Deposition, clearance, and translocation of chrysotile asbestos from peripheral and central regions of the rat lung.

We investigated the pulmonary deposition, clearance, and translocation of chrysotile asbestos in the context of our previously developed model of asbestosis in the rat. Adult male rats were exposed for 3 hr to an aerosol of chrysotile asbestos. Subgroups were sacrificed up to 29 days postexposure and the lungs of the animals fixed. Peripheral and central regions of the left lung were resected, digested, and analyzed for fiber content by scanning electron microscopy. Pulmonary deposition did not differ between peripheral and central regions. There was no evidence of translocation of fibers from central to peripheral regions. The average diameter of retained fibers decreased over time, consistent with longitudinal splitting. The average length of retained fibers increased over time, consistent with slower clearance of longer fibers. We employed a novel counting scheme to ensure accurate fiber number measurements, allowing the calculation of clearance rates for fibers 0.5 to greater than or equal to 16 microns in length. Fibers of length greater than or equal to 16 microns were cleared slowly, if at all. These findings could have important implications for the pathogenesis of asbestos-related pleural disease. Many fibers are deposited in the peripheral region, and the longest (greater than or equal to 16 microns) will persist there for extended periods.