Quantitative risk analyses for RCF: survey and synthesis

Refractory ceramic fiber (RCF) is a high-temperature insulating fiber used principally in industrial applications. Epidemiological studies on occupationally exposed cohorts have not indicated that exposure leads to fibrosis, increased lung cancer, or mesothelioma. However, inhalation bioassays with rats and hamsters have shown that these animals can develop each of these endpoints when exposed to high levels of RCF-particle mixtures. This work summarizes relevant quantitative risk analyses based upon analysis of the rat bioassay studies, which lead to predicted unit risks that range nearly three orders of magnitude. Additionally, we identify key assumptions that affect the risk estimates and provide additional estimates using the benchmark dose methodology favored by the U.S. EPA in cases where mechanistic models are inadequate or not available. We show that a key determinant of risk is how lung burdens are normalized (e.g., in terms of the number of fibers per square centimeter of lung surface or the number of fibers per milligram dry lung) for species conversion. Plausible values of unit potency/risk range from 1.4 × 10−4 to 7.2 × 10−4, neglecting any allowance for the effects of particulate material in the RCF tested in the bioassay experiments.