Low Levels of Exposure to Libby Amphibole Asbestos and Localized Pleural Thickening
Libby Amphibole asbestos (LAA) is a mixture of amphibole fibers present in ore from the vermiculite mine near Libby, MT (1). Workers and community residents were exposed to LAA at the mining operations in Libby, MT (2, 3), as well as at vermiculite processing facilities in Marysville, OH (4, 5), and Minneapolis, MN (6). Epidemiologic studies of these populations indicate that exposure to LAA is associated with increased risk of adverse health effects including lung cancer, mesothelioma, and non-cancerous radiographic abnormalities (2, 3, 5, 7, 8). One example of such a radiographic abnormality is pleural thickening, a condition in which the pleural lining around the lungs (visceral pleura) or along the chest wall and diaphragm (parietal pleura) thickens due to fibrosis and collagen deposits (9). This thickening has been characterized as either localized (LPT) or diffuse (DPT), with diffuse thickening representing a health endpoint of greater severity. In the current (2000) International Labour Organization (ILO) classification, LPT is focal areas of pleural thickening of the parietal pleura along the chest wall in which the CPA is not blunted, or on the diaphragm (10). LPT is considered to be the most ‘sensitive’ health endpoint, occurring sooner after exposure to LAA compared to other radiographic outcomes (11). Further, LPT is of clinical and public health importance because it represents an irreversible pathologic change, and although evidence is mixed, some studies indicate LPT may be associated with decrements in lung function (12-17) and chest pain (18). Three recent studies have used quantitative exposure estimates to evaluate the relationship between LAA exposure and risk of LPT. These include two studies of occupationally exposed cohorts—the study of workers exposed to LAA at the mining and milling facilities in Libby, MT (19), and the study of workers exposed at the O.M. Scott facility in Marysville, OH (5). In each case, workers were exposed to vermiculite ore from the mine in Libby, MT, which contained amphibole asbestos fibers. The third study was conducted not among workers, but among community residents living near the Western Minerals facility in Minneapolis, MN, which also handled vermiculite ore mined in Libby, MT (20). While each study evaluated the association between LAA exposure and increased risk of LPT, there were differences in the exposure scenario, study population and design, and analytic approach. The objective of this review is to compare these three studies and evaluate the overall conclusions on the risk of LPT at low levels of exposure to LAA.