Asbestos is a general name given to a group of naturally occurring silicate minerals with a tendency to separate into fibers or fiber bundles. The fibers have high tensile strength, low heat transfer, chemical resistance, and heat resistance. These properties make asbestos useful for a number of industrial applications, including thermal insulations and fireproofing, friction materials such as automotive brake pads, and fiber reinforcement in cementitious materials…
Although asbestos is a versatile material with many commercial applications, it also a known human carcinogen. Epidemiological data consistently indicate an increased incidence of cancer in occupationally exposed individuals. Asbestos exposure occurs primarily through inhalation of fibers in asbestos dust. Animal inhalation studies show consistently similar findings for lung cancer and mesothelioma. Animal and epidemiological ingestion study data are insufficient to judge carcinogenicity due to ingestion. Asbestos regulation has been based on a linear dose-response relationship between exposure and adverse health effects (risk increases as total dose increases) and on the lack of a known exposure threshold below which no asbestos-related health effects have been observed. Much of the available epidemiological data cover occupational exposures, which are frequently higher than environmental exposures.
Since asbestos is a naturally-occurring mineral, however, there are areas of the United States in which geological deposits of asbestos minerals pose a potential environmental exposure risk. Asbestos also occurs as a contaminant in some commercially mined minerals, such as vermiculite. The most well-known case of exposure to naturally-occurring asbestos may be the case of Libby, Montana. Asbestos-contaminated vermiculite was mined in Libby from 1919 until the mine was closed in 1990. In response to local concerns and media coverage of the local population’s exposure to the asbestos-contaminated vermiculite, EPA sent an emergency response team to Libby in 1999 to collect air, soil, dust, and insulation samples from businesses and homes. Libby was added to EPA’s Superfund National Priorities List in 2002. Asbestos-related lung diseases have been observed in the Libby population. Exposure scenarios in this case include occupational exposures in the mining process, exposure of family members through “take-home” dust, environmental exposures due to ambient airborne asbestos concentrations, and exposure of residents due to vermiculite-containing insulations and soil conditioners used in and around their homes.
Although the Libby, Montana, situation may be the best known case of exposure to naturally occurring asbestos in the United States, there are other areas of the country in which asbestos deposits result in potential exposure. The presence of naturally occurring asbestos in exposed soils in El Dorado Hills, California, has been well documented by State and Federal agencies. In response to a citizen’s petition to evaluate asbestos-related health risks in the community, EPA contracted to conduct a multimedia assessment of the area in 2003 to evaluate the potential for inhalation exposure to naturally occurring asbestos in disturbed soils. That assessment concluded through activity-based sampling that airborne asbestos concentrations were elevated in the breathing zone for both children and adults when soils were disturbed (Ladd, 2005).
Unlike occupational asbestos exposures, which may be controlled with personal protective equipment and specialized work practices, exposure to naturally occurring asbestos in native soils is not easily controlled. Exposed individuals may not even realize they have been exposed during outdoor activities. While occupational exposures generally affect adults of working age, exposure to naturally occurring asbestos minerals may also affect children and the elderly. Adverse health effects resulting from exposure to asbestos have been anecdotally documented as far back as ancient Rome, where slaves weaving asbestos fibers into textile products became weakened due to breathing problems and suffered premature death. More recent awareness of escalating asbestos-related respiratory disorders in the 1960s and early 1970s led EPA to add asbestos in 1971 to the list of materials regulated by the National Emissions Standard for Hazardous Air Pollutants (NESHAP), and to promulgate regulation under the Asbestos Hazard Emergency Response Act (AHERA) in 1986 to address asbestos in schools. AHERA covers asbestos-containing materials inside school buildings and, therefore, works to protect a susceptible subpopulation (children).
While there is strong evidence of a causal link between inhalation of asbestos particles and the development of debilitating respiratory disease and cancers, the specific mechanisms by which asbestos minerals cause disease are still not fully understood. The roles that morphology, fiber length, chemistry, and solubility in biological fluids (biopersistence) play in asbestos toxicity are still an area of vigorous debate. As noted by Fubini and Fenoglio (2007), particle toxicology is a distinct study area. Particle toxicants, in which surface chemistry and surface topography play a significant role in interaction with living tissues, behave differently than molecular toxicants. A particle’s surface structure and surface chemistry are affected by factors such as the mechanical processes that generate the particle, weathering processes, and adsorption of chemical contaminants onto the particle surface. For this reason, two particles with the same general chemical composition may have different surface chemistry.
In the case of mineral particles, properties relevant to toxicity include fibrous morphology, surface features such as sharp edges or fracture faces, surface reactivity related to covalent and ionic bonds, the presence of surface contaminants, and biopersistence. Asbestos particles have some toxicity characteristics that are different from other mineral dusts. Although fibrous morphology plays a part in toxicity, not all mineral fibers are equally toxic. There is some evidence that carcinogenic potency varies with asbestos mineral type and the geographic area from which the asbestos originates (EPA IRIS). It is generally agreed that chrysotile asbestos is less toxic than the other regulated asbestos minerals in relationship to mesothelioma, a cancer of the lining of the lungs and abdominal cavity.