Asbestos-related disease in the workplace and the environment: controversial issues

Asbestos-related diseases continue to be sources of controversy for epidemiologist, clinician, and pathologist. Most investigators agree that the different fiber types behave differently in the lung, with chrysotile being rapidly removed, and amphibole persisting. These differences in biologic behavior probably account for the much greater disease potential of amphibole (amosite and crocidolite) compared with chrysotile asbestos, particularly in regard to mesothelioma induction in man. Asbestosis is defined as diffuse interstitial fibrosis of the lung caused by asbestos exposure, and this is the only condition to which the term asbestosis should be applied. The classical pathologic diagnostic criteria for asbestosis, namely the presence of diffuse interstitial fibrosis resembling usual interstitial pneumonia, and asbestos bodies visible in ordinary tissue sections, have proved to withstand the test of time. Cases without asbestos bodies visible in routine or iron-stained tissue sections almost never turn out to be asbestosis. It should be remembered that workers with asbestos exposure develop all of the interstitial lung diseases to which the remainder of the populace is subject; some of these conditions are treatable and should not be misdiagnosed as asbestosis, which is not treatable. There is strong epidemiologic and pathologic evidence that the only association of asbestos exposure and lung cancer is the association of asbestosis and lung cancer. Thus, a lung cancer should only be attributed to asbestos exposure when asbestosis is present on clinical or pathologic grounds. The histologic type and location of the tumor are irrelevant in this regard. Analytical electron microscopy indicates that chrysotile asbestos does induce mesothelioma in man, but that extremely high levels of retained fibers, levels as high as those seen in cases of asbestosis, are required for this event to occur. The weight of the evidence suggests that exposure of the general population to the very low levels of chrysotile that are found in some public building (levels not greatly different from ambient air) will never produce mesothelioma, asbestosis, or lung cancer because these diseases all appear to require quite high-level occupational chrysotile exposure. Even if one accepts the ideas (probably wrong) that any level of asbestos exposure carries a risk of cancer, and that mathematical extrapolation of risk from high-level occupational exposure to low-level building exposure is scientifically valid, the calculated risks are much smaller than real everyday risks such as driving to work. Thus, exposure to asbestos at environmental levels appears to produce no real dangers to health.