Asbestos-Related Disease

William G. Hughson

The term asbestos refers to a group of naturally occurring, fibrous hydrated silicates that share characteristics of heat and chemical resistance. Chrysotile, or serpentine asbestos, is characterized by curvilinear fibrils. The other types have straight fibers, and are referred to as amphibole asbestos. The most important amphiboles are amosite and crocidolite. Asbestos has been used for multiple purposes, including textiles, thermal insulation, building materials, and friction products such as brake linings.

The risk of asbestos-related disease is related to the amount of exposure, described using the term fiber/cc-year, which is the amount of asbestos that would be inhaled by working for a year in an atmosphere where the time-weighted average level of asbestos was 1 fiber/cc of air. The year in this term is a working year (8 hours/day for 250 days), and only fibers longer than 5 μm are counted. Historically, exposure levels were very high. For example, insulators working in shipyards and commercial construction were exposed to approximately 10 fibers/cc. Exposures for chrysotile miners and textile workers were in the range of 10 to 100 fibers/cc. In contrast, automobile mechanics doing brake repair had an average exposure of 0.1 fibers/cc. The threshold limit value for asbestos was 5 million particles per cubic foot (∼15 fibers/cc) until 1968. Since then, it has been progressively lowered to the current permissible exposure limit of 0.1 fiber/cc.

The risk of asbestos-related disease is also related to fiber type. Chrysotile asbestos is cleared rapidly from the lungs. In contrast, amphibole asbestos is retained to a much greater extent. As a result, the risk of disease in chrysotile-exposed populations is lower on a fiber/cc-year basis. For example, the chrysotile miners and millers of Quebec had little evidence of increased mortality up to a cumulative exposure of approximately 1,000 fiber/cc-years. In contrast, the risk of mesothelioma from crocidolite or amosite is increased by exposures in the range of 2 to 5 fiber/cc-years. Approximately 90% of the asbestos used in North America was chrysotile. Amosite was used primarily in insulation products, and crocidolite was used mainly for asbestos cement. In Europe and Australia, amphibole use was more common. Those areas have experienced a much higher rate of asbestos-related malignancies.

Although the high exposure levels of the 1900s are a thing of the past, the widespread use of asbestos-containing construction materials has led to concern that occupants of public buildings and schools may be harmed. Asbestos fibers are present in the air of many buildings, typically in the range of 0.001 to 0.0001 fibers/cc. Most are chrysotile, and there is no evidence that the general public is at increased risk from these environmental or background levels of exposure. There have been reports of family members developing asbestos-related diseases caused by exposure to fibers brought home on the clothes of asbestos workers. Most of those cases involved amphibole asbestos, and lung tissue analysis has demonstrated that the family members had lung tissue concentrations similar to people with occupational exposures.

Regulatory agencies such as the Occupational Safety and Health Agency and Environmental Protection Agency (EPA) use a linear no-threshold model for risk assessment. An inevitable consequence of this model is that any level of asbestos exposure, no matter how small, can be calculated to cause some degree of risk. However, there is ample evidence that low levels of exposure, particularly to chrysotile asbestos, do not cause an increased risk of asbestos-related disease. For example, residents in the Quebec chrysotile mining townships were exposed to ambient levels between 0.1 and 3 fibers/cc prior to 1970. The estimated lifetime exposure at these levels would be approximately 25 fiber/cc-years. There is no evidence of increased lung cancer risk in the townships, and almost all the reported cases of mesothelioma had either occupational exposure in the mining industry, or lived with family members who were employed in the industry. The EPA’s model significantly overestimated the risk of lung cancer and mesothelioma in the Quebec mining townships.

The term asbestosis refers to parenchymal fibrosis caused by asbestos, and is characterized by the presence of interstitial fibrosis and an increased number of asbestos ferruginous bodies and uncoated asbestos fibers in lung tissue. The ferruginous body is an asbestos fiber coated with proteinaceous iron-staining material; it is visible on light microscopy. Using lung digestion techniques, uncoated fibers can be identified and counted. Patients with asbestosis typically have concentrations of ferruginous bodies and uncoated fibers that are 100 to 1,000 times higher than the general population. Asbestosis usually begins subpleurally in the lung bases. As the disease progresses, it can involve both lungs diffusely as a fine fibrosis. In the final stages, the lungs can acquire a cystic honeycomb appearance and can be indistinguishable radiographically from other forms of severe interstitial fibrosis.

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