A review of the literature on chronic inhalation studies in which rats were exposed to mineral fibres at known fibre number concentrations was undertaken to examine the specific roles of fibre length and composition on the incidences of both lung cancer and mesothelioma. For lung cancer, the percentage of lung tumours (y) could be described by a relation of the form y = a + bf + cf2, where f is the concentration of fibre numbers and a, b, and c are fitted constants. The correlation coefficients for the fitted curves were 0.76 for > 5 microns f/ml, 0.84 for > 10 microns f/ml, and 0.85 for > 20 microns f/ml. These seemed to be independent of fibre type. It has been shown that brief inhalation exposures to chrysotile fibre produces highly concentrated fibre deposits on bifurcations of alveolar ducts, and that many of these fibres are phagocytosed by the underlying type II epithelial cells within a few hours. Churg has shown that both chrysotile and amphibole fibres retained in the lungs of former miners and millers do not clear much with the years since last exposure. Thus, lung tumours may be caused by that small fraction of the inhaled fibres that are retained in the interstitium below small airway bifurcations where clearance processes are ineffective. By contrast, for mesothelioma, the (low) tumour yields seemed to be highly dependent upon fibre type. Combining the data from various studies by fibre type, the percentage of mesotheliomas was 0.6% for Zimbabwe (Rhodesian) chrysotile, 2.5% for the various amphiboles as a group, and 4.7% for Quebec (Canadian) chrysotile. This difference, together with the fact that Zimbabwe chrysotile has 2 to 3 orders of magnitude less than tremolite than Quebec chrysotile, provides support for the hypothesis that the mesotheliomas that have occurred among chrysotile miners and millers could be largely due to their exposures to tremolite fibres. The chrysotile fibres may be insufficiently biopersistent because if dissolution during translocation from their sites of deposition to sites where more durable fibres can influence the transformation or progression to mesothelioma.
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