing cutback asphalts are mainly used as surface treatments, prime coats, tack coats, mix-in-place road mixtures, and patching mixtures (Speight, 1992; Roberts et al., 1996).
• Emulsified asphalt: a mixture of two normally immiscible components (asphalt and water) and an emulsifying agent (usually soap). It is used for seal coats on asphalt pavements, built-up roofs, and other waterproof coverings (Stein, 1980; AI, 1990b; Speight, 1992; Roberts et al., 1996). Emulsified asphalts are further graded according to their setting rate (i.e., rapid, medium, and slow). Rapid-setting grades are used for surface treatment, seal coating, and penetration macadams; medium-setting grades are used for patch mixtures; and slow-setting grades are used for mix-in-place road mixtures, patch mixtures, tack coats, fog coats, slurry seals, and soil stabilization (Speight, 1992; Roberts et al., 1996).
• Hot-mix asphalt: paving material that contains mineral aggregate coated and cemented together with asphalt cement (AI, 1990b).
• Mastic asphalt: a mixture of asphalt and fine mineral material in proportions such that it may be poured hot into place and compacted by hand-troweling to a smooth surface (AI, 1990b).
• Asphalt-based paints: a specialized cutback asphalt product that contains relatively small amounts of other materials that are not native to asphalt or to the diluents typically used in cutback products, such as lampblack, aluminium flakes, and mineral pigments. They are used as a protective coating in waterproofing operations and other similar applications (AI, 1990b).
• Hard bitumens: produced using "similar processes to penetration grades but have lower penetration values and softening points." They are mainly used to manufacture bitumen paints and enamels. They are normally classified by their softening point and "designed by a prefix, H (hard) or HVB (high vacuum bitumen)" (CONCAWE, 1992).

The European Committee for Standardization (CEN, 2000) has published a recommended terminology for bitumen and bituminous binders. The main classes of bitumens include paving bitumen, modified bitumen, special bitumen, industrial bitumen, petroleum cut-back bitumen, petroleum fluxed bitumen, and bitumen emulsion.

Performance specifications, not chemical composition, direct asphalt production. To meet these performance specifications, the asphalt may be air-blown or further processed by solvent precipitation or propane deasphalting. Additionally, the products of other refining processes may be blended with the asphalt to achieve the desired performance specifications. Therefore, the exact chemical composition of asphalt is dependent on the chemical complexity of the original crude petroleum and the manufacturing process. Crude petroleum consists mainly of aliphatic compounds, cyclic alkanes, aromatic hydrocarbons, PACs (a class of chemicals that includes PAHs and heterocyclic derivatives in which one or more of the carbon atoms in the PAH ring system have been replaced by a heteroatom of nitrogen [N-PAC], oxygen [O-PAC], or sulfur [S-PAC]; Vo-Dinh, 1989), and metals (e.g., iron, nickel, and vanadium). The proportions of these chemicals can vary greatly because of significant differences in crude petroleum from oil field to oil field or even at different locations in the same oil field (AI, 1990a).

While the manufacturing process may change the physical properties of asphalt dramatically, the chemical nature of the asphalt does not change unless thermal cracking occurs. Raising the temperature will increase the likelihood of cracking and cause more volatiles and even higher-boiling components to be released from the residuum. Solvent precipitation (usually using propane or butane) removes high-boiling components from a vacuum-processed asphalt, which are then used to make other products. Solvent precipitation results in a harder asphalt that is less resistant to temperature changes and often blended with straight-reduced or vacuum-processed asphalts. The air-blowing process can be a continuous or batch operation. Since the continuous operation is faster and results in a softer asphalt, a continuous operation is preferred for processing paving asphalts (Speight, 1992; Roberts et al., 1996). Air blowing combines oxygen with hydrogen in the asphalt, producing water vapour. This decreases saturation and increases cross-linking within and between different asphalt molecules. The process is exothermic (heat producing) and may cause a series of chemical reactions, such as oxidation, condensation, dehydration, dehydrogenation, and polymerization. These reactions cause the amount of asphaltenes (hexane-insoluble materials) to increase and the amounts of polar aromatics (hard resins), cycloalkanes, and non-polar aromatics (soft resins) to decrease, while the amounts of aliphatic compounds (oils and waxes) remain about the same (Table 1); at the same time, the oxygen content of the asphalt increases (Moschopedis & Speight, 1973; Corbett, 1975; Puzinauskas & Corbett, 1978; Boduszynski, 1981; Speight, 1992; Roberts et al., 1996).

Table 1: Changes in physical properties and chemical classes in an asphalt^a with increasingly longer air-blowing times.^b

^a Straight-reduced Arkansas asphalt.

^c T₀ = no air-blowing time; T₀ < T₁ < T₂ < T₃.

Although no two asphalts are chemically identical and chemical analysis cannot be used to define the exact chemical structure or chemical composition of asphalt, elemental analyses indicate that most asphalts contain 79–88 wt% carbon, 7–13 wt% hydrogen, traces to 3 wt% nitrogen, traces to 8 wt% sulfur, and traces to 8 wt% oxygen (examples shown in Table 2) (Speight, 1992). While heteroatoms (i.e., nitrogen, oxygen, and sulfur) make up only a minor component of most asphalts, the heteroatoms profoundly influence the differences in physical properties of asphalts from different crude sources (Speight, 1992; Roberts et al., 1996).

Table 2: Elemental analysis of asphalts from different crude sources.^a

^a Adapted from Speight (1992).

