Contents

Foreword

As part of its mandate to "provide a safe and healthful workplace for working women and men," the National Institute for Occupational Safety and Health (NIOSH) critically evaluates the scientific data on potentially hazardous occupational exposures or work conditions and makes recommendations that address measures for minimizing the risk from the hazard. This document, Hazard Review: Health Effects of Occupational Exposures to Asphalt, is an evaluation of the health effects and other relevant data that have become available since publication of the 1977 NIOSH document Criteria for a Recommended Standard: Occupational Exposure to Asphalt Fumes. It includes an assessment of chemistry, health, and exposure data from studies in animals and humans exposed to raw asphalt, paving and roofing asphalt fume condensates, and asphalt-based paints. Most important, the document serves as a basis for identifying future research to reduce occupational exposures to asphalt.

The complex chemical composition of asphalt makes it difficult to identify the specific component(s) responsible for adverse health effects observed in exposed workers. Known carcinogens have been found in asphalt fumes generated at work sites. Observations of acute irritation in workers from airborne and dermal exposures to asphalt fumes and aerosols and the potential for chronic health effects, including cancer, warrant continued diligence in the control of exposures.

NIOSH and its labor and industry partners are making great strides in reducing worker exposures to paving and roofing asphalt fumes. The partnership has succeeded because the partners set aside key differences to focus on the development of engineering and other control measures to reduce workplace exposures. A major success occurred when 100 percent of the asphalt paving industry voluntarily agreed to install new controls on all new highway pavers produced after July 1997— effectively reducing asphalt fume exposure. Other aspects of the partnership have encouraged collaborative laboratory and field research and the development of communication materials for workers and contractors on methods for reducing workplace exposures. Representatives of industry, labor, government, and academia met in Cincinnati, OH, on September 11 and 12, 2000, and identified research needed to assess completely the health risks associated with exposure to asphalt. Through these and other efforts of the partnership, effective workplace measures can be implemented to reduce worker exposure to asphalt fumes.

Linda Rosenstock, M.D., M.P.H.
Director, National Institute for Occupational Safety and Health
Centers for Disease Control and Prevention

Executive Summary

In 1977, the National Institute for Occupational Safety and Health (NIOSH) reviewed the available data on the health effects of occupational exposure to asphalt and asphalt fumes. NIOSH determined the principal adverse health effects to be irritation of the serous membranes of the conjunctivae and mucous membranes of the respiratory tract. NIOSH also acknowledged that evidence from animal studies indicated that asphalt left on the skin for long periods of time could result in local carcinomas but that no comparable reports of these effects existed for humans. On the basis of this evidence, NIOSH recommended an exposure limit (REL) for asphalt fumes of 5 milligrams per cubic meter of air (5 mg/m3) measured as total particulates during any 15-minute period. In testimony to the Department of Labor in 1988, NIOSH recommended that asphalt fumes also be considered a potential occupational carcinogen. Since then, additional data have become available from studies of animals and humans exposed to asphalt, paving and roofing asphalt fume condensates, and asphalt-based paints. This document evaluates the health effects data that have become available since publication of the 1977 NIOSH criteria document; it also assesses exposures associated with occupations that involve the use of roofing and paving asphalts and asphalt-based paints.

Asphalt is a dark brown to black, cement like semisolid or solid produced by the nondestructive distillation of crude oil during petroleum refining. The three major types of asphalt products are paving asphalts, roofing asphalts, and asphalt-based paints. Performance specifications—not chemical composition—direct the type of asphalt produced. Most of the asphalt produced in the United States is used in paving and roofing operations. Only about 1% is used for waterproofing, damp-proofing, insulation, paints, or other activities and products. 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 1,500 to 2,000 workers are exposed to asphalt fumes in approximately 100 roofing manufacturing plants.

The exact chemical composition of asphalt depends on the chemical complexity of the original crude petroleum and the manufacturing processes. The proportions of the chemicals that constitute asphalt (mainly aliphatic compounds, cyclic alkanes, aromatic hydrocarbons, and heterocyclic compounds containing nitrogen, oxygen, and sulfur atoms) can vary because of significant differences in crude petroleum from various oil fields and even from various locations within the same oil field. Further analysis of the chemical data indicates that paving and roofing asphalts are qualitatively and quantitatively different; therefore, the vapors and fumes from these asphalt products may also be different. Other factors that increase the variability of asphalt vapors and fumes include temperature and mixing during the manufacturing process, and temperature and extent of mixing during laboratory generation or field operations. Studies indicate that the composition of asphalt fumes generated in the laboratory may differ qualitatively and quantitatively from asphalt fumes generated during field operations. However, one study showed that it is possible to generate asphalt fumes in the laboratory that are representative of field fumes.

