Montana & Mesothelioma

 Mining, handling, processing, and personal or commercial use of asbestos-contaminated vermiculite have led to widespread contamination of the Libby, Montana, area. We initiated a medical testing program in response to reports of respiratory illness in the community. The purpose of this analysis was to identify and quantify asbestos-related radiographic abnormalities among persons exposed to vermiculite in Libby and to examine associations between these outcomes and participants' self-reported exposures. A cross-sectional interview and medical testing were conducted in Libby from July through November 2000 and from July through September 2001. A total of 7,307 persons who had lived, worked, or played in Libby for at least 6 months before 31 December 1990 completed the interview. Of those, 6,668 participants [greater than or equal to] 18 years of age received chest radiographs to assess the prevalence of pleural and interstitial abnormalities. We observed pleural abnormalities in 17.8% of participants and interstitial abnormalities in 1% of participants undergoing chest radiography. We examined 29 occupational, recreational, household, and other exposure pathways in the analysis. The prevalence of pleural abnormalities increased with increasing number of exposure pathways, ranging from 6.7% for those who reported no apparent exposures to 34.6% for those who reported [greater than or equal to] 12 pathways. The factors most strongly associated with pleural abnormalities were being a former W.R. Grace worker, being older, having been a household contact of a W.R. Grace worker, and being a male. In addition to being a former W.R. Grace worker, environmental exposures and other nonoccupational risk factors were also important predictors of asbestos-related radiographic abnormalities. Key words: asbestos-related disease, medical screening, pleural plaques, radiographic opacities, radiography, tremolite-actinolite, vermiculite. Environ Health Perspect 111:1753-1759 (2003). 

Mining and processing of vermiculite from the world's largest deposit near Libby, Montana, began in the early 1920s and continued for 70 years. In 1963, W.R. Grace acquired the mine from Zonolite Company and operated it until it was closed in 1990. These operations included open-pit mining and on-site milling of the mineral; transporting the raw material by truck and rail to two processing plants in Libby and to plants throughout the United States and Canada, where it was heated and expanded (exfoliated); and finally, shipping the finished product by truck and rail to distribution centers nationally (Myers 1960). Vermiculite is a silicate mineral with the unique property of expanding 8- to 12-fold in volume upon heating (Meisinger 1980; Myers 1960; Otis 1960). Because of its fire-resistant and absorptive properties, vermiculite is used commercially in the construction industry as insulation and filler material, and in agriculture as a soil additive and carrier agent for fertilizers and other chemicals (Lockey 1981; Meisinger 1980). Vermiculite from Montana has been shown to be contaminated with tremolite, actinolite, and other forms of the amphibole series of asbestos, ranging from 2 to 26% in the raw form (Amandus et al. 1987b; Atkinson et al. 1982; Bank 1980; Dixon et al. 1985; Moatmed et al. 1986). Thus, mining, handling, processing, and personal or commercial use of vermiculite have led to widespread contamination of the Libby area with asbestos-contaminated vermiculite (Dixon et al. 1985).

Although no serious health effects from vermiculite alone have been reported to date (Addison 1995), health effects such as pleural thickening, pleural calcifications, pleural effusions, asbestosis, mesothelioma, and lung cancer from occupational exposure to asbestos have been well documented. Early evidence of pulmonary fibrosis among Libby mine and mill employees was described in an X-ray survey of miners in 1959 (McDonald et al. 1986a). Additional evidence came from a report of 12 cases of pleural effusions over a 12-year period among employees of an Ohio fertilizer plant that processed vermiculite from Libby, followed by a cross-sectional study of workers in the plant that demonstrated a relationship between cumulative fiber exposure and radiographic changes and pleuritic chest pain (Lockey et al. 1984). Two separate but parallel cohort studies of workers at the vermiculite mine in Libby showed excess mortality from lung cancer, malignant mesothelioma, and nonmalignant respiratory disease (Amandus and Wheeler 1987; McDonald et al. 1986a). These same investigators also conducted radiographic studies of then-current workers and found that the prevalence of pleural thickening and small opacities increased with increasing levels of cumulative fiber exposure (Amandus et al. 1987a; McDonald et al. 1986b). Together, these findings provide substantial evidence that exposure to the amphibole type of asbestos from the Libby vermiculite mine results in adverse respiratory health effects similar to those seen with exposure to other forms of asbestos.

Reported cases of mesothelioma among household contacts of asbestos workers led to a growing concern that asbestos exposure and the risk of later disease could spread beyond the workplace to the home and community (Anderson et al. 1976; Berry 1997; Hansen et al. 1993, 1998; Magnani et al. 2000, 2001; Newhouse and Thomson 1965; Wagner et al. 1960). Several studies have reported an increased prevalence of pleural thickening, pleural plaques, and calcifications as well as parenchymal opacities among household members of asbestos workers (Anderson et al. 1976, 1979; Kilburn et al. 1985). Libby area physicians have reported cases of asbestos-related pulmonary disease among household contacts of former mine workers and other residents of the community who were not directly associated with the mining or processing operations (Whitehouse 2000).

Although household contacts may have been exposed by workers taking home asbestos on their clothes, shoes, and hair, numerous other sources of environmental asbestos exposure exist for Libby community members. Air sampling in downtown Libby in 1975 and at several points in the 1980s detected levels of asbestos well above the Occupational Safety and Health Administration's occupational limit of 0.1 fiber/[cm.sup.3] over 8 hr of exposure (Atkinson et al. 1982; Dixon et al. 1985; U.S. Department of Labor 1994). Residents could have also been exposed to asbestos through the use of vermiculite in gardening activities, in home insulation, as aggregate in driveways, or through other uses around the home. Additionally, Libby residents have described other activities that involved close contact with vermiculite, such as playing in piles of vermiculite at the processing facilities, expanding or "popping" vermiculite at home by heating pieces of the ore over the stove, playing at the baseball field located adjacent to the vermiculite expansion plant, and playing along Rainey Creek Road that leads from town to the vermiculite mine.

A community-based medical testing program was initiated in response to reports of illness among people exposed to asbestos-contaminated vermiculite in Libby. In this article we outline the results from radiographic testing and self-reported exposure pathways for 7,307 persons who participated in this program. The main objectives were to a) identify and quantify possible asbestos-related pleural and interstitial abnormalities among participants and b) examine associations between these outcomes and the participants' exposure histories.