Fibre content of lung in amphibole- and chrysotile-induced mesothelioma: implications for environmental exposure.

Using 9 pairs of exposure-period-matched shipyard and insulation workers (amphibole exposure) and chrysotile-industry workers (chrysotile exposure) with mesothelioma, and an additional 9 pairs of workers with asbestosis, we found that the chrysotile workers with mesothelioma had 400 times the median lung fibre burden of the shipyard and insulation workers with mesothelioma. Mesothelioma in the chrysotile workers was associated with a 3 times greater median fibre burden than asbestosis, whereas in the shipyard and insulation workers mesothelioma was associated with only 1/35 the median amphibole burden seen in cases of asbestosis. In the chrysotile workers, the tremolite:chrysotile ratio and the mean fibre sizes were the same for both mesothelioma and asbestosis cases. These data suggest that total fibre load is crucial to the induction of mesothelioma by chrysotile, and that this phenomenon requires, on average, as high a fibre burden as induction of asbestosis by chrysotile. By contrast, for amphibole exposure, mesothelioma appears at a much lower fibre burden than asbestosis. The fibre types appear to differ by at least two orders of magnitude in their potential for inducing mesothelioma. Estimates of risk from environmental exposure must take these differences into account.