Data are limited regarding the presence of carcinogens in asphalt fumes generated at U.S. work sites. The occasional detection of benzo(a)pyrene, B(a)P, in asphalt fumes generated at work sites as well as the more frequent detection of B(a)P and other carcinogenic polycyclic aromatic compounds in laboratory-generated asphalt fumes indicate that under some conditions, known carcinogens are likely to be present. Moreover, asphalt fumes generated at high temperatures are probably more likely to generate carcinogenic polycyclic aromatic hydrocarbons (PAHs) than fumes generated at lower temperatures.

Studies of the acute toxic effects of asphalt fume exposures in workers have repeatedly reported irritant symptoms of the serous membranes of the conjunctivae (eye irritation) and the mucous membranes of the upper respiratory tract (nasal and throat irritation). These health effects are best described in asphalt road pavers and typically appear to be mild in severity and transient in nature. Similar symptoms were also reported in workers exposed to asphalt fumes during the manufacture of asphalt roofing shingles and fluorescent lights, the insulation of cables, and exposure to a malfunctioning light fixture in an office environment. Workers employed in five segments of the asphalt industry (hot-mix plants, terminals, roofing, paving, and roofing manufacturing) experienced mild transient symptoms of nasal and throat irritation, headache, and coughing. In addition to mucosal irritation, workers with differing occupational exposures to asphalt fumes (e.g., paving operations, insulation of cables, and manufacturing of fluorescent light fixtures) also reported skin irritation, pruritus, rashes, nausea, stomach pain, decreased appetite, headaches, and fatigue. Such nonspecific symptoms require further investigation to clarify and establish the nature of causal relationships with asphalt fume exposure.

Results from recent studies indicated that some workers involved in asphalt paving operations experienced lower respiratory tract symptoms (e.g., coughing, wheezing, and shortness of breath) and pulmonary function changes. Irritant symptoms were noted in workers involved in open-air paving operations whose average personal exposures were generally below 1.0 mg/m3 total particulates and 0.3 mg/m3 benzene-soluble particulates calculated as a full-shift time-weighted average (TWA). Although an exposure-response relationship has not yet been established in these studies, the identification of health effects related to higher mean personal exposures during underground asphalt paving* indicates that such a relationship may exist. Bronchitis that is possibly related to lower respiratory tract irritation has also been reported among asphalt workers and highway maintenance workers; however, the data are insufficient to conclude that the bronchitis was caused by occupational exposure to asphalt fumes.

A recent meta-analysis of epidemiologic studies of roofers indicates an excess of lung cancer among roofers, but it is uncertain whether this excess is related to asphalt and/or to carcinogens such as coal tar or asbestos. Data from studies in animals and in vitro assays indicate that laboratory-generated roofing asphalt fume condensates are genotoxic and produce skin tumors in mice. Known carcinogenic PAHs have been identified in roofing asphalt fumes.

In contrast to the studies of roofers, epidemiologic studies of pavers exposed to asphalt fumes have yielded contradictory results regarding lung cancer. Although some of the studies reported an elevated risk for lung cancer among pavers exposed to asphalt, design limitations of these studies precluded any strong conclusions. Confounders included smoking and coexposure to coal tar and other potential lung carcinogens (e.g., diesel exhaust, silica, and asbestos). Furthermore, a recently conducted meta-analysis of these studies failed to find overall evidence for a lung cancer risk among pavers exposed to asphalt. However, carcinogenic PAHs have been detected in asphalt paving fumes—although at lower concentrations than those found in fumes from roofing asphalt. No published data examine the carcinogenic potential of paving asphalt fumes or fume condensates in animals.

A few studies reported an association between cancer at sites other than the lungs (e.g., bladder, kidneys, brain, and liver) with occupations having potential exposure to asphalt. Since the interpretation of these findings is limited by the study designs and the lack of good exposure data and consistent findings, no association can be made at this time. Further confirmation is needed by studies with better control of confounding variables and better identification of asphalt exposures.

