Exposure to airborne amphibole structures and health risks: Libby, Montana

Our plain-English summary of Exposure to airborne amphibole structures and health risks: Libby, Montana, by B. Price (Regulatory Toxicology and Pharmacology, 2008).

The study

• Author: B. Price
• Journal: Regulatory Toxicology and Pharmacology
• Published: 2008;52(1 Suppl):S97-S109
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Libby, Montana is the site of a large vermiculite deposit that was mined between 1920 and 1990 to extract vermiculite for commercial applications such as insulation, gardening products, and construction materials. The Libby vermiculite deposit also contains amphibole minerals including tremolite, actinolite, richterite, and winchite. Historically, Libby mine workers experienced high exposures to amphibole structures, and, as a group, have experienced the health consequences of those occupational exposures. It has been suggested that Libby residents also have been and continue to be exposed to amphibole structures released during the vermiculite mining operations and therefore are at increased risk for disease. The Agency for Toxic Substance and Disease Registry (ATSDR) conducted two epidemiological-type studies of residents living in Libby and the surrounding areas to assess these risks. The Environmental Protection Agency (EPA) collected and analyzed exposure data in Libby and used those data to project risks of asbestos-associated disease for Libby residents. The EPA has placed the Libby Asbestos Site, which includes the mine and the town of Libby, on its National Priority List of hazardous waste sites in need of clean up. This article presents a review of the exposure studies conducted in Libby and an analysis of health risks based on the data collected in those studies. Libby mine workers have experienced elevated levels of asbestos-associated disease as a consequence of their occupational exposures to amphibole structures. Libby residents' exposures typically are substantially lower than mine workers' historical exposures, and the health risk projections for residents are, accordingly, substantially lower.

In plain English

This article was the author's review of exposure and health studies from Libby, Montana, where vermiculite containing amphibole minerals was mined from 1920 to 1990. The article calls these minerals 'Libby Asbestos' and names tremolite, actinolite, soda tremolite, richterite and winchite. The review included the author's re-analyses of published mine worker data and of an ATSDR medical screening data file. Re-analysing two mine worker studies, the author estimated lung cancer potency at 0.0025 to 0.0077 per f-yr/cc, below the 0.01 value EPA uses. In the screening data, pleural abnormalities were reported in 186 of 365 former mine workers (51.0%). The figure was 785 of 3936 for people with other occupational or household exposure (19.9%), and 215 of 2367 for people with neither (9.1%). In the two lower-exposure groups, higher body mass was statistically linked to a positive reading. The author suggested this could produce false positives. The author concluded that the Libby studies had not produced sufficient evidence that environmental exposure, apart from miners' occupational exposure, is associated with increased disease. The author disclosed consulting for, and paid court testimony at the request of, W.R. Grace & Co.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review of exposure and health studies in Libby, including the author's re-analyses of previously collected data (mine worker lung cancer data and the ATSDR Screening Study data file)
• Who was studied: Libby vermiculite mine worker cohorts from two earlier studies: 575 workers (Amandus) followed through 1981 and 406 workers (McDonald) followed through 1999. Also 542 decedents in ATSDR's Libby-area mortality analysis for 1979 to 1998, 6668 Libby-area participants with chest X-rays in ATSDR's 2000 to 2001 medical testing program, and EPA air sampling for Libby residents.
• Exposure: Libby Asbestos (LA): amphibole minerals in the Libby vermiculite, a mix of fibers and cleavage fragments. Exposures came from mine work, other occupational or household (domestic) contact, and environmental/residential activities. Exposure was measured in fiber-years per cc (f-yr/cc).
• Main results:
  • In the re-analysis of mine worker data, estimated lung cancer potency (unit risk) for LA ranged from 0.0025 to 0.0077 per f-yr/cc. This was below the 0.01 value EPA uses in its IRIS risk assessment.
  • For the Amandus mine worker data, the exposure needed to double lung cancer risk compared with the external reference group exceeded 150 f-yr/cc in all models.
  • Mesothelioma among mine workers: Amandus recorded 2 cases (proportional mortality ratio 1.2%), and McDonald recorded 12 (4.2%). The author compared these with an estimated U.S. male figure of approximately 0.2%.
  • ATSDR's Libby asbestosis mortality excess was based on 12 asbestosis deaths. Of these, 11 were men previously employed at the Libby mine, and 1 was a woman who was a household contact of a mine worker.
  • The ATSDR data showed a statistically significant excess of lung cancer in males compared with Montana. The author recalculated after excluding 21 lung cancer decedents who were former mine employees, assuming all were male. The resulting male SMRs did not show statistically significant excesses.
  • Pleural abnormalities in the ATSDR screening data were reported in 186 of 365 former mine workers (51.0%). The figure was 785 of 3936 for those with other occupational or domestic exposure (19.9%), and 215 of 2367 for those with neither (9.1%). The overall figure was 1186 of 6668 (17.8%).
  • Among the 1186 pleural cases, at least one X-ray reader checked the 'FAT?' box for 339 (28.6%, per the table). By group, this was 18.3% of mine workers, 30.1% of Group 2 and 32.1% of Group 3. The author suggested the false positive rate may be near 30%.
  • Among mine workers, body mass index was not correlated with a pleural abnormality diagnosis (p-value 0.86). It was correlated in Group 2 and Group 3 (p-value less than 0.001). In Group 3, 51.2% of those diagnosed positive were obese, compared with 30.8% of those diagnosed negative.
  • From EPA's activity scenarios, the maximum estimated lifetime cumulative exposure for a Libby resident (excluding the contractor scenario) was 0.0425 f-yr/cc. The author called the resulting health risks negligible.
• The authors' conclusion: The author concluded that the excess risk applies to Libby mine workers with high historical occupational exposure, and that there is no evidence LA is more potent than other types of asbestos. In the author's view, the studies have not produced sufficient evidence that environmental exposure to LA, independent of miners' occupational exposure, is associated with increased pleural or parenchymal abnormalities, lung cancer, or mesothelioma.
• Limitations the authors state:
  • The EPA air sampling design was not detailed enough to claim the results represent Libby residents' exposures.
  • The mesothelioma data from the mine worker studies were insufficient to estimate risk models.
  • The mine worker cohort was too small to adopt the suggested mesothelioma threshold value for all risk management purposes.
  • McDonald's updated data were not available for re-analysis.
  • The fiber size data were not collected according to a formal statistical design and may not be representative of LA.
  • The lung cancer recalculation assumed that the 21 former mine worker decedents were all male, because ATSDR did not report their sex.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Libby vermiculite mine, Montana

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis, Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Amphibole Group, vermiculite, Libby Vermiculite, Tremolite, Actinolite, richterite, winchite, Occupational Exposure.

Related studies

• A review of scientific literature examining the mining history, geology, mineralogy, and amphibole asbestos health effects of the Rainy Creek igneous complex, Libby, Montana, USA — B.R. Bandli, M.E. Gunter (2006)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Exposure to asbestos-containing vermiculite ore and respiratory symptoms among individuals who were children while the mine was active in Libby, Montana — L.C. Vinikoor, T.C. Larson, et al. (2010)
• A review of the federal government's health activities in response to asbestos-contaminated ore found in Libby, Montana — K. Horton, V. Kapil, et al. (2006)
• Potency factors for risk assessment at Libby, Montana — S.H. Moolgavkar, J. Turim, et al. (2010)
• Trees as reservoirs for amphibole fibers in Libby, Montana — T.J. Ward, T. Spear, et al. (2006)

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