LUNG PROLIFERATIVE AND CLEARANCE RESPONSES TO INHALED PARA-ARAMID RFPIN EXPOSED HAMSTERS AND RATS - COMPARISONS WITH CHRYSOTILE ASBESTOS FIBERS

This study compared pulmonary effects of para-aramid respirable-sized, fiber-shaped particles (RFP) (p-aramid fibrils) and chrysotile asbest os fiber exposures in rats. Additional p-aramid inhalation studies wer e conducted in hamsters to compare species responses. The hamster resu lts are preliminary. The parameters studied were clearance/biopersiste nce of inhaled p-aramid RFP or size-separated asbestos fibers as well as pulmonary cell proliferation and inflammation indices after 2-week inhalation exposures. Rats were exposed nose only to chrysotile asbest os fibers at concentrations of 459 and 782 fibers/ml or to p-aramid RF P at 419 or 772 fibrils/ml. Hamsters were exposed whole body to p-aram id RFP at concentrations of 358 and 659 fibrils/ml. Subsequently, anim als were assessed immediately (time 0) as well as 5 days (10 days for hamsters), 1, 3, 6, and 12 months postexposure. Lung burdens for the p aramid-exposed rats were 4.8x10(7) and 7.6x10(7) fibrils/lung, with si milar numbers of chrysotile fibers >5 mu m recovered from the lungs of asbestos-exposed rats. in comparison, (sic).4x10 fibrils/lung were re covered in the high-dose hamster group. Biopersistence studies in para mid-exposed rats and hamsters demonstrated an initial increase (relati ve to time 0) in retained p-aramid fibrils during the first month post exposure, which indicated breakage or shortening of inhaled fibrils. T his result was associated with a progressive reduction, and increased residence time in the lung, in the mean lengths of the fibrils, which signified biodegradability of inhaled p-aramid fibrils in both species . In contrast, clearance of short chrysotile asbestos fibers was rapid , but clearance of the long chrysotile fibers was slow or insignifican t, as evidenced by a progressive increase over time in the mean length s of fibers recovered from the lungs of exposed rats. Two-week, high-d ose exposures to p-aramid in both rats and hamsters produced transient increases in pulmonary inflammatory and cell proliferative responses. in contrast. inhalation of size-separated chrysotile asbestos fibers in rats produced persistent increases in cell labeling indices of airw ay, alveolar, and subpleural cells measured through a period of 1 to 3 months postexposure. These results suggest that inhaled p-aramid RFP are biodegradable in ti-e lungs of exposed rats and hamsters. In contr ast, exposures to chrysotile asbestos fibers in rats resulted in a sel ective pulmonary retention of long chrysotile fibers.