Conflicting results were obtained when raw roofing asphalts were applied dermally to mice. In one study, the raw roofing asphalt was weakly carcinogenic and caused malignant skin tumors in mice. In the other study, the raw roofing asphalt was not carcinogenic. Available data also indicate that several formulations of asphalt-based paints cause benign and malignant skin tumors in mice. However, these paints were not mutagenic in the Ames Salmonella mutagenicity assay, either with or without metabolic activation. Several other asphalt-based paints caused the formation of DNA adducts in the skin and lungs of treated mice and in fetal and adult human skin cultures.

Conclusions

In this hazard review, NIOSH has evaluated the scientific evidence concerning the potential health effects of occupational exposure to asphalt. On the basis of available data from studies in animals and humans, as well as in in vitro studies, NIOSH concludes the following about the acute health effects of asphalt exposure:

• The findings of this hazard review continue to support the assessment of the 1977 NIOSH criteria document on asphalt fumes, which associated exposure to asphalt fumes from roofing, paving, and other uses of asphalt with irritation of the eyes, nose, and throat. Furthermore, in studies conducted since the publication of the 1977 criteria document, these symptoms have also been noted among workers exposed to asphalt fumes at geometric mean concentrations generally below 1 mg/m3 total particulates and 0.3 mg/m3 benzene-soluble or carbon disulfide-soluble particulates, calculated as a full-shift TWA. Recent studies also report evidence of acute lower respiratory tract symptoms among workers exposed to asphalt fumes. These data are currently being further analyzed to assess the relationship between lower respiratory tract symptoms and asphalt fume exposure. The available data on chronic pulmonary effects (such as bronchitis) are insufficient to support an association with asphalt fume exposures.

In 1988, NIOSH recommended to OSHA that asphalt fumes be considered a potential occupational carcinogen based on the results of an animal study in which laboratory-generated roofing asphalt fume condensates induced malignant skin tumors in mice. Since then, investigators have described differences in chemical composition, physical characteristics, and biological activity between asphalt fumes collected in the field and those generated in the laboratory. The relevance of these differences in ascribing adverse health effects in humans is unknown. Data from studies in humans indicate that some workers exposed to asphalt fumes are at an elevated risk of lung cancer; however, it is uncertain whether this excess is related to asphalt and/or other carcinogens in the workplace. Although carcinogenic PAHs have been identified in asphalt fumes at various work sites, the measured concentrations and the frequency of their occurrence have been low.

Based on evaluation of these data, the following conclusions were drawn regarding the carcinogenicity of asphalt under several conditions of use:

• Data regarding the potential carcinogenicity of paving asphalt fumes in humans are limited. Only one study identified B(a)P in field fumes, but it was unclear whether paving asphalt fumes were the source of the B(a)P. Chrysene has been identified only in laboratory-generated paving asphalt fumes. The available data from studies in humans have not provided consistent evidence of carcinogenic effects in workers exposed to asphalt fumes during paving operations. No animal studies have examined the carcinogenic potential of either field- or laboratory-generated samples of paving asphalt fume condensates. Although genotoxicity assays (but no carcinogenicity assays) using laboratory-generated and field-generated (storage tank paving asphalt) fumes have been conducted, only the laboratory-generated fumes were genotoxic. Therefore, NIOSH concludes that the collective data currently available from studies on paving asphalt provide insufficient evidence for an association between lung cancer and exposure to asphalt fumes during paving. The available data, however, do not preclude a carcinogenic risk from asphalt fumes generated during paving operations.
• The results from epidemiologic studies indicate that roofers are at an increased risk of lung cancer, but it is uncertain whether this increase can be attributed to asphalt and/or to other exposures such as coal tar or asbestos. Data from experimental studies in animals and cultured mammalian cells indicate that laboratory-generated roofing asphalt fume condensates are genotoxic and cause skin tumors in mice when applied dermally. Furthermore, a known carcinogen, B(a)P, was detected in field-generated roofing fumes. The collective health and exposure data provide sufficient evidence for NIOSH to conclude that roofing asphalt fumes are a potential occupational carcinogen.
• The available data indicate that although not all asphalt-based paint formulations may exert genotoxicity, some are genotoxic and carcinogenic in animals. No published data examine the carcinogenic potential of asphalt-based paints in humans, but NIOSH concludes that asphalt-based paints are potential occupational carcinogens.

