Db. Warheit et al., LUNG PROLIFERATIVE AND CLEARANCE RESPONSES TO INHALED PARA-ARAMID RFPIN EXPOSED HAMSTERS AND RATS - COMPARISONS WITH CHRYSOTILE ASBESTOS FIBERS, Environmental health perspectives, 105, 1997, pp. 1219-1222
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.