Interpretation of the carcinogenicity of amosite asbestos and ferroactinolite on the basis of retained fiber dose and characteristics in vivo.

Rats were exposed to amosite asbestos and ferroactinolite fibers by intrapleural inoculation and intratracheal instillation. The ferroactinolite sample was found to be more carcinogenic in both exposures than the amosite sample on the basis of total fiber dose or fiber dose expressed for any size category of hypothetical greatest carcinogenic potency. Quantitative transmission electron microscope analysis of low-temperature ashed whole lung samples collected at different times following intratracheal instillation of fibers demonstrated that concentrations and sizes of fibers retained in rat lungs were greatly influenced by the relative ability of each mineral to undergo longitudinal splitting as a consequence of dissolution in vivo. Ferroactinolite fibers rapidly split to produce many thin fibers so that the number of ferroactinolite fibers retained in the lung 2 years after intratracheal instillation was four times greater than the number of fibers originally instilled. The number of short, thin ferroactinolite fibers retained (10-fold more than amosite) after in vivo splitting best explains the greater lung carcinogenicity of ferroactinolite compared to amosite.