Low-Level Cumulative Exposure to Libby Amphibole and Pleural Disease

The mineralogical composition and morphology of amphiboles deposited within the vermiculite ore from Libby, Montana are best described in decreasing order of abundance as winchite, richterite, and tremolite. Of these amphiboles, only tremolite meets the regulatory definition of asbestos. With mechanical force these minerals fracture into respirable size fibers with high aspect ratios.

A 1980 investigation identified a cluster of bloody pleural effusions in workers utilizing vermiculite that contained Libby amphiboles at a facility manufacturing lawn care products. Predominantly pleural changes consistent with asbestos exposure were identified in 2.2% of the overall cohort and 8.4% within the highest cumulative exposure group (>10 fiber/cc-year). A 2004 follow-up study of the original cohort 25 years after discontinued use of Libby vermiculite demonstrated pleural and interstitial changes in 28.7% and 2.9% of participants, respectively. Pleural changes were seen in 54.3% of those in the highest exposed quartile (2.2–19.0 fiber/cc-year) and 20% of those with less than 2.2 cumulative fiber/cc-year. A mortality study had limited power in relationship to detecting lung cancer risks, but three mesothelioma cases were recently identified.

Conclusions: Libby amphiboles are associated with pulmonary changes consistent with commercial asbestos exposure. These amphiboles cause progressive pleural changes as well as malignant mesothelioma years after past exposure at relatively low levels. This study represents a public health concern in view of the historically wide distribution of this ore throughout the United States for both residential and commercial use.