Chrysotile, tremolite and mesothelioma

Although it has been demonstrated that there is a very weak association between chrysotile exposure and mesothelioma, the presence of occasional fibrous tremolite, an amphibole mineral, in some chrysotile ore body has been cited as a potential risk factor amongst chrysotile workers. The available evidence, however, shows that mesotheliomas in chrysotile mining populations are extremely rare relative to rates in amphibole-exposed populations. In fact, less than 40 mesothelioma cases over several decades have been reported amongst chrysotile miners and millers (McDonald et al. 1993). In their analysis of the implications of the 37 mesothelioma cases identified up until 1992 in the 11,000 person cohort, McDonald & McDonald (1995) found that they were concentrated in workers from specific areas of the mines. Further post-mortem lung tissue analysis showed that workers in these areas had tremolite lung content four times higher than those workers in other areas of the mines studied, suggesting that the rare cases of mesothelioma among chrysotile miners are mainly, if not wholly, due to tremolite exposure. The authors note that it should be kept in mind that these mesothelioma cases occurred as a result of long, heavy exposures 20 to 70 years ago. They conclude: "The geological distribution of tremolite within the Québec chrysotile ore body may well vary in time and place and, at present levels of environmental controls, any mesothelioma risk from exposure (...) would be far below the limits of epidemiological detection." The previous review by Dr. Andrew Churg, a pathologist at the University of British Colombia in Canada, supports this conclusion. Churg (1988) writes, "whether tremolite or chrysotile be the critical agent, these observations suggest that chrysotile ore, in both crude and processed forms, does cause mesothelioma in man, but that it is an extremely weak carcinogen and that in today's terms, the doses required are extremely high. As a practical matter, the data indicate that chrysotile will not produce mesotheliomas in those exposed to any current or recently regulated number of fibers..."