The effect of fibre size on the in vivo activity of UICC crocidolite.

Standard (UICC) crocidolite was subjected to ball milling to reduce the length of the fibre. These milled materials and the original standard sample were injected into the pleural cavity of rats to determine their ability to induce mesothelioma. Previous in vitro work on the same materials had suggested that biological activity was related to fibres greater than 6.5 microns in length and that the material milled for 4 and 8 h did not contain fibres in this range and was biologically inactive. The results of the animal work, however, did not follow this pattern; mesotheliomas occurred in rats in all treatment groups including the 4 and 8 h milled samples. Examination of the tissues and the dust recovered from them showed the presence of fibres greater than the suggested threshold. Attention is drawn to the problems associated with drawing conclusions from size distributions and in vitro studies without considering in vivo mechanisms.