Current data are considered insufficient for quantifying the acute and chronic health risks of exposure to asphalt, asphalt-based paint, or asphalt fumes and vapors. However, data from at least two studies of acute effects are currently being evaluated to determine their usefulness in deriving an REL. Additional studies of workers exposed to asphalt fumes, vapors, and aerosols (e.g., during paving, roofing, and painting operations) are needed to better characterize exposures and to evaluate the risk of chronic disease, including lung cancer. Also required are experimental animal studies that use laboratory generation methods to produce fumes and vapors representative of asphalt roofing and paving operations. Until the results of these studies become available, NIOSH recommends minimizing possible acute or chronic health effects from exposure to asphalt, asphalt fumes and vapors, and asphalt-based paints by adhering to the current NIOSH REL of 5 mg/m3 during any 15min period and by implementing the following practices:

• Prevent dermal exposure.
• Keep the application temperature of heated asphalt as low as possible.
• Use engineering controls and good work practices at all work sites to minimize worker exposure to asphalt fumes and asphalt-based paint aerosols.
• Use appropriate respiratory protection (see Appendix C).

Contributors

Mary Ann Butler, Ph.D.
Douglas Sharpnack, Ph.D.
Gregory Burr, C.I.H.
John Snawder, Ph.D.
David Dankovic, Ph.D.
Leslie Stayner, Ph.D.
R. Alan Lunsford, Ph.D.
Marie Haring Sweeney, Ph.D.
Aubrey Miller, M.D.
Alexander Teass, Ph.D.
Mimi Nguyen, M.P.H.
Joann Wess, M.S.
Larry Olsen, Ph.D.
Ralph Zumwalde, M.S.

Acknowledgements

In addition to the other contributors, the following staff members of the National Institute for Occupational Safety and Health are gratefully acknowledged for their assistance in preparing this document: Heinz Ahlers, J.D.; Chris Ellison; Jerome Flesch; Dennis Lynch; Tom Reid, Ph.D.; Paul Schulte, Ph.D.; Richard Niemeier, Ph.D.; Leroy Mickelsen; Kenneth Wallingford, Ph.D.; Elizabeth Ward, Ph.D.; Vincent Castranova, M.D.; Tong-Man Ong, Ph.D.; Ainsley Weston, Ph.D.; and Frank Hearl, Ph.D.

Priscilla Wopat edited the document, Diane Felice oversaw production, Susan Kaelin and Jane Weber provided editorial support, Pat Ulakovic provided desktop publishing, JudyCurless provided word processing and production support, and Vanessa Becks and Ken Strunk contributed to the cover design.

We wish to thank the following individuals and organizations for their review of this document.

Selected Abbreviations

Glossary

Aggregate: Graduated fragments of hard, inert mineral material that are mixed with asphalt. Aggregate includes sand, gravel, crushed stone, and slag [Stein 1980].

Asphalt (CAS number 8052-42-4): The product of the nondestructive distillation of crude oil in petroleum refining; it is a dark brown to black cement-like semisolid or solid. Depending on the crude oil used as a feedstock, the distillation residuum may be further processed, typically by air blowing (sometimes with a catalyst) or solvent precipitation, to meet performance specifications for individual applications [AI 1990b]. It is a mixture of paraffinic and aromatic hydrocarbons and heterocyclic compounds containing sulfur, nitrogen, and oxygen [Sax and Lewis 1987].

Asphalt cement: Asphalt that is refined to meet specifications for paving, roofing, industrial, and special purposes [AI 1990b].

Asphalt, cutback: An asphalt liquefied by the addition of diluents (typically petroleum solvents). Cutback asphalts are used in both paving and roofing operations depending on whether a paving or roofing asphalt is liquefied [AI 1990b; Roberts et al. 1996; Speight 1992a].

Asphalt, emulsified: A mixture of two normally immiscible components (asphalt and water) and an emulsifying agent (usually soap, but may be starch, glue, gum, colloidal clay, or other materials with similar properties) that allows the asphalt and water to mix. Emulsified asphalts are either cationic (electro positively charged micelles containing asphalt molecules or anionic (electro-negatively charged micelles containing asphalt molecules) depending on the emulsifying agent. Emulsified asphalts are used for seal coats on asphalt pavements, built-up roofs, and for other waterproof coverings. Emulsified asphalts are also called asphalt emulsions [AI 1990b; Roberts et al. 1996; Speight 1992a; Stein 1980].

Asphalt fumes: The cloud of small particles created by condensation from the gaseous state after volatilization of asphalt [NIOSH 1977a].

