Objective: Low cumulative fiber exposure (CFE) has been associated with health effects in a cohort exposed to Libby vermiculite. This study refines the original 1980 exposure estimates and compares the CFE results.
Methods: Cumulative fiber exposure estimates were developed using three times more industrial hygiene measurements and long-term workers' input. New adjustments included vermiculite ore source, seasonal overtime hours, time spent in various tasks, and recollection of historical dustiness.
Results: The overall mean (95% confidence interval) CFE (n = 513) in 1980 (0.80 [0.69 to 0.93]) was statistically similar to the overall mean (95% confidence interval) CFE in 2010 (0.74 [0.61 to 0.90]). The mean CFE in the lowest exposure category (<2 fiber-years/cm3) decreased from 0.36 to 0.22 fiber-years/cm3 (P < 0.05). The 2010 CFE estimate extended the upper bound of the range of previous estimates from 28.10 to 106.31 fiber-years/cm3.
Conclusions: The range of CFE values was expanded. These estimates may impact the understanding of Libby vermiculite health outcomes.