Chest
Volume 93, Issue 3, March 1988, Pages 621-628
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Review
Chrysotile, Tremolite, and Malignant Mesothelioma in Man

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The question of whether chrysotile asbestos ever causes mesothelioma in man has become a major public and occupational health issue. Review of the literature suggests that only 53 acceptable cases of chrysotile-induced mesothelioma have ever been reported; of these, 41 cases have occurred in individuals exposed to chrysotile mine dust, all of it naturally contaminated with tremolite. Ten cases have occurred in secondary industry workers, but here the suspicion of amosite or crocidolite contamination is high. Analysis of lung asbestos content indicates that induction of mesothelioma by chrysotile requires, on average, as great a lung fiber burden as induction of asbestosis by chrysotile, whereas amphibole (amosite or crocidolite)-induced mesotheliomas appear at a several hundred-fold smaller lung burden. Tremolite alone has definitely produced mesothelioma in man, particularly when exposure has been to long, high aspect ratio, fibers. Analysis of tremolite:chrysotile fiber ratios in human lung suggests that some, but not all tremolite is removed in milling chrysotile ores. The low incidence of mesothelioma in secondary chrysotile users may reflect the small amount of tremolite left in the product. These observations indicate that although chrysotile asbestos can produce mesothelioma in man, the total number of such cases is small and the required doses extremely large. The data are consistent with the idea that mesotheliomas seen in chrysotile miners and some secondary industry workers are produced by the tremolite contained in the chrysotile ore, but that the short length and low aspect ratio of the tremolite make its carcinogenicity quite low. However, these data are very indirect, and a role for the chrysotile fiber itself is still possible.

Section snippets

THE EXISTENCE OF CHRYSOTILE-INDUCED MESOTHELIOMA IN MAN

Table 1 presents a survey of the literature reporting chrysotile-induced mesotheliomas. There are a total of 142 mesotheliomas in this whole set of reports, of which, for reasons to be considered shortly, at most 53 can be accepted as chrysotile-induced tumors. The table is grouped in sections: reports of mesothelioma occurring in persons exposed either directly or indirectly to chrysotile mine dust; reports of secondary application workers in largely chrysotile-using factories where some

MESOTHELIOMAS ASSOCIATED WITH TREMOLITE EXPOSURE IN MAN

Cases of mesothelioma induced by tremolite not associated with chrysotile in man are rare (Table 2). McDonald et al29 reported a group of workers mining tremolite-contaminated vermiculite in which four mesothelomas (2.4 percent of deaths) were found. Tremolite has been found as an environmental contaminant and in the lungs of persons in a local area in Greece in which the death rate from mesothelioma is reported to be 1 percent.30,31 Brown et al32 described a mesothelioma in a miner of New York

AMOUNT OF EXPOSURE REQUIRED FOR INDUCTION OF MESOTHELIOMA BY CHRYSOTILE

The small number of chrysotile-associated mesotheliomas, as well as the possibility of amosite and crocidolite contamination, make any estimate of dose extremely difficult. At the low end of the scale, however, it is worth emphasizing that there is no evidence of an increased mesothelioma risk in long-term residents of the Quebec chrysotile mining towns, despite the fact that these persons are exposed for a lifetime to several hundred times the ambient chrysotile levels found in most North

THE ROLE OF TREMOLITE VS CHRYSOTILE IN MESOTHELIOMA INDUCTION

It has been suggested that the fiber which causes mesothelioma in those exposed to chrysotile is the tremolite contaminant rather than the chrysotile itself.7,8 Given that amosite and crocidolite are powerful mesothelial carcinogens in man, and, as shown in Table 2, that long fiber tremolite in the absence of chrysotile also produces a high yield of deaths or tumors, this idea is appealing, but direct proof is difficult.

If tremolite is the agent of mesothelioma, disease incidence should be

CONCLUSIONS

Despite great interest in the topic, and despite the current furor in the United States over the dangers of mesothelioma from exposure to chrysotile asbestos at very low levels in buildings, an extensive search of the literature reveals only 53 possible cases of chrysotile induced mesothelioma, of which 41 occurred in individuals exposed directly to chrysotile mine dust. For those exposed to chrysotile products, the risk appears to be much lower, and here it is likely that some of the reported

AUTHOR'S NOTE

In compiling this manuscript, I have unintentionally left out the study of Yazicioglu et al. Pleural calcifications, pleural mesotheliomas, and bronchial cancers caused by tremolite dust Thorax 1980; 35:564-69. These authors report what appears to be a high incidence of mesothelioma in persons living in a village where a tremolite containing whitewash and stucco is made from outcrops of local rock. Their illustrations suggest that the tremolite is a fairly long, high aspect ratio, fiber.

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    Supported by grants from the National Cancer Institute of Canada ana the Medical Research Council of Canada.

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