Mesothelioma Associated With Commercial Use of Vermiculite Containing Libby Amphibole

Objectives: To describe asbestos-related mortality among manufacturing workers who expanded and processed Libby vermiculite that contained amphibole fiber. Methods: Standardized mortality ratio was calculated for 465 white male workers 31 years after last Libby vermiculite exposure. Results: Two workers died from mesothelioma, resulting in a significantly increased standardized mortality ratio of 10.5 (95% confidence interval, 1.3 to 38.0). These workers were in the upper 10th percentile of cumulative fiber exposure, that is, 43.80 and 47.23 fiber-years/cm(3), respectively. One additional worker with cumulative fiber exposure of 5.73 fiber-years/cm(3) developed mesothelioma but is not deceased. There were no other significantly increased standardized mortality ratios. Conclusions: Workers expanding and processing Libby vermiculite in a manufacturing setting demonstrated an increased risk for the development of mesothelioma following exposure to the amphibole fiber contained within this vermiculite ore source.