Differences in pathogenic potential of fibre types

The results of human epidemiological studies and lung mineral content analyses demonstrate that amphiboles (crocidolite and amosite) are more strongly associated with mesothelioma than is chrysotile. Comparative analysis of fibre durability and chemical composition are helping to explain the greater toxicity of amphiboles. Of the thousands of asbestos-related mesotheliomas reported, virtually all can be directly attributed to exposure to amphiboles. In his widely cited 1988 review of evidence related to mesothelioma causation, Dr. Andrew Churg found that only 53 cases of chrysotile-related mesothelioma had ever been reported from the tens of thousands of workers studied. Of these, ten cases were observed in secondary industry workers for which there was a strong suspicion of amphibole contamination, and 41 cases have occurred in individuals exposed to chrysotile mine dust, which contained traces of the naturally occurring amphibole; tremolite (Churg, 1988). Other evidence of the extremely weak association between chrysotile exposure and mesothelioma has been revealed through the cohort study of some 11,000 Québec chrysotile miners born between 1891 and 1920. The last follow-up of this cohort found that only 37 mesothelioma deaths had been identified among 8,000 deaths from all causes (McDonald et al., 1993). No cases were detected in workers with less than two years of exposure. In addition, unlike crocidolite mining towns, there has been no indication of environmentally-related mesothelioma in chrysotile mining communities. Also in contrast to amphiboles, the risks to household members of chrysotile workers through non-occupational contact appear to be extremely low, as only 2 or 3 isolated cases allegedly related to this "second hand" exposure have been reported. According to Churg, the research data indicates that although chrysotile asbestos can produce mesothelioma in man, the total number of such cases is small and the required doses extremely large. Another important factor is that while in general, amphiboles have been shown to cause lung disease and cancer after short but intense exposures, chrysotile-related illness is associated with very high, long-term exposures only.