Asphalt-based paints: A specialized cutback asphalt product that can contain small amounts of other materials such as lampblack, aluminum flakes, or mineral pigments. They are used as a protective coating in waterproofing operations and other similar applications [AI 1990b].

Asphalt, hot mix (HMA): Paving material that contains mineral aggregate coated and cemented together with asphalt cement [AI 1990b].

Asphalts, liquids: These are asphalts that are liquids at ambient temperatures. Liquid asphalts include cutback and emulsified asphalts [Roberts et al. 1996; Speight 1992a].

Asphalt, mastic: A mixture of asphalt and fine mineral material in such proportions that it may be poured hot into place and compacted by hand-troweling to a smooth surface [AI 1990b]. It is similar to hot-mix asphalt, but it is a finer aggregate.

Asphalt, oxidized (blown or air-refined) [CAS number 64742-93-4]: Asphalt 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, under sealing for Portland cement concrete pavements, hydraulic applications, membrane envelopes [AI 1990b], and the manufacture of paints [Speight 1992a].

Asphalt, roofing: Asphalt that is refined or processed to meet specifications for roofing. Asphalt, paving: Asphalt that is refined to meet specifications for paving. Bitumen: The term more commonly used in Europe to refer to asphalt.

Coal tar: A tar that contains polycyclic aromatic compounds and is produced by the destructive distillation of bituminous coal [Bingham et al. 1980]. Distillation of coal-tar produces a variety of compounds such as coal tar pitch, creosote, and other chemicals or oils [NIOSH 1977b]. It is used in roofing, roads, waterproofing, paints, pipe coatings, sealants, insulation, and pesticides [Sax and Lewis 1987].

Coal tar pitch (CTP): A black or dark brown cementitious solid that is obtained as a residue in the partial evaporation or fractional distillation of coal tar [Bingham et al. 1980]. CTP is used in coatings, paints, roads, roofing, coal briquettes, and sealants [Sax and Lewis 1987].

Coal tar pitch volatiles (CTPV): Volatile matter emitted into the air when coal tar, coal tar pitch, or their products are heated [NIOSH 1977b].

Fog coat: Light application of slow-setting asphalt emulsion diluted with water. Fog coats are used to renew old asphalt surfaces and seal small cracks and surface voids [Stein 1980].

International Agency for Research on Cancer (IARC) categorization of agents as to their carcinogenicity:

• Group 1—The agent is carcinogenic to humans.
• Group 2A—The agent is probably carcinogenic to humans.
• Group 2B—The agent is possibly carcinogenic to humans.
• Group 3—The agent is not classifiable as to its carcinogenicity to humans.
• Group 4—The agent is probably not carcinogenic to humans.

Penetration macadam: Roadway consisting of a liquid asphalt sprayed onto a coarse aggregate (usuallycrushed gravel, slag, or stone) of uniform size [Stein 1980].

Polycyclic aromatic compound (PAC): A class of chemical compounds that contains two or more fused benzenoid rings. This class of compounds includes polycyclic aromatic hydrocarbons (PAHs) and heterocyclic derivatives where one or more of the carbon atoms in the benzenoid rings have been replaced by a heteroatom of nitrogen (N-PAC), oxygen (O-PAC), or sulfur (SPAC) [Vo-Dinh 1989].

Polycyclic aromatic hydrocarbons (PAH): A class of chemical compounds that only contain carbon and hydrogen in two or more fused benzenoid rings [Vo-Dinh 1989].

Prime coat: Application of a viscous liquid asphalt by spraying onto an absorbent surface. It is used to prepare an untreated base for an asphalt overlay. The prime penetrates the base, filling voids, and hardens the top so that the asphalt overlay will bond [Stein 1980].

Seal coat: A liquid asphalt treatment used to waterproof and improve the texture of an asphalt wearing surface. Many seal coats are covered with an aggregate [Stein 1980].

Slurry seal: A mixture of a slow-setting emulsified asphalt, fine aggregate, and mineral filler with enough water added to form a slurry [Stein 1980].

Surface treatments: The addition of an asphaltic material to any road surface, with or without a covering of aggregate, that increases the thickness of the surface by less than 1 inch [Stein 1980].

Tack coat: A light application (usually by spraying) of a liquid asphalt cement to an existing pavement so that a bond can form with the new asphalt pavement [FAA 1991].

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