Asphalt is used for paving, roofing, industrial, and special purposes. Oxidized asphalt is used in roofing operations, pipe coating, undersealing for concrete pavements, hydraulic applications, membrane envelopes, some paving-grade mixes (AI, 1990b), and the manufacture of paints (Speight, 1992).

From a scientific point of view, asphalts probably should be classified as to whether or not they have been oxidized. However, most publications have classified asphalts according to the performance specifications for which they were manufactured (e.g., paving asphalts and roofing asphalts). This greatly complicates our understanding of the chemistry of asphalts and the presentation of materials in this document, because most asphalts used in paving are not made from oxidized asphalts, but most asphalts used in roofing are made from oxidized asphalts (Speight, 1992; Roberts et al., 1996). The situation is further complicated by the addition of additives and modifiers, differences in application temperatures, and work practices.

Three asphalt products are used in paving processes: asphalt cements, cutback asphalts, and emulsified asphalts. Cutback and emulsified asphalts are also called liquid asphalts because they are liquid at ambient temperatures. As mentioned previously, most asphalts used in paving operations are not oxidized. The asphalts are heated to about 149–177 °C and mixed with heated (143–163 °C) mineral aggregate. Once transported to the worksite, the hot-mix asphalt is applied to the roadway. The temperature of application is generally between 112 and 162 °C (AI, 1990a; FAA, 1991; Speight, 1992; Roberts et al., 1996).

Oxidized asphalts may or may not be used in roofing manufacturing plants to produce shingles, roll goods, built-up roofing felts, and underlayment felts; these asphalts are shipped hot and kept hot until used in the manufacturing process (AREC, 1999). In addition, some cutback and emulsified asphalts are also used in roofing operations (Speight, 1992). However, most oxidized asphalts are used to produce "mopping-grade" roofing asphalts. These asphalts are generally shipped as a solid and heated in a kettle at the worksite until they become a liquid. Table 3 lists the recommended application temperatures (Appendix C in AI, 1990a) and the recommended maximum heating temperatures (AREC, 1999) for these asphalts.

Table 3: Recommended application temperatures and recommended maximum heating temperatures used with "mopping-grade" roofing asphalts.

^a Adapted from AI (1990a, Appendix C) and AREC (1999).

^b Adapted from AREC (1999).

Differences in the way in which asphalts are handled during paving and roofing operations probably influence the composition of asphalt fumes and vapours. When a hot-mix paving asphalt arrives at the worksite, the asphalt has been cooling since leaving the plant and may not be used immediately when it arrives at the worksite. Conversely, roofing asphalts are heated continuously and stirred occasionally at the worksite until the asphalt is needed.

Since the compositions of asphalts and asphalt fumes and vapours vary depending on temperature, manufacturing process, presence of additives and modifiers, and work practices, it should be no surprise to learn that laboratory-generated asphalt fumes that mimic asphalt fumes in the environment are difficult to produce. Researchers (Kriech & Kurek, 1993; Kriech et al., 1999) have shown how generation conditions can affect the composition of fumes. Using a variety of analytical techniques — gas chromatography with flame ionization detection (GC/FID), GC with flame photometric detection, GC with atomic emission detection, and GC with mass spectrometry (GC/MS) — they compared laboratory-generated asphalt fumes with fumes collected from the headspace in a storage tank at a hot-mix plant (paving asphalt), from the headspace in roofing kettles, and from PBZ samples. They concluded that temperature, rate of stirring, and pulling versus pushing the collection air all affected the chemical composition of the fumes.

When asphalts are heated, vapours are released; as these vapours cool, they condense. As such, these vapours are enriched in the more volatile components present in the asphalt and would be expected to be chemically and potentially toxicologically distinct from the parent material. Asphalt fumes are the cloud of small particles created by condensation from the gaseous state after volatilization of asphalt (NIOSH, 1977). However, because the components in the vapour do not condense all at once, workers are exposed not only to asphalt fumes but also to vapours. The physical nature of the fumes and vapours has not been well characterized, but the fume should be fairly viscous. The asphalt fume particles may collide and stick together, making it difficult to characterize the fume particle size. Some of the vapours may condense only to the liquid phase, thus forming a viscous liquid with some solids. Table 4 shows the results of a chemical analysis of a laboratory-generated oxidized asphalt fume (Lunsford & Cooper, 1989). A chemical analysis of non-oxidized paving asphalt fumes, PBZ samples from two sites, identified many of the same chemical classes as shown for the oxidized asphalt fume in Table 4 (Kriech et al., 2002a).

Table 4: Analysis by GC/MS of chemical composition of asphalt fume fractions A–E from an oxidized "mopping-grade" (Type III) roofing asphalt collected during laboratory generation at 316 °C.^a

^a Adapted from Lunsford & Cooper (1989).

^d – = Not observed.

This section is not intended to be an all-inclusive list of the analytical sampling and analysis methods available for characterizing asphalt fumes and vapours. Emphasis is placed on validated methods that have been used in multiple studies.

