Asbestos General Aspects of Exposure

Asbestos, a group of naturally occurring, fibrous silicate minerals, include the serpentine mineral, chrysotile, and five amphibole minerals (actinolite, amosite, anthophyllite, cro-cidolite, and tremolite)64 (Table 18.2). Chrysotile, anthophyl-lite, amosite, and crocidolite are the only forms that have been used commercially.65 However, tremolite can be a contaminant in chrysotile and talc deposits, and actinolite is a common contaminant in amosite deposits.2 Occupational exposure to chrysotile, amosite, anthophyllite, and mixtures containing crocidolite has resulted in lung carcinomas, and mesothelioma has been observed after occupational exposure to crocidolite, amosite, and, to a lesser extent, chrysotile asbestos.66-68 Due to asbestos-related cancers in occupational settings, consumption of asbestos in the United States has been declining since 1973.2

Asbestos is released into the environment from natural and man-made sources and has been detected in indoor and outdoor air, soil, drinking water, food, and medicines. Significant exposure to any type of asbestos increases the risk of lung cancer, mesothelioma, and nonmalignant lung and pleural disorders, including asbestosis, pleural plaques, pleural thickening, and pleural effusions. This conclusion is based on observations of these diseases in workers with cumulative exposures ranging from about 5 to 1,200 fibers-year/mL, which results from 40 years of occupational exposure to air concentrations of 0.125 to 30 fibers/mL. These conclusions are supported by results from animal and mechanistic studies.69

Evidence for asbestos carcinogenicity in humans comes from epidemiologic studies as well as from numerous clinical reports on workers exposed to asbestos in a variety of occupational settings.68,70 Most recently, tremolite asbestos exposure has been associated with an increased incidence of disease in vermiculite miners and millers from Libby, Montana. Vermiculite is a clay mineral used in concrete aggregate, fertilizer carriers, insulation, potting soil, and soil conditioners. The Libby mine opened in 1921 and at one point accounted for 80% of the world's vermiculite production. The Libby vermiculite deposit is unique in the sense that it contains an average amphibole asbestos content of 4% to 6%,71 including tremolite and actinolite.69 Miners, millers, and some residents of Libby were exposed to high levels of asbestos-containing dust and developed nonmalignant respi

TABLE 18.2. Types and physicochemical properties of asbestos fibers (defined as having at least a 3:1 length-to-diameter ratio).

Type

Composition

Morphology

Sources

Chrysotile

(Mg)6(OH)8Si4Oio (±Fe)

Curly

95% of asbestos usage historically

Crocidolite

Na2(Fe3+)2(Fe2+)3(OH)2Si8O22 (±Mg)

Straight/rodlike

Mined in Australia and South Africa

Amosite

Fe7(OH)2Si8O22 (± Mg, Mn)

Straight/rodlike

Mined in Australia and South Africa

Tremolite

Ca2Mgs(OH)2Si8O22 (± Fe)

Straight/rodlike

Contaminant of certain chrysotile deposits

Anthophyllite

(Mg, Fe)7(OH)2Si8O22

Straight/rodlike

Mined in northern Europe

Actinolite

Ca2Fes(OH)2Si8O22 (± Mg)

Straight/rodlike

Contaminant of certain chrysotile deposits

Source: Data from Guthrie,64 by permission of BookCrafters, Inc.; 1993.

Source: Data from Guthrie,64 by permission of BookCrafters, Inc.; 1993.

ratory diseases, lung cancer, and mesothelioma.69 A mortality review that compared asbestos-related deaths in Libby versus Montana versus the United States found that, for the 20-year period examined (1979-1998), mortality from asbesto-sis was approximately 40 times higher than the rest of Montana and 60 times higher than the rest of the U.S. population. Lung cancer mortality was 1.2 to 1.3 times higher than expected compared to Montana and the United States, and the mesothelioma mortality was also elevated.72 The Libby vermiculite mine closed in 1990, but its products are still on the market.71 In a recent medical testing program to identify and quantify asbestos-related radiographic abnormalities among persons exposed to vermiculite in Libby, cross-sectional interview and medical testing was conducted in 7,307 persons who have lived, worked, or played in Libby for at least 6 months before December 31, 1990. Of these, 6,668 participants received chest radiographs to assess the prevalence of pleural and interstitial abnormalities. The study showed 17.8% of pleural abnormalities and less than 1% interstitial abnormalities.

Tremolite asbestos, a hydrated calcium magnesium silicate [Ca2Mg5Si8O22(OH)2], is also a known contaminant of chrysotile and fibrous talc.73-75 The differences in cancer potential between chrysotile asbestos, a serpentine mineral, and amphibole asbestos have been debated extensively. Many studies show that chrysotile is cleared from the lungs more rapidly than amphiboles.76 Due to the clearance and dissolution of chrysotile, it has been suspected that contaminating tremolite might be responsible for mesotheliomas occurring in chrysotile miners and millers.75 It also has been suggested that processed chrysotile contains little or no tremolite.77 Amosite asbestos was the most common fiber type found in the lungs of asbestos-exposed mesothelioma patients.78 In a more recent study by Roggli et al.,75 312 cases of mesothelioma were analyzed for fiber types in lung parenchyma by scanning electron microscopy. Tremolite was identified in 53% of the cases and was increased in 26% of the cases. Fibrous talc was identified in 62% and correlated strongly with the tremolite content (P = 0.0001). Chrysotile was only identified in 10% of the cases, and amounts correlated with the proportion of tremolite. In 4.5% of the cases, noncommercial amphibole fibers (tremolite, actinolite, and/or anthophyllite) were the only fiber types found existing above background (control individual) levels. They concluded that tremolite in lung tissue samples from mesothelioma victims were derived from contamination of talc and chrysotile, and that tremolite accounted for a considerable fraction of the excess fiber burden in end-users of asbestos products.

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