CHRONIC INHALATION AND BIOPERSISTENCE OF REFRACTORY CERAMIC FIBER IN RATS AND HAMSTERS

Lifetime ''nose-only'' inhalation studies were conducted in rats using four types of refractory ceramic fibers (FCF), 1 mu m in diameter x 2 2 to 26 mu m length: High Purity, Kaolin, Zirconia, and After-Service; and on hamsters using Kaolin RCF. For comparison, animals also were e xposed to chrysotile fibers. Rats were exposed 6 hr/day, 5 days/week f or 24 months to concentrations ranging between 3 and 30 mg/m(3). Time- and dose-dependent lesions in the rat included the development of int erstitial fibrosis, pleural fibrosis, pulmonary tumors, and mesothelio ma. Exposure to 3, 9, or 16 mg/m(3) produced no excess lung tumors; no fibrosis was seen at 3 mg/m(3). A significant increase in lung tumors and interstitial fibrosis was observed at 30 mg/m3(.) A single mesoth elioma was observed in rats exposed to 9 mg/m3, while two occurred at 30 mg/m(3). Hamsters were similarly exposed to 30 mg/m(3) Kaolin RCF f or 18 months; no lung tumors were induced, but pulmonary and pleural f ibrosis were observed and there was a 42% incidence of mesothelioma. M ultiple interim sacrifices together with recovery animals allowed deta iled assessment of the lung burden of RCF, which was found to be dose related and, at the high doses, exceeded 10(5) fibers/mg of dry lung. During the various recovery periods there was a clear reduction in fib er burden. Mathematical modeling of these data for deposition, clearan ce, and retention and for species is currently underway.