Although a variety of sample collection and analytical methods are available for evaluating asphalt fume exposures, most of them are non-specific and cannot be used to characterize total asphalt fume exposure. Many studies have focused on TP and BSP determination for assessing asphalt fume exposures. NIOSH Methods 0500 (NIOSH, 1984, 1994) and 5023 (NIOSH, 1984) have commonly been used to determine these analytes, but on different samples. Using NIOSH Method 0500, the TP sample is collected by drawing a known volume of air through a tared polyvinyl chloride (PVC) filter; using NIOSH Method 5023, the BSP is collected by drawing a known volume of air through a polytetrafluoroethylene (PTFE) filter. The PVC filter is analysed gravimetrically to determine the TP, and the PTFE filter is analysed by extracting with benzene and gravimetrically determining the BSP. In some recent studies, because NIOSH Method 5023 had been withdrawn and because both TP and BSP can be determined on the same sampler, NIOSH Method 5042 (NIOSH, 1998) has been used. In this method, the TP and BSP sample is collected by drawing a known volume of air through a tared PTFE filter. After the tared PTFE filter is analysed gravimetrically to determine the TP, the filter is reanalysed by extracting with benzene and gravimetrically determining the BSP. The working range is 0.13–2 mg/m³ for a 1000-litre sample. The limit of detection (LOD) and limit of quantification (LOQ) for TP are 0.04 and 0.13 mg per sample, respectively; the LOD and LOQ for BSP are 0.04 and 0.14 mg per sample, respectively.

While other solvents have been used (such as cyclohexane, acetonitrile, and methylene chloride) to measure the soluble particulate, the results should not be compared, because the extraction capabilities of these solvents vary. In addition, these methods do not measure distinct chemical components or even a distinct class of chemicals in the asphalt fume sample. Although many researchers have reported results for PAHs in asphalt fumes, results obtained using high-performance liquid chromatography (HPLC)/fluorescence and GC/FID methods are suspect. Because asphalt fumes are composed of many alkylated isomers of PAHs, along with O-PACs and S-PACs, with the exception of naphthalene and some three-ring PAHs, they are so chemically complex that they cannot be separated into discrete compounds. The greater the lack of resolution between compounds, the less reliable the quantification results. Because of the poor resolution obtained with asphalt fume samples, quantification is unreliable when these methods are used. Moreover, an alternative method (such as GC/MS or HPLC/MS) is required to confirm the identity of any suspected PAHs, including naphthalene and other possible baseline-resolved PAHs. Any compounds reported using these methods are tentative identifications at best, and the more complex the matrix, the more unreliable these identifications become. Furthermore, since chromatographic software programs assign peak identification based on the largest peak in a given time window and not on retention time, the wrong peak may be assigned and analysed. Also, for HPLC, a gradient elution (e.g., mobile-phase composition varies during the chromatographic run) is used, which might result in varying retention times, thus further complicating the selection of the correct peak for identification and analysis (NIOSH, 2000).

NIOSH Method 5800 (NIOSH, 1998) can be used to estimate the total PAC content of asphalt fumes. This method uses an HPLC pump to provide a mobile phase into which a sample is injected. The sample then passes through two fluorescence detectors. Since no liquid chromatographic column is used, the entire sample reaches the flow cell at once, resulting in a rapid and sensitive analysis of the sample. The two fluorescence detectors are used to monitor different excitation and emission wavelengths. One set of wavelengths is more sensitive to two- and three-ring PACs, and the second set of wavelengths is more sensitive to four- and higher-ring PACs (NIOSH, 2000; Neumeister et al., 2003).

Readily accessible body fluids and/or physiological functions have been sampled or monitored for biomarkers of exposure to asphalt fumes. Biomarkers specific to asphalt fume exposures have not yet been identified. However, urinary 1-hydroxypyrene (1-OHP) (Hatjian et al., 1995a,b, 1997; Toraason et al., 2001, 2002), DNA strand breaks and oxidative damage in peripheral blood leukocytes (Toraason et al., 2001, 2002), and DNA or protein adducts (Herbert et al., 1990; Lee et al., 1991) have been used with limited success as general indicators of exposure to asphalt fumes and PAHs.

Natural asphalt deposits occur in various parts of the world, mainly as a result of mineral oil seepage from the ground. The best known natural deposit is Trinidad’s Pitch Lake; asphalt deposits can also be found in Venezuela, the Dead Sea, Switzerland, and the Athabasca oil sands in northeastern Alberta (IPCS, 1982; Budavari, 1989; Lewis, 1993). In addition, asphalt is produced from crude petroleum, and it is these petroleum-based asphalts that are the focus of this document.

A broad spectrum of asphalt modifiers and additives spanning categories such as antioxidants, antistripping agents, extenders, fibres, fillers, hydrocarbons, oxidants, plastics, rubbers, waste materials, and miscellaneous products are also employed with the various asphalts (Speight, 1992; Roberts et al., 1996). Their presence may affect the composition of asphalt fumes and vapours and worker exposure.

The major types of asphalt products are paving asphalts and roofing asphalts. Asphalt is also used in asphalt-based paints as protective coatings to prevent corrosion of metals; in lining irrigation canals, water reservoirs, dams, and sea defence works; in adhesives in electrical laminates; and as a base for synthetic turf (Lewis, 1993).

In the USA, approximately 30 million tonnes of asphalt materials were produced in 2000 for paving and non-paving applications (AI, 2001). In 2001, approximately 16 million tonnes of bitumen (asphalt) were produced in Western Europe, of which 14 million tonnes were used in road pavement applications (D. Lyall, Eurobitume, Brussels, personal communication, 2002).

No specific data are available relating to transport and distribution among media, environmental transformation and degradation, interaction with physical, chemical, or biological factors, and bioconcentration. However, a recent report by CONCAWE (2001) indicates that although constituents of bitumen (asphalt) have octanol/water partition coefficient (log K_ow) values in excess of 6 and are potentially bioaccumulative, in practice, their very low water solubilities and high relative molecular masses (ranging from 500 to 15 000) are such that their bioavailability to aquatic organisms is expected to be limited. The bioaccumulation of bitumen components would therefore be highly unlikely.

