CHRONIC INHALATION TOXICITY OF A KAOLIN-BASED REFRACTORY CERAMIC FIBER IN SYRIAN GOLDEN-HAMSTERS

Kaolin-based refractory ceramic fiber (RCF) is a man-made vitreous fib er used primarily in industrial high-temperature applications, especia lly for insulation of furnaces and kilns. Because of its increasing us e and potential for human exposure, a chronic toxicity/carcinogenicity inhalation study was conducted in Syrian golden hamsters. Two groups of 140 weanling male hamsters were exposed via nose-only inhalation to either HEPA-filtered air (chamber controls) or 30 mg/m(3) (similar to 220 fibers/cm(3)) of ''size-selected'' RCF fibers (1 mu m in diameter and similar to 25 mu m in length) for 6 h/day, 5 days/wk for 18 mo. T hey were then held unexposed until similar to 20% survival (20 mo). A positive control group of 80 hamsters was exposed to 10 mg/m(3) chryso tile asbestos (0.09 mu m average diameter and 2.2 mu m average length) . Groups of 3 hamsters were sacrificed at 3, 6, 9, 12, 15, and 18 mo t o follow the progression of lesions. Additional groups of 3 hamsters w ere removed from exposure at 3, 6, 9, and 12 mo and were held until 18 mo (recovery groups). Treatment-related lesions were restricted to th e lungs. RCF exposure resulted in macrophage infiltration, bronchioliz ation of proximal alveoli, and microgranuloma formation by 3 mo of exp osure. interstitial and focal pleural fibrosis were observed at 6 mo. The pulmonary lesions progressed in severity until 12 mo after which t hey plateaued. In contrast, pleural fibrosis progressed until the end of the study. In recovery animals, there was no further progression of either pulmonary or pleural lesions following cessation of exposure. While no pulmonary neoplasms were observed in the RCF exposed hamsters , 42 of 102 (41.2%) had pleural mesotheliomas. A greater severity of f ibrosis was observed in tile lungs of the positive (chrysotile asbesto s) controls, but no neoplasms were observed in either the lung or pleu ra. No neoplasms were found in tile lungs or pleura of the chamber air controls.