Vermiculite Worker Mortality: Estimated Effects of Occupational Exposure to Libby Amphibole

Abstract

Objective: To examine the relationship between cumulative fiber exposure (CFE) and mortality in a retrospective cohort study of vermiculite workers exposed to Libby amphibole (n ϭ 1862). Methods: Extended Cox regression was used to estimate the hazards associated with CFE as a timedependent covariate of multiple-cause mortality. Results: The Cox models for mesothelioma, asbestosis, lung cancer, and non-malignant respiratory disease were significant with rate ratios that increased monotonically with CFE. The model for deaths due to cardiovascular disease was also significant (rate ratio for CFE Ն44.0 f/cc-y vs Ͻ1.4 f/cc-y was 1.5; 95% confidence interval ϭ 1.1 to 2.0). Conclusions: By using a within-cohort comparison, the results demonstrate a clear exposure-response relationship between CFE and mortality from asbestos-related causes. The finding of an association between CFE and cardiovascular mortality suggests persons exposed to Libby amphibole should be monitored for this outcome. L ibby, Montana was the site of a vermiculite mining and processing operation from the 1920s to 1990. Although vermiculite causes no known adverse health effects, Libby vermiculite is contaminated with a mixture of amphibole fibers. These amphiboles have been collectively called tremolite by the community, popular press, and in some scientific literature 1 but Libby vermiculite also contains actinolite 2 and unregulated asbestos-like fibers, including winchite and richterite. 1,3 The only chemical difference between these amphiboles is their iron content; this mineralogic difference is somewhat arbitrary and may have no biological significance. 2 Regardless, the raw vermiculite ore mined in Libby contained up to 26% amphibole fibers by mass. 4 Libby is thought have produced 80% of the world's supply of vermiculite during the 20th century. 5 Because many of its years of operation predated significant regulation of occupational asbestos exposure, the amphibole exposures at the Libby vermiculite operation were quite high, especially before engineering controls were put in place in the late 1960s and early 1970s. The resulting health effects of working at the Libby vermiculite operation have been well documented, demonstrating excesses of asbestos-related morbidity and mortality. 6 -8 An excess of asbestos-related mortality among Libby vermiculite workers is not surprising given their occupational amphibole exposures. In this study, we use mortality data on the entire vermiculite worker cohort updated through the end of 2006. Our objective was to conduct an exposure-response study to obtain estimates of the hazard of asbestos-related mortality associated with cumulative asbestos exposure.

Vascular and Thrombogenic Effects of Pulmonary Exposure to Libby Amphibole

“…mesothelioma (Sullivan 2007). In this series of experiments, systemic adverse vascular effects of pulmonary exposure to LA in healthy and cardiovascular compromised rat models were examined in order to gain insight into recent epidemiological findings suggesting a correlation between human exposure to LA and increased autoimmune diseases (Noonan et al 2006) and increased rate of mortality associated with cardiovascular events (Larson et al 2010). Since epidemiological findings indicate that individuals with cardiovascular disease are increasingly sensitive to the effects of inhaled PM (Cheng et al 2009;Krewski et al 2005;Kwon et al 2001;Pope et al 2004), rat models of CVD (SH, SHHF) were included along with healthy strains (WKY, F344).…”

“…LA asbestos exposure was linked to an increase in the rate of systemic autoimmune disease and mortality associated with cardiovascular causes (Larson et al 2010;Noonan et al 2006). Since asbestos fibers are known to deposit within the lung and produce lung injury, chronic inflammation, fibrosis, and mesothelioma, these fibers provide an interesting test material for study of systemic, prothrombotic, and vascular effects.…”

“…Although systemic and more specifically adverse immune effects were postulated to occur following asbestos exposure (Goldsmith 1982;Lemaire et al 1986;Otsuki et al 2007), these effects secondary to pulmonary inflammation and disease are less well studied, likely due to the dominance of pulmonary disturbances (Bunderson-Schelvan et al 2011). However, more recently greater emphasis has been given to asbestos-induced adverse systemic and vascular health effects (Fukagawa et al 2008;Larson et al 2010;Noonan et al 2006) based on the linkage between ambient particulate matter (PM) exposure and the occurrence of systemic and vascular disease in healthy and compromised individuals ( Clark et al 2011;Jimenez et al 2011;Krewski et al 2005;Mills et al 2007).…”

Use of Direct Versus Indirect Preparation Data for Assessing Risk Associated with Airborne Exposures at Asbestos-contaminated Sites

“…(2) Numerous studies of workers from the Libby mine have demonstrated increased morbidity and mortality due to malignant and non-malignant diseases associated with exposure to LA. (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) Increased risk for the development of mesothelioma among manufacturing workers who expanded and processed Libby vermiculite has been reported, (15) as has increased morbidity in workers from an Ohio facility that used Libby vermiculite in the manufacturing of lawn care and other agricultural products. (16,17) Additionally, epidemiological investigations of Libby residents with no known occupational exposure to LA-contaminated vermiculite have documented respiratory symptoms, radiographic abnormalities, and adverse health effects.…”

Analysing health data from rare occupational cohorts

“…Early work by Marsh et al 11 provided coding rubrics based on alternative codes, or sets of codes, considered to be indicative of an underlying cause of death from mesothelioma. Sullivan5 had a trained nosologist review a targeted set of death certificates for any mention of mesothelioma and reclassified the cause of death accordingly; Larson et al 12 reviewed all workers’ death certificates for any mention of mesothelioma. These methods have allowed for the inclusion of additional data on mesothelioma mortality which would otherwise have gone missing.…”