Bitumens (asphalts) would not be readily degradable. However, basing toxicological conclusions on the activity of single components may not be relevant to the physical/chemical interactions of a complex mixture such as asphalt.

Cooper & Kratz (1997) determined the components of runoff from asphalt pavement in fish (rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, and Paiute sculpin Cottus beldingi) and invertebrates from streams in California, USA. Concentrations of the PAH analytes in fish and invertebrate tissues were below the detection limit of 0.2 mg/kg. While concentrations of lead and cadmium in fish tissues were below the detection limits of 0.5 and 0.05 mg/kg, respectively, concentrations of zinc were higher in invertebrate tissues than in fish tissues and also significantly elevated at downstream relative to upstream sites (P = 0.05), ranging from 26 to 98 mg/kg. Invertebrate tissue concentrations of cadmium were independent of collection sites within streams and ranged from below detectable limits to 0.28 mg/kg. However, there is a potential for contributions from fuel, combustion, and crankcase deposits, as well as metals contained in tire tread rubber dust from tire abrasions (see also section 6).

Limited data are available on the concentration of asphalt in environmental media.

Asphalt fractions, including polars, aromatics, and saturates, were characterized in airborne particles and air samples collected 2.0–83.6 m from a highway in Denmark and in plant samples (grass, leaves, and wheat straw) collected 2.0–10.0 m from the highway (Kebin et al., 1996). The percentage of asphalt in these airborne particles was 1.61–11.02%. Concentrations of asphalt fractions in air samples were 0.54–3.96 × 10^–3 mg polars/m³ air, 1.77–9.50 × 10^–4 mg aromatics/m³ air, and 0.21–1.23 × 10^–4 mg saturates/m³ air. Concentrations of asphalt fractions for polars, aromatics, and saturates in mg/g dry plant material were: 0.96, 0.89, and 0.37 for grass; 0.93 and 3.07, 2.91 and 3.89, and 1.28 and 1.53 for leaves; and 1.19 and 0.29, 1.38 and 1.30, and 0.63 and 0.56 for wheat straw (at 5 m and 10 m, respectively), respectively. However, diesel and gasoline exhaust from nearby traffic may have contributed to the composition of these fractions.

An assessment was made of the effects of runoff from asphalt pavement on streams in California, USA (Cooper & Kratz, 1997). Concentrations of PAHs and selected heavy metals (lead, zinc, cadmium) were determined in water samples collected from water draining road surfaces and from waters upstream and downstream from the point where water discharged from road surfaces into stream sites. Results of analyses indicate that concentrations of all PAH analytes in all stream and road runoff samples were below the detection limit of 0.5 µg/litre. Although detectable levels of heavy metals were present in stream and runoff water, the authors concluded that no significant upstream versus downstream differences existed in the concentrations of any heavy metal across all streams. Furthermore, concentrations of metals were elevated in runoff waters from the road surfaces relative to upstream samples. Elevated metal concentrations could be due to sources other than asphalt (i.e., vehicle emissions, crankcase oil drippings, industrial operations, etc.).

Kriech et al. (2002b) conducted a laboratory study to determine 29 PACs in leachate water of six paving asphalt and four roofing asphalt samples. Samples were leached according to US Environmental Protection Agency (EPA) method SW846-1311. Results indicated that none of the roofing samples tested leached any of the 29 PACs. While four of the paving samples did not leach any of the 29 PACs, leachate of two paving samples contained detectable amounts of naphthalene and phenanthrene; however, the levels were well below drinking-water limits (0.015 mg/litre) in the USA. Similarly, Brantley & Townsend (1999) performed a series of leaching tests on samples of reclaimed asphalt from facilities in Florida, USA. None of 16 EPA priority pollutant PAHs were detected in the water leachates of any of these samples. The authors pointed out that during normal use of pavement, the asphalt may come in contact with vehicle exhaust, lube oils, gasoline, and metals from brake pads. In addition, Brandt & DeGroot (2001) demonstrated that PAH concentrations in leachate water from 10 asphalts were well below the European maximum tolerable concentration for potable water (0.1 μg/litre).

Quantitative information on levels of asphalt in drinking-water and foodstuffs has not been identified. However, experiments conducted to determine whether the use of asphalt seal coating in ductile-iron pipe would contribute significant concentrations of PACs in drinking-water indicated that the highest concentration found in three experiments was 5 ng/litre (Miller et al., 1982). The significance of these experiments is unclear, since they represented a worst-case scenario and the pipes were aged for only 1 month in a laboratory setting.

In the USA, approximately 300 000 workers are employed at hot-mix asphalt facilities and paving sites (APEC, 1999); an estimated 50 000 workers are employed in asphalt roofing operations; and about 1500–2000 workers are employed in approximately 100 roofing manufacturing plants (AREC, 1999). In Western Europe, there are approximately 4000 asphalt mixing plants employing 5–10 individuals per plant. Approximately 100 000 members of paving crews apply these asphalt mixes to road surfaces across Western Europe (Burstyn, 2001).

Data collected between 1994 and 1997 during seven paving surveys conducted in the USA by NIOSH (2000) indicated that, in general, most TWA PBZ air concentrations for both TP and BSP were below 0.5 mg/m³. Geometric mean (GM) full-shift PBZ samples for TP and BSP ranged from 0.041 to 0.48 mg/m³ and from 0.073 to 0.12 mg/m³, respectively. However, GM data collected during paving operations in a tunnel in Boston, Massachusetts, USA (Sylvain & Miller, 1996), indicated that PBZ exposures to TP and BSP were about 3 times higher than exposures measured during the seven NIOSH surveys at open-air roadway paving sites (NIOSH, 2000). Personal exposures to TP and BSP ranged from 1.09 to 2.17 mg/m³ and from 0.30 to 1.26 mg/m³, respectively (Sylvain & Miller, 1996).

