Assessment of the Risk of Human Cancer from Exposure to Refractory Ceramic Fibers Using Animal Inhalation Data

Although refractory ceramic fibers have been subject to considerable toxicological evaluation, only the pooled results from two rat inhalation studies can be used to assess the occupational risk from exposure. Even in these studies, there is good evidence that the maximum tolerated dose was exceeded and that pulmonary overload occurred in the test animals. Despite these defects, we adopted a conservative approach to estimate the risk using a biologically-based model, the two-stage clonal expansion model (TSCE). Although other statistical models are available, we argue that the circumstances favor the use of the TSCE. The model yields a maximum likely estimate for RCF excess risk of 4.6 x 10 (95% upper confidence limit = 9.2 x 10). This implies that the risk from occupational exposure to RCF at 1 fiber/ml for a typical working lifetime would not exceed 1/10,000.