Animal experiments on biological effects of mineral fibres.

The papers presented in this session are summarized. Although asbestos fibres produce tumours in a number of animal species tested, rats appear to be the most susceptible, in terms of latent period and numbers of tumours produced. The deposition, translocation and clearance of different types of fibres in the lung have been investigated in a number of experiments, and it has been shown that many of them migrate more readily than was previously thought; their penetration into the gut was the object of further investigation. The syncarcinogenicity with asbestos of various substances, such as benzo[a]pyrene, N-nitrosodiethylamine, cigarette smoke or radiation, is described. Experiments on the different carcinogenicities of different fibres are summarized; although it is pointed out that there is much controversy in this area. A hypothesis is presented whereby the carcinogenic potency of a fibre is dependent on various size parameters, based on length, diameter and length:diameter ratio. On the basis of this hypothesis, the carcinogenic potency of short fibres may be weak, but many short fibres may induce a tumour as easily as a few long fibres. Finally, a plea is made for a far greater number of well-defined standard samples of asbestos and man-made mineral fibres than exists at present, since there are currently great difficulties in comparing and interpreting results.