Other studies examined exposures to asphalt not only at road paving sites, but also at hot-mix plants, refineries and terminals, roofing manufacturing plants, and roofing application sites in the USA (Hicks, 1995; Exxon, 1997; Gamble et al., 1999). GM exposures for TP and BSP at these sites are presented in Table 5. GM exposures for TP and BSP varied across all industry types: TP ranged from 0.18 to 1.40 mg/m³, and BSP ranged from 0.05 to 0.27 mg/m³. Heikkilä et al. (2002) reported GM exposures for TP from asphalt (described by the author as bitumen fume) of 0.4, 0.5, and 4.1 mg/m³ for paving operator, screed operator, and manual mastic paver, respectively. Similarly, Burstyn et al. (2000) reported higher GM asphalt fume exposures (described by the author as bitumen) during mastic laying operations (2.29 mg/m³) compared with exposures during paving operations (0.28 mg/m³). These values indicate that exposures may be higher in situations such as mastic laying.

Table 5: Geometric mean of personal exposures for total particulates (TP) and benzene-soluble particulates (BSP).

^a Adapted from Hicks (1995).

^b Adapted from Exxon (1997) and Gamble et al. (1999).

Several investigators have attempted to assess asphalt exposure by the dermal route. Wolff et al. (1989) collected dermal wipe samples by wiping a 3 × 3 cm area of the forehead of workers exposed to asphalt during the application of hot asphalt to roofs in order to evaluate the extent to which dermal absorption of PAHs may contribute to the total body burden. These dermal wipe samples were analysed for specific PAHs. In the Wolff et al. (1989) study, PAH residues per square centimetre of skin were higher in postshift samples (6.1–31 ng/cm²) than in preshift samples (0.44–2.2 ng/cm²). However, workers monitored during the entire roofing application were potentially exposed to PAHs during both the removal of the old coal tar pitch roof and the application of hot asphalt for the new roof. Hicks (1995) collected dermal wipe samples by wiping a 4 × 8 cm area from the back of the hand or forehead of workers at the various asphalt sectors described in Table 5. The PAH concentrations determined from these postshift samples ranged from 2.2 to 520 ng/cm². Workers in paving operations produced the largest number of PAHs detected (12 of 16), while refinery and roofing workers had the fewest (2 of 16). However, the HPLC/fluorescence technique used by these authors cannot reliably identify and quantify components of asphalt; their results are presented for completeness only.

Toraason et al. (2001, 2002) examined urinary 1-OHP concentrations at the beginning and end of the same work week (4 days later) in seven roofers who applied hot asphalt products but had no coal tar exposure during the preceding 3 months. All seven workers were smokers at the time of the study. Urinary 1-OHP concentrations were statistically significantly increased(P < 0.05) at the end of the work week (start of work week 0.26 ± 0.13 µmol/mol creatinine; end of work week 0.58 ± 0.29 µmol/mol creatinine). The average weekly TWA exposure for TP and BSP for a crew of six asphalt-only roofers was 0.24 ± 0.10 mg/m³ and 0.08 ± 0.02 mg/m³, respectively. The TWA exposures for TP and BSP for a seventh roofer in another crew were 0.31 mg/m³ and 0.18 mg/m³, respectively.

Heikkilä et al. (2002) measured preshift and postshift urinary 1-OHP concentrations in 32 road pavers participating in a study to evaluate asphalt fume exposures of workers employed at 13 paving sites where 11 different asphalt mixtures were applied. The mean TP exposure for the 11 asphalt mixtures ranged from 0.2 to 4.2 mg/m³ (AM [arithmetic mean] = 0.6 mg/m³; GM = 0.5 mg/m³). The mean TP exposure for all mixtures was below 0.5 mg/m³, with the exception of manual mastic paving (4.2 mg/m³) and stone mastic asphalt (2.0 mg/m³). The control group consisted of 78 smoking and non-smoking unexposed office workers obtained from a national reference group for 1-OHP in Finland. The authors reported that mean 1-OHP concentrations were statistically significantly higher (P < 0.05) among pavers (AM = 6.6 nmol/litre, standard deviation [SD] = 9.8) than in controls (AM = 1.6 nmol/litre, SD = 2.6) and twice as high among pavers who were smokers (preshift: AM = 8.5 nmol/litre, SD = 10.5) as among pavers who were non-smokers (preshift: AM = 4.0 nmol/litre, SD = 8.0) (P < 0.05) (P. Heikkilä, personal communication, Finnish Institute of Occupational Health, Helsinki, 2003). A similar trend was observed in postshift data (data not shown). There was no difference between non-smoking road pavers or non-smoking referents (data not shown), suggesting that smoking strongly influences urinary 1-OHP concentrations and may not be a sensitive measure of occupational asphalt fume exposure.

No studies that report exposures to cutback asphalts, emulsified asphalts, or asphalt-based paints (products applied at or near ambient temperatures) have been found. Because these products are liquids, workers may be exposed via inhalation and dermal contact.

Mixtures do not lend themselves to kinetic analyses. Because asphalt is a complex mixture, its pharmacokinetic pattern will vary depending upon the properties and interactions of the individual constituents. The pharmacokinetics of some asphalt components, particularly the PAHs, have been studied in considerable detail (Syracuse Research Corporation, 1985).

