In the 1960s and 1970s, well designed case-referent studies put beyond doubt
that exposure to airborne asbestos fibres was a cause of malignant
mesothelioma. Some 35 cohort mortality studies in a large variety of industries
during the 20-year period, 19744, showed a wide range of outcomes, but in
general that the risk was higher in exposures which included amphiboles rather
than chrysotile alone. Real progress began, however, with discoveries along
several lines: the link between pleural changes and mineralogy, the concept and
importance of biopersistence, the developments in counting and typing mineral
fibres in lung tissue, and data on amphibole mining in South Africa and
Australia for comparison with that on chrysotile in Canada and Italy. This led
to the recognition of the potential contamination in North America of
chrysotile with tremolite. A survey in Canada in 19808 and other surveys
demonstrated that crocidolite, amosite, and tremolite could explain almost all
cases of mesothelioma. Effective confirmation of this was finally achieved with
data on vermiculite miners in Libby, Montana, in the years 19839, where
exposure was to tremoliteactinolite and/or other amphibole fibres alone.

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AplusA-online.de - Source: Annals of Occupational Hygiene