Pulmonary effects in rats inhaling size-separated chrysotile asbestos fibres or p-aramid fibrils: differences in cellular proliferative responses.

This study was designed to compare the pulmonary cellular proliferative effects of inhaled, size-separated preparations of chrysotile asbestos fibres with similar aerosol fibre concentrations of para-aramid fibrils. Following fibre preparation, rats were exposed for 2 weeks to aerosols of p-aramid fibrils or chrysotile asbestos fibres at design fibre concentrations of 750 and 400 f/cc. Two week exposures to p-aramid fibrils produced transient pulmonary inflammatory and cell labeling responses in terminal bronchiolar and subpleural regions. Similar to p-aramid, exposure to chrysotile produced a transient increase in neutrophils. In contrast, however, substantial increases compared to controls in pulmonary cell labeling indices were measured on terminal bronchiolar, parenchymal, subpleural, and mesothelial surfaces immediately after exposure, and some increases persisted for 3 months postexposure. In complementary studies we demonstrated that p-aramid is biodegradable in the lungs of exposed rats; in contrast, the clearance of long chrysotile fibres was slow or insignificant, resulting in a pulmonary retention of long chrysotile asbestos fibres. The dimensional changes of asbestos fibres as well as the pulmonary cell labeling data indicate that chrysotile asbestos fibres may produce greater long-term pulmonary effects when compared to inhaled para-aramid fibrils.