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, respectively. One additional worker with cumulative fiber exposure of 5.73 fiber-years/cm 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.