The long-chain aliphatic hydrocarbons constitute major components of asphalt; routes of uptake include inhalation, ingestion, and dermal uptake. Data indicate that following inhalation, hydrocarbons with 9–16 carbons were absorbed in the blood, brain, liver, kidneys, and fat of rats (ATSDR, 1998). Aerosols of hydrocarbons with more than 16 carbons were absorbed in liver and lungs of mice. These long-chain aliphatic compounds may be oxidatively metabolized via cytochrome P450 oxidases. Aliphatic hydrocarbons with between five and eight carbons may be oxidized to several alcohol, ketone, and carboxylic acid derivatives. Aliphatic hydrocarbons with 9–16 carbons are oxidatively metabolized via cytochrome P450 isozymes to fatty acids and alcohols. Evidence indicates that metabolism of these hydrocarbons may be quite slow. In general, these compounds are slowly eliminated in the urine and faeces.

The major routes of uptake of PAHs in humans are the lungs and respiratory tract after inhalation of PAH-containing aerosols or of particulates to which a PAH in the solid state has become absorbed; the gastrointestinal tract after ingestion of contaminated food or water; and the skin as a result of contact with PAH-bearing materials (IPCS, 1998). In general, the oxidative metabolism of PAHs involves epoxidation of double bonds, a reaction catalysed by cytochrome P450-dependent mono-oxygenases, rearrangement or hydration of the epoxides to yield phenols or diols, respectively, and conjugation of the hydroxylated derivatives with glutathione, sulfate, or glucuronic acid. However, in certain cases, radical cations and sulfate esters of hydroxymethyl derivatives may also be important (IPCS, 1998). Whole body distribution of PAHs has been studied in rodents. These studies have demonstrated that detectable levels of PAHs occur in almost all internal organs and that organs high in adipose tissue can serve as storage depots from which the PAHs are generally released (IPCS, 1998). In general, these compounds are eliminated by urinary or biliary excretion of metabolites.

In vivo and in vitro animal studies have evaluated the genotoxicity, carcinogenicity, and other toxic effects of asphalt-based paints and asphalt fumes. Because of the difficulty in obtaining a sufficient quantity of paving and roofing asphalt fumes in the field, many of the studies used laboratory-generated asphalt fume condensates.

Irritation studies (eye, skin, respiratory tract) have been reviewed previously in NIOSH (1977), IPCS (1982), and IARC (1985).

Exposure of rabbits to asphalt vapours was reviewed (NIOSH, 1977). The asphalts in the study were from the USA and England, with no further details provided. Additional experimental details (temperature of vapour generation, concentrations of the vapour, duration and frequencies of exposures) were not provided. Exposure to asphalt vapours caused only minor, transient conjunctivitis in the eyes of rabbits. After frequent exposures, a slight infiltration of the cornea was sometimes noted; however, this disappeared several days after exposures ceased. No other toxic effects were observed in the rabbits (NIOSH, 1977).

In a skin painting study summarized in IARC (1985), Swiss albino mice were exposed to samples of eight different bitumens (class 1). They received biweekly applications of 25 µl of bitumen solution (10% in benzene) to shaved areas of their backs for approximately 81 weeks. Skin effects included epidermal hyperplasia, along with inflammatory infiltration of the dermis and cutaneous ulceration with abscess formation.

In another study (Hueper & Payne, 1960), 30 guinea-pigs (Strain 13) and 65 Bethesda black rats were placed in chambers and exposed to roofing asphalt fumes and vapours for 5 h/day, 4 days/week, for 2 years. These fumes and vapours were derived from an air-blown petroleum asphalt by placing 700–10 000 g of the asphalt into an evaporating dish and heating it to 120–135 °C. Fresh asphalt was placed in the evaporating dish once a week, while on other days only the amount lost was replaced. (Asphalt typically would not be heated repeatedly during the course of a week; therefore, these fumes and vapours may not be representative of a typical exposure. Further experimental details were not provided.) Exposure to these asphalt fumes and vapours caused "extensive chronic fibrosing pneumonitis with peribronchial adenomatosis" (Hueper & Payne, 1960).

While exposure conditions in Simmers (1964) are not representative of real-world exposures, results are included for completeness. "The asphalt used in this study was a pooled sample from six different California refineries and contained both steam and air-blown samples." In the first experiment, 20 C57 Black mice were exposed to an asphalt aerosol made from an asphalt emulsion. Mice were exposed to this aerosol 30 min/day, 5 days/week, for up to 410 treatments. (Three mice survived 410 treatments, while 10 mice survived 280 or more treatments.) Effects included congestion, acute bronchitis, pneumonitis, bronchial dilatation, and some peribronchial round cell infiltration. In the second experiment, asphalt smoke was generated by placing 250–350 g of the asphalt sample into a tin container and heating to 120 °C, causing the asphalt to boil and give off a yellowish-brown smoke. Thirty C57 Black mice were exposed for 6–7.5 h/day, 5 days/week, for 21 months. Effects included peribronchial round cell infiltration, bronchitis, pneumonitis, loss of cilia, and epithelial atrophy.

