Chronic Inhalation Study of Size-Separated Rock and Slag Wool Insulation Fibers in Fischer 344/N Rats

Abstract

This study was designed to investigate the potential pathogenic effects in Fischer 344/N rats of two different types of man-made vitreous fibers (MMVF). Eight-week-old male rats were exposed in nose-only inhalation chambers, 6 hlday, 5 dayslwk, lor 24 mo to 3 concentrations (3, 16, and 30 mg/m¹) each of the two MMVFs: a basalt-based rock wool (stone wool), and a slag wool (blast furnace). Crocidolite asbestos (10 mg/m³) was used as a positive control. The experimental groups were compared to unexposed (chamber) controls. The MMVFs used in this study were size selected to be largely respirable in rats. Interim sacrifices took place at 3- and 6-mo intervals to monitor the progression of pulmonary changes. Fibers were recovered from digested lung tissue for determination of changes in fiber number and morphology. Exposure to crocidolite asbestos was terminated after 10 mo because of increased morbidity/mortality. Exposure to rock and slag wool, while producing a dose-related nonspecific inflammatory response (rock and slag) and minimal local pulmonary fibrosis (rock only), showed no evidence of carcinogenic activity in either the lung or pleura, in contrast to crocidolite asbestos, which induced neoplasms in both tissues. Since workplace airborne levels are several orders of magnitude lower than even the lowest exposure level to which the animals were exposed, these results suggest that these MMVFs do not pose a significant health risk to humans at airborne levels found in the workplace, during installation or for the in-place product.