Mineral fibre carcinogenesis: experimental data relating to the importance of fibre type, size, deposition, dissolution and migration.

Fibre type, fibre size, deposition, dissolution and migration are all factors of importance in mineral fibre carcinogenesis. These factors are, however, so interrelated that only fibre size can be considered on its own to any extent. When dusts are injected into the pleural or peritoneal cavities, the most carcinogenic samples, producing the most mesotheliomas, are those containing the most long, thin fibres. When very short fibre samples of both amosite and chrysotile recently became available for comparison with long fibre preparations of the same materials, short fibres were found to be much less fibrogenic and carcinogenic than long fibres. The same studies provided important information on fibre deposition and dissolution. Short fibre samples of both asbestos varieties penetrated to the pulmonary parenchyma more easily than long ones but, after deposition, short fibres were cleared more quickly. Very much less chrysotile was present in lung tissue at the end of one year’s dusting and clearance during the following 6 months was very much faster. The long fibre chrysotile, which would be expected to be resistant to mechanical clearance, was removed from the lungs much more quickly than short fibre amosite, which was easily phagocytosed by macrophages. This indicates that rapid chrysotile removal from lung tissue is due at least in part to fibre dissolution. The phenomenon of chrysotile dissolution probably explains why this asbestos type has been shown to be extremely carcinogenic in rats but seems less carcinogenic than the amphiboles in humans. Fibres may remain in lung tissue for the 1-2 years necessary to cause tumours in rats but this is too short a time for the much longer lived humans. Only very few fibres penetrate the walls of the gut following massive asbestos ingestion, although a few of these can subsequently be found disseminated to other organs. Fibres are disseminated to other organs much more effectively after inhalation. One area where fibre dissemination has been suggested as being very important is that of transport from the lung tissue to the pleural cavity, but in rats, direct fibre penetration to the pleura occurs very rarely and the exact mechanism by which inhaled fibres reach the sites where they can produce mesotheliomas remains one of the most important subjects for future research.