A dose-response model for refractory ceramic fibers

Refractory ceramic fibers (RCFs) are man-made vitreous fibers commonly used in insulation applications above 1000degreesC. Although they have been subjected to considerable toxicologic evaluation, only the pooled results from two rat inhalation studies provide data that may be suitable for performing a numerical risk assessment. Even in these inhalation studies, good evidence exists that the maximum tolerated dose (MTD) was exceeded and that pulmonary overload occurred, a condition that will cause tumors whatever the dust responsible. Indeed, a significant yield of tumors was only obtained at the highest dose tested. If these results are omitted, there is no statistically significant evidence of carcinogenicity within the RCF results. Although there is little evidence that overload-related tumors are relevant to human risk, we adopted a conservative approach to obtain the estimates of risk regardless of overload, using a biologically based model, the two-stage clonal expansion model, as well as various statistical models, including the benchmark dose model. We argue that the data favor the use of a biologically based model, which gives the best fit when the highest dose RCF exposures are omitted. Continuing with this model, we show that available data from the RCF experiment, less outliers, coupled with results from other experiments with man-made mineral fibers (MMVFs), demonstrate that all MMVFs are potentially carcinogenic, with any risk mediated by the fibers' biopersistence. Application of this "all MMVF data set" model yields a maximum likely estimate for RCF excess unit risk of 4.6 x 10(-5) (95% upper confidence limit = 9.2 x 10(-5) per fiber/ml). This implies that the risk from occupational exposure to RCFs at 1 fiber/ml for a typical working lifetime would not exceed 10(-4).