Mesotheliomas and chrysotile

Mesotheliomas are rare. While most are reported to be associated with exposure to durable fibres, a proportion are not caused by the inhalation of fibres at all. Reports of individual cases and studies of small groups are unreliable as evidence of cause because: (a) diagnosis is often unreliable; (b) even if chrysotile fibres are found in lung tissues, the mesothelioma may still be spontaneous. Present knowledge has not progressed much since 1964 when the absence of cases of mesothelioma in amosite and chrysotile miners in South Africa and the very low incidence in Canadian mines was known but not believed. Now we know this information was correct. The significance of low-level amphibole exposures in predominantly chrysotile mixes was not appreciated until studies of fibres in lungs using electron microscopy showed the high lung burden of amphibole fibres in the 1970s and 1980s. The effect of these findings is to make most European and U.S.A. factory cohorts inappropriate for the evaluation of chrysotile mesothelioma risk. Reviewing the current evidence published and unpublished, it seems likely that chrysotile uncontaminated by tremolite may not have caused any mesotheliomas even at high cumulative life-time exposures. Information on the mesothelioma risk among chrysotile user populations using fibres not containing tremolite is badly needed.