A study to evaluate possible toxic effects of asphalt fumes after inhalation exposure of male and female Wistar WU rats was conducted by the Fraunhofer Institute (Fraunhofer, 2001) to determine concentrations and a maximally tolerated dose for a future carcinogenicity study. The composition of the asphalt fumes was designed to mimic exposure during road paving in Germany (Pohlmann et al., 2001). Rats (16 per group) were exposed nose only to clean air (control) or to target concentrations of 4, 20, or 100 mg/m³ of asphalt fumes for 6 h/day, 5 days/week, for 14 weeks. The mean actual concentrations (aerosol + vapour phase) analysed by infrared spectroscopy were 3.95, 20.12, and 106.55 mg/m³. The composition of the exposure atmosphere (% particulate/% vapour) was 24.6/75.4, 42.9/57.1, and 68.1/31.9 for 4, 20, and 100 mg/m³, respectively. The number median aerodynamic diameter as measured with the scanning mobility particle sizer system was 105 nm in the 4 mg/m³, 82 nm in the 20 mg/m³, and 86 nm in the 100 mg/m³ asphalt fumes. No mortality related to the asphalt fume exposure occurred. Results indicate that exposure to 100 mg/m³ asphalt fumes caused a significantly lower body weight in male rats and statistically significant (P-values not presented) exposure-related histopathological changes (e.g., hyalinosis, basal cell hyperplasia, mucous cell hyperplasia, inflammatory cell infiltration) in the nasal and paranasal cavities. Under the experimental conditions described above, the no-observed-adverse-effect level for asphalt fumes is 20 mg/m³.

8.2.1 Mutagenic effects

A number of studies evaluated potential mutagenic effects of paving and roofing asphalt and asphalt-based paints using the Ames Salmonella assay. An evaluation of available data indicates that asphalt fumes collected at 146–157 °C from the headspace of an asphalt storage tank at a hot-mix asphalt production plant were not mutagenic in the modified Ames Salmonella assay, while fume condensates generated in the laboratory at 149 °C and 316 °C were mutagenic (Reinke & Swanson, 1993; Reinke et al., 2000). Asphalt fume condensates generated at 316 °C were more mutagenic than the fumes generated at 149 °C. In contrast, a study by Heikkilä et al. (2003) demonstrated that the particulate fractions of asphalt fumes collected in the PBZ of workers during paving operations were mutagenic in the Ames Salmonella assay, recycled asphalts being more mutagenic than the particulate fractions of new asphalt. Additionally, another study did not demonstrate any mutagenicity in mice exposed by nose only to paving asphalt fumes (Micillino et al., 2002). Asphalt fume samples collected above an open port of the heated cement storage tank at hot-mix plants were not mutagenic using a spiral Salmonella mutagenicity assay (Burr et al., 2002). In other studies, paving and roofing asphalt fumes generated in the laboratory under a variety of conditions were also mutagenic (AI, 1990a; NTP, 1990; Machado et al., 1993; De Méo et al., 1996). None of the asphalt-based paints examined by Robinson et al. (1984) demonstrated mutagenic activity in either the presence or absence of metabolic activation (S9).

8.2.2 Micronuclei formation and chromosomal aberrations

Condensates of Type I and Type III roofing asphalt fumes generated in the laboratory at 316 °C using the same methodology as in Sivak et al. (1989) and roofing asphalt fumes generated by Sivak et al. (1989) (information on the methodology can be found in section 8.4) caused a dose-related increase in micronucleus formation in exponentially growing Chinese hamster lung fibroblasts (V79 cells) (Qian et al., 1996, 1999). The authors suggested that Type I and Type III roofing asphalt fume condensates are aneuploidogens and possess some clastogenic activities. These condensates caused mainly cytogenetic damage by spindle apparatus alterations in cultured mammalian cells. Ma et al. (2002) exposed male Sprague-Dawley rats intratracheally to asphalt fume condensates (saline control, 0.45 mg/kg body weight, or 8.8 mg/kg body weight) collected at the top of a paving storage tank (160 °C). Exposure to 0.45 mg asphalt fume condensate/kg body weight caused a non-significant increase in micronuclei formation, while 8.8 mg asphalt fume condensate/kg body weight (the highest concentration tested) caused a statistically significant (P < 0.05) increase in micronuclei formation in bone marrow polychromatic erythrocytes. However, all results were negative when three paving asphalt fume condensates generated in the field and in the laboratory were tested at 5, 10, 15, 20, 30, 40, 60, 80, and 120 µg/ml in a chromosomal aberration assay using Chinese hamster ovary cells (Reinke & Swanson, 1993; Reinke et al., 2000).

8.2.3 DNA adduct formation

De Méo et al. (1996) and Genevois et al. (1996) tested paving asphalt fume condensates generated in the laboratory at 160 and 200 °C for their ability to induce DNA adduct formation in vitro and in vivo,respectively. All of the fume condensates induced DNA adduct formation in vitro when added to calf thymus DNA, although no specific DNA adducts were identified (De Méo et al., 1996). Additionally, the same paving asphalt fume condensates induced DNA adducts in the skin, lungs, and lymphocytes of BD4 rats treated with them dermally, but specific types of DNA adducts were not identified (Genevois et al., 1996). In a later study, Genevois-Charmeau et al. (2001) exposed three BD6 rats by nose only to paving asphalt fume condensates. A DNA adduct was detected only in the lungs of the exposed rats.

Male Parkes mice that received multiple topical applications of asphalt-based paints showed accumulations of DNA adducts in both skin and lung tissue (Schoket et al., 1988a). After topical application, asphalt-based paints also induced DNA adduct formation in adult and fetal human skin samples maintained in short-term tissue culture. A single 15-mg dose per skin patch of asphalt-based paint induced 0.22 fmol adducts (Schoket et al., 1988b). However, the specific types of DNA adducts were not identified in either study.

