Radiographic abnormalities and asbestos exposure: Libby, Montana.

Peipins et al. (2003) described the study conducted by the Agency for Toxic Substances and Disease Registry (ATSDR 2001, 2002a) to analyze the association between the prevalence of radiographic abnormalities and asbestos exposure pathways for residents of Libby, Montana. Although Peipins et al. presented many detailed results, they failed to explicitly state the obvious conclusion of their analysis: excess risk of asbestos-related disease for Libby residents is a consequence of occupational exposure, and risk associated with low-level exposure is negligible. This finding is extremely important for guiding future public health assessments of exposure to vermiculite from the Libby mine and exposure to amphibole asbestos in general.

Peipins et al. (2003) made a clear case for the importance of occupational exposure. They reported the following statistically significant factors for predicting pleura] abnormalities: being a former mine worker; being older; having been a household contact of a mine worker; and being male. These results associate pleural abnormalities with high occupational exposure groups--mine workers and household contacts of mine workers. Only one environmental exposure pathway was statistically significant--played in vermiculite pile, which may involve exposures as high as occupational exposures.

The ATSDR data (ATSDR 2001, 2002a) indicate that 17.8% of the 6,668 subjects with X-ray films had pleural abnormalities. This percentage appears large in comparison to 6.7% of subjects with pleural abnormalities in the internal control group (no exposure). However, occupational exposures inflate the difference. I reanalyzed the data by forming three exposure groups: mine workers, residents with other occupational or domestic exposure, and residents with no occupational or domestic exposure (the "environmental exposure only" group) (Price B. In press). Pleura] abnormalities were recorded for 51% of mine workers, 19.9% of residents with other occupational or domestic exposure, and 9.1% of residents with no occupational or domestic exposure.

These results again demonstrate the importance of occupational exposure for pleural abnormalities. The percentage for the "environmental exposure only" group is close to the percentage for the internal controls. Other confounding factors mentioned by Peipins et al. (2003) could further reduce the difference. One of those factors is the false positive bias due to subpleural fat, which would be expected to be greater in the environmental exposure group than in the mine worker group (Price B. In press).

Peipins et al. (2003) implied that asbestos exposure was high in Libby:

   Air sampling in downtown Libby in 1975 and at
   several points in the 1980s detected levels well
   above the Occupational Safety and Health
   Administration's [OSHA] 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).

However, the cited references do not contain data for samples collected in 1975. Dixon et al. (1985) reported that a total of four stationary samples were collected, each for 2 hr (Atkinson et al. 1982). These samples, analyzed by two laboratories, produced the following measurements, respectively: 0.08 and 0.50 fiber/[cm.sup.3], 0.10 and 0.02 fiber/[cm.sup.3], 0.03 fiber/[cm.sup.3] and none detected, and 0.03 and 0.02 fiber/[cm.sup.3]. Comparison of these 2-hr sampling results to OSHA's 8-hr limit is not meaningful. Also, because the measurements vary considerably between laboratories, the largest measurement is not a reliable estimate of exposure in Libby.

Recently, the U.S. Environmental Protection Agency (U.S. EPA 2001, 2002) measured airborne asbestos levels in Libby and estimated exposure for Libby residents engaged in activities that disturbed household dust, vermiculite attic insulation, and soil. The results, stated as lifetime average daily exposure, ranged from 0.00007 to 0.005 fiber/[cm.sup.3]. Risk levels calculated by the U.S. EPA for these exposures were between 1 x [10.sup.-6] and 1 x [10.sup.-4], the acceptable range defined by the U.S. EPA (1989) for regulatory decisions.

The study described by Peipins et al. (2003) is one of a number of studies addressing asbestos exposure and risk in Libby (Amandus and Wheeler 1987; ATSDR 2000, 2001, 2002a, 2002b; McDonald 2001; McDonald et al. 1986; McDonald JC. Unpublished data; U.S. EPA 2001, 2002). The perception that mine workers' disease rates apply to all Libby residents gained credibility through the ATSDR (2000) asbestosis mortality study: "... mortality in Libby resulting from asbestosis was approximately 40-60 time higher than expected." Later, the ATSDR (2002b) explained that the excess mortality was a consequence of occupational exposure. As noted by Peipins et al. (2003), the ATSDR observed a total of 12 asbestosis deaths: 11 males who were previously employed at the Libby mine and 1 female who was a household contact of a mine worker.

The results presented by Peipins et al. (2003) and results from other studies of asbestos in Libby indicate that occupational exposure--not low-level environmental exposure--is the most significant risk factor for asbestos-related disease. Peipins et al. should have stated that conclusion explicitly, taking the first step toward correcting misperceptions about asbestos disease in Libby and, more generally, the risk of disease associated with low-level exposures to amphibole asbestos.

The author declares he has no competing financial interests.

Bertram Price

Price Associates, Inc.

White Plains, New York

E-mail: [e-mail]

REFERENCES

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ATSDR. 2000. Health Consultation, Mortality from Asbestosis in Libby, Montana. Atlanta, GA:Agency for Toxic Substances and Disease Registry. Available: http:// www.atsdr.cdc.gov/HAC/PHA/libby/lib_toc.html [accessed 7 January 2004].

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U.S. Department of Labor. 1994. Occupational Exposure to Asbestos. 29CFR parts 1910, 1915, 1925. Final Rule. Fed Reg 59:40964-41162.

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U.S. EPA. 2001. Memorandum from Chris Weis, U.S. EPA, to Paul Peronard, U.S. EPA. Amphibole Mineral Fibers in Source Materials in Residential and Commercial Area of Libby Pose an Imminent and Substantial Endangerment to Public Health. Washington, DC:U.S. Environmental Protection Agency, 20 December 2001.

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