ABSTRACT
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.
Subject(s)
Aluminum Silicates/adverse effects , Extraction and Processing Industry , Inhalation Exposure/adverse effects , Lung Neoplasms/mortality , Mesothelioma/mortality , Occupational Exposure/adverse effects , Adolescent , Adult , Aged , Aged, 80 and over , Cause of Death , Cohort Studies , Confidence Intervals , Female , Humans , Lung Neoplasms/chemically induced , Male , Mesothelioma/chemically induced , Middle Aged , Respiratory Tract Diseases/chemically induced , Respiratory Tract Diseases/mortality , Young AdultABSTRACT
RATIONALE: From 1921 to 1990, vermiculite ore from Libby, Montana, was shipped worldwide for commercial and residential use. A 1980 study of a manufacturing facility using Libby vermiculite was the first to demonstrate a small but significant prevalence of pleural chest radiographic changes associated with amphibole fibers contained in the ore. OBJECTIVES: This follow-up study of the original cohort evaluated the extent of radiographic changes and cumulative fiber exposure (CFE) 25 years after cessation of exposure. METHODS: From the original cohort of 513 workers, 431 (84%) were living and available for participation and exposure reconstruction. Of these, 280 (65%) completed both chest radiographs and interviews. Primary outcomes were pleural and/or interstitial changes. MEASUREMENTS AND MAIN RESULTS: Pleural and interstitial changes were demonstrated in 80 (28.7%) and 8 (2.9%) participants, respectively. Of those participants with low lifetime CFE of less than 2.21 fiber/cc-years, 42 (20%) had pleural changes. A significant (P < 0.001) exposure-response relationship of pleural changes with CFE was demonstrated, ranging from 7.1 to 54.3% from the lowest to highest exposure quartile. Removal of individuals with commercial asbestos exposure did not alter this trend. CONCLUSIONS: This study indicates that exposure within an industrial process to Libby vermiculite ore is associated with pleural thickening at low lifetime CFE levels. The propensity of the Libby amphibole fibers to dramatically increase the prevalence of pleural changes 25 years after cessation of exposure at low CFE levels is a concern in view of the wide national distribution of this ore for commercial and residential use.