8.2.4 Intercellular communication

The five laboratory-generated asphalt roofing fume fractions used by Sivak et al. (1989) were tested for inhibition of intercellular communication. All fractions inhibited intercellular communication in Chinese hamster lung fibroblasts (V79 cells) (Toraason et al., 1991). Similarly, Wey et al. (1992) examined the effect of these fractions on intercellular communication in human epidermal keratinocytes. All fractions inhibited intercellular communication in a concentration-dependent fashion. Modulation of gap functional intercellular communication has been implicated as an important effect of tumour promoters. The inhibition of intercellular communication by a tumour promoter is believed to isolate an initiated or preneoplastic cell from the regulatory signals of surrounding cells, leading to the development of neoplasms (NIOSH, 2000).

Ma et al. (2002) exposed male Sprague-Dawley rats intratracheally to asphalt fume condensates (saline control or 0.45, 2.22, or 8.8 mg/kg body weight) collected at the top of a paving asphalt storage tank (160 °C). Exposure to 8.8 mg asphalt fume condensate/kg body weight, the highest concentration tested, caused a statistically significant (P < 0.05) dose-dependent increase in both the level and activity of CYP1A1 in the lung. However, CYP2B1 levels and activity were not significantly affected.

Several studies have reported carcinogenicity in mice following applications of laboratory-generated asphalt roofing fume condensates (Thayer et al., 1981; Niemeier et al., 1988; Sivak et al., 1989, 1997), raw roofing asphalt (Sivak et al., 1989, 1997), and asphalt-based paints (Robinson et al., 1984; Bull et al., 1985) to the skin of mice. However, in another study (Emmett et al., 1981), raw roofing asphalt applied dermally to mice was not carcinogenic.

Thayer et al. (1981) and Niemeier et al. (1988) investigated the tumorigenicity of fume condensates generated in the laboratory at 232 and 316 °C from Type I and Type III roofing asphalt³ and applied biweekly for 78 weeks to the skin of male CD-1 and C₃H/HeJ mice. Eighteen groups of 50 mice per strain received these applications. Half of each group was exposed to simulated sunlight. Tumours were produced in both strains of mice by fume condensates of both types of asphalt (see Tables 6 and 7). The majority of benign tumours were papillomas; the majority of malignant tumours were squamous cell carcinomas. Both strains of mice exposed to asphalt fumes had significantly (P = 0.01) more tumours than the control groups, although the C₃H/HeJ mice demonstrated a greater tumorigenic and carcinogenic response than did the CD-1 mice. The C₃H/HeJ mice showed a significant increase (P = 0.01; Fisher-Irwin exact test) in tumorigenic response for both types of condensed asphalt fumes generated at 316 °C compared with tumours generated at 232 °C. The mean time to tumour appearance was longer for all groups of CD-1 mice compared with the corresponding C₃H/HeJ groups. The mean latency period ranged from 39.5 to 56.1 weeks among the C₃H/HeJ groups and from 47 to 76.5 weeks among the CD-1 groups treated with the asphalt fume condensates. Niemeier et al. (1988) concluded that the carcinogenic activity of the asphalt fume condensates may have been due to the high concentrations of aliphatic hydrocarbons, which have co-carcinogenic effects, and that higher generation temperatures may increase carcinogenic effects.

Table 6: Final histopathology of tumours induced in CD-1 mice treated dermally with roofing asphalt fume condensates.^a

Table 7: Final histopathology of tumours induced in C₃H/HeJ mice treated dermally with roofing asphalt fume condensates.^a

Sivak et al. (1989, 1997) heated Type III roofing asphalt from the same lot used by Niemeier et al. (1988) at 316 °C, generated fume condensates, and separated them by HPLC (see Belinky et al., 1988, for a description of this procedure). The chemical composition of fractions A through E, as analysed by GC/MS, can be found in Table 4. Raw roofing asphalt, neat asphalt fumes, asphalt heated to 316 °C with fumes allowed to escape, reconstituted asphalt fumes, and the asphalt fume fractions — individually and in various combinations — were then tested for their carcinogenic and tumour-promoting activity in male C₃H/HeJ and Sencar mice. Fractions A through E were dissolved in a 1:1 solution of cyclohexane and acetone to yield concentrations proportional to their presence in the unfractionated (neat) asphalt fume condensate, i.e., 64.1%, 8.3%, 10.5%, 11.5%, and 5.6%, respectively. They were then applied biweekly to 40 groups of male C₃H/HeJ mice and two groups of male Sencar mice (30 mice per group) for 104 weeks (2 years). Table 8 shows all the treatment groups, the number of papillomas and carcinomas per group, the number of tumour-bearing mice, and the average time (in weeks) to carcinoma development. The raw roofing asphalt and neat asphalt fumes induced carcinomas (local skin cancers) in 3 of 30 and 20 of 30 C₃H/HeJ mice, respectively. However, the heated asphalt with fumes allowed to escape did not induce any tumours. Fractions B and C induced carcinomas in 10 of 30 and 17 of 30 C₃H/HeJ mice, respectively, while fractions A, D, and E failed to induce any carcinomas when applied alone. All the combinations of the fractions induced carcinomas only if they included B or C. The A and D combination, the A and E combination, and the A, D, and E combination failed to induce any carcinomas. Furthermore, fractions A, D, and E failed to act as either tumour promoters or co-carcinogens. Eighteen of the 30 Sencar mice treated with the asphalt fume condensate developed carcinomas. Fractions contained PACs that included PAHs, S-PACs, and O-PACs, such as alkylated aryl thiophenes, alkylated phenanthrenes, alkylated acetophenones, and alkylated dihydrofuranones. Fraction B contained most of the S-PACs, and only a few were carried over to fraction C. Fraction C contained a small amount of four-ring PACs.

Table 8: Tumorigenic response in all treatment groups.^a