Asbestos

CAS RN:1332-21-4

Toxicity Summary

IDENTIFICATION AND USE: Asbestos minerals are divided into two major groups: Serpentine asbestos and amphibole asbestos. Serpentine asbestos includes the mineral chrysotile, which has long, curly fibers that can be woven. Chrysotile asbestos is the form that has been used most widely in commercial applications. Amphibole asbestos includes the minerals actinolite, tremolite, anthophyllite, crocidolite, and amosite. Amphibole asbestos has straight, needle-like fibers that are more brittle than those of serpentine asbestos and are more limited in their ability to be fabricated. Domestically used asbestos fibers are classified into seven quality categories or grades. Grades 1, 2, and 3 include the longer, maximum-strength fibers and generally are used in the production of textiles, electrical insulation, and pharmaceutical and beverage filters. Grades 4, 5, and 6 are medium-length fibers used in the production of asbestos cement pipes and sheets, clutch facings, brake linings, asbestos paper, packaging, gaskets, and pipe coverings. Grade 7 includes short fibers generally used as reinforcers in plastics, floor tiles, coatings and compounds, some papers, and roofing felts. By the 1990s, chrysotile accounted for more than 99% of U.S. asbestos consumption. By 2008, chrysotile was the only type of asbestos used in the United States; 64% of chrysotile used was categorized as grade 7 asbestos (with fiber lengths less than 3 mm), followed by grades 4, 5, and 3. HUMAN STUDIES: Asbestos causes three forms of lung disease: asbestosis, lung cancer, and malignant mesothelioma. Asbestosis is a form of pulmonary fibrosis with characteristically diffuse collagen foci and the presence of asbestos fibers, either free or coated with proteinaceous material (asbestos bodies). Lung cancer develops in workers in the asbestos mining industry and smoking of cigarettes greatly enhances risk. Malignant mesothelioma is a rare form of cancer that develops mainly in the pleural mesothelium, the protective lining that covers the lungs, diaphragm, and interior of the chest wall. Unlike lung cancer, mesothelioma is not associated with smoking history. The data reported over the last several decades consistently support the conclusions that exposure to fibers longer than 10 um and perhaps 20 um are required to significantly increase the risk of developing asbestos-related disease in humans and that there is very little, if any, risk associated with exposure to fibers shorter than 5 um. Studies of exposed asbestos workers, residentially exposed Turkish villagers, mesothelioma patients, and lung cancer patients suggest that asbestos is genotoxic. The number of chromosomal aberrations and the rate of sister chromatid exchange were significantly elevated in the peripheral blood lymphocytes of asbestos workers compared to a control population. The mean sister chromatid exchange rate was significantly increased in nonsmoking asbestos insulators compared to a control population. An inherited predisposition has been suggested to explain multiple cases of malignant pleural mesothelioma in the same family and the observation that not all individuals highly exposed to asbestos develop the tumor. Germline mutations in BAP1 are responsible for a rare cancer predisposition syndrome that includes predisposition to mesothelioma. Cancers of ovary, and possibly other organs have been described as well. ANIMAL STUDIES: In early experiments, it was demonstrated that guinea pigs and monkeys exposed by inhalation to 4 commercial types of asbestos developed fibrotic lesions affecting lung and pleura. In more recent experiments, this finding has been confirmed in rats and hamsters. All commercial forms of asbestos tested are carcinogenic in mice, rats, hamsters and rabbits. The size and shape of fibers influence the incidence of tumors; fibers <0.5 um in diameter are more active in producing tumors. Pregnant mice were given 4, 40 or 400 mg asbestos/kg bw (1.43, 14.3 or 143 mg asbestos/mL) in their drinking water during days 1-15 of gestation. Water consumption did not vary between the different dosage groups. There was also no difference in embryo survival between the treatment groups and the controls, which received only tap water. There were no signs of maternal toxicity. Chrysotile, amosite, and anthophyllite showed no mutagenic activity toward tester strains of Escherichia coli or Salmonella typhimurium. A large number of studies indicate that asbestos fibers can cause chromosomal aberrations in Chinese hamster ovary (CHO) and Syrian hamster embryo (SHE) cells. The aberrations include aneuploidy (usually polyploidy), fragmentation, breaks, rearrangements, gaps, dicentrics, inversions, and rings. Asbestos causes DNA damage and apoptosis, some of the implicated mechanisms include the generation of iron-derived reactive oxygen species (ROS) as well as reactive nitrogen species (RNS), alteration in the mitochondrial function, and activation of the death receptor pathway.
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