2005/08/12 | 石棉[Asbestos]
类别(℃〖化学〗) | 评论(0) | 阅读(35) | 发表于 20:35
Natural asbestos is found in two varieties: serpentine asbestos and amphibole asbestos. Approximately 90% of serpentine is the variety chrysotile, while amphibole asbestos includes crocidolite, amosite, anthophyllite asbestos, actinole asbestos, and tremolite asbestos.

Asbestos has been observed to cause four health disorders. Asbestosis results in stiffening of the lung, and has resulted in the deaths of many miners. Lung cancer has a higher incidence in miners who also smoke, with the chance of developing cancer roughly proportional to the amount smoked. Asbestos-induced cancer is found only rarely in nonsmokers. Among the various type of asbestos, chrysotile workers have the lowest incidence of cancer. Mesthelioma involves the development of a fatal tumor. The time between diagnosis and original exposure is commonly 30 years or more. Family members of miners are also at risk. Among the general population, 70-80% of all mesothelioma cases are caused by exposure to asbestos. A staggering 18% of all mortalities in crocidolite workers are the result of mesothelioma. Benign pleural changes also occur to an extent proportional to exposure, but rarely cause functional impairment.

Judging from this data, there appears to be a difference in the detrimental effects of asbestos depending on the variety. In fact, "chrysotile-exposed workers have shown an appreciable lung burden of amphibole fibers. In contrast, chrysotile has been found post-mortem in smaller amounts than expected in the lungs of asbestos workers" (Mossman 1990, p. 296).

"Data suggest that amphiboles are the major cause of mesotheliomas in asbestos workers." On the basis of medical studies, "amphiboles are more potent than chrysotile in the induction of fibrotic lung disease and associated lung cancer." The reason for the difference in virulence is that "rod-like amphiboles appear to penetrate the peripheral lung more readily than chrysotile fibers, which are curly" (Mossman 1990, pp. 294-295). Furthermore, according to the Stanton hypothesis, "fibers longer than 8 m and less than 0.25 m in diameter have the most marked carcinogenic potential." This is presumably a result of the differing shapes, which means that "chrysotile fibers, in comparison to amphibole fibers, are cleared more readily from human lungs."

"The available experimental and epidemiological data indicate that both fiber types are important determinants of the pathogenicity of asbestos." "Recent epidemiological data are concordant with the suggestion that exposure to chrysotile at current occupational standards does not increase the risk of asbestos-associated diseases." However, "federal policy in the United States does not differentiate between different types of asbestos." This makes little sense, since, "with few exceptions, the type of asbestos fiber found predominantly in buildings is chrysotile." (Mossman, p. 247). Furthermore, removal will cost $53 billion, but estimates range up to $100-150 billion (Mossman 1990, p. 94).

"Recent epidemiological studies of deaths from mesothelioma in the general population also suggest that risk from asbestos in buildings is minuscule." "Risks of asbestos-related total deaths (both lung cancers and mesothelioma) due to exposure in schools are magnitudes lower than commonplace risks in modern day society," for example, 0.005-0.096 deaths/million compared to 6 deaths/million for airplane accidents. "The available data and comparative risk assessments indicate that chrysotile asbestos, the type of fiber found predominantly in U. S. schools and buildings, is not a health risk in the nonoccupational environment. Clearly, the asbestos panic in the U. S. must be curtailed" (Mossman 1990, p. 299).

"About 95% of the commercial asbestos now used in the United States is chrysotile." Chrysotile asbestos used in the U. S. is mined mainly from vast deposits in Quebec. Amosite in mined only in South Africa, crocidolite only in South Africa, western Australia, and Bolivia, and anthophyllite only in Finland. Russia is the world's largest chrysotile producer, followed by Canada and South Africa (Ross 1981, p. 281).

Ross comes to similar conclusions about the health risks associated with cancer. "Pleural cancer seems to be caused by crocidolite asbestos but not by chrysotile or anthophyllite asbestos. Lung cancer is caused by chrysotile, anthophyllite, amosite, and crocidolite asbestos in asbestos workers who smoke cigarettes. Evidence for excess lung cancer in nonsmoking asbestos workers is weak. Two completely different substances, asbestos and cigarette smoke, combine to produce a very significant risk to many asbestos workers, particularly those who are heavily exposed to asbestos dusts."

"Generally, asbestos related diseases appear in asbestos workers only after many years have elapsed since first exposure. A significant increase in the lung cancer death rate appears 10 to 14 years after first exposure and peaks at 30 to 35 years. The mesothelioma death rate becomes significant 20 years after the first exposure, but continues to climb even after 45 years have elapsed. The asbestosis death rate becomes significant 15 to 20 years after first exposure and apparently peaks at 40 to 45 years" (Ross 1981, p. 303).

"Increased risk of lung cancer due to asbestos exposure in non-smokers is very low. There appears to be no relationship between smoking habits and the incidence of mesothelioma" (Ross 1981, p. 306). "Analysis shows a positive correlation between lung cancer and mesothelioma mortality. Individuals who did not come into contact with crocidolite have very low mesothelioma mortality" (Ross 1981, p. 311). "There is no question that those exposed to heavy concentrations of chrysotile and anthophyllite dust over long periods of time have suffered a significant excess mortality due to lung cancer and asbestosis--but not to mesothelioma. For men exposed for over 20 years [to] low and medium dust [concentrations], there was a slight excess of risk for lung cancer" (Ross 1981, p. 313).

"Crocidolite asbestos is much more hazardous than chrysotile, anthophyllite, and amosite. Mesothelioma deaths have been reported among the residents of these areas who are not employed in the mines or mills" (Ross 1981, p. 314). However, "mesothelioma is very rare where amosite is mined" (Ross 1981, p. 315).

"Of the six forms of asbestos, only four have been used to any significant degree in commerce. These are amosite, crocidolite, anthophyllite, and chrysotile" (Ross 1981, p. 316). "Lung cancer can be caused by exposure to chrysotile, anthophyllite, amosite, and crocidolite asbestos; however, increased risk of this disease is probably found only in those who smoke cigarettes. Asbestosis is also caused by heavy and prolonged exposure to all four forms of asbestos. Mesothelioma is caused principally by exposure to crocidolite asbestos. There is good evidence that anthophyllite and chrysotile asbestos do not cause any significant increase in mesothelioma mortality, even after heavy exposure for many years" (Ross 1981, p. 317).

In conclusion, "chrysotile miners working a lifetime under present dust levels should not be expected to suffer any measurable excess cancer" (Ross 1981, p. 318).
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