Follow-Up of the Libby, Montana Screening Cohort: A 17-Year Mortality Study

Our plain-English summary of Follow-Up of the Libby, Montana Screening Cohort: A 17-Year Mortality Study, by T.C. Larson, L. Williamson, V.C. Antao (Journal of Occupational and Environmental Medicine, 2020).

The study

• Authors: T.C. Larson, L. Williamson, V.C. Antao
• Journal: Journal of Occupational and Environmental Medicine
• Published: 2020;62(1):e1-e6
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objective: To evaluate mortality patterns among participants in a community-based screening program for asbestos-related disease.

Methods: We calculated standardized mortality ratios (SMRs) and stratified results by exposure group (three occupational exposure groups, household contacts and residents without occupational asbestos exposure) and by radiographic abnormality presence.

Results: All-cause mortality (15.8%; 1,429/8,043) was statistically lower than expected. Asbestosis was statistically elevated in all exposure groups. Lung cancer was moderately associated with vermiculite miner/miller employment. Mesothelioma was elevated in that same exposure group and among residents. Systemic autoimmune disease mortality was also elevated. Radiographic parenchymal abnormalities were associated with lung cancer mortality.

Conclusion: In addition to asbestos-related mortality in occupational exposure groups, this initial follow-up of this cohort also shows elevated mortality for some asbestos-related causes in non-occupational exposure groups.

In plain English

The authors followed 8,043 people screened in Libby, Montana, during 2000 to 2008. Vermiculite mined there contained Libby amphibole asbestos. Deaths through 2016 were compared with US rates. There were 1,429 deaths (17.8%). Overall deaths were lower than expected (SMR 0.86, 95% CI 0.82 to 0.91). Asbestosis deaths were 128.15 times expected (95% CI 98.25 to 164.29), and mesothelioma deaths were 6.21 times expected (95% CI 3.48 to 10.25). Whole-cohort lung cancer deaths were not significantly raised but were elevated among W.R. Grace vermiculite workers (SMR 1.77, 95% CI 1.09 to 2.70). Mesothelioma was also elevated among residents without job exposure (SMR 4.25, 95% CI 1.16 to 10.89), and asbestosis was elevated in every exposure group. Deaths involving rheumatoid arthritis, scleroderma or systemic lupus erythematosus (16 deaths) were 4.30 times expected (95% CI 2.19 to 6.40). Parenchymal (lung-tissue) changes on chest X-ray were associated with higher lung cancer death rates. The authors noted crude exposure groups and possibly heightened asbestos awareness among local death certifiers as limitations. They concluded that some asbestos-related causes of death were elevated in non-occupational groups too.

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: Cohort mortality follow-up of a community screening program. Deaths were identified through the National Death Index and compared with US mortality rates (standardized mortality ratios, SMRs). Exposure subgroups were compared internally (standardized rate ratios, SRRs).
• Who was studied: 8,043 participants in asbestos screening programs in Libby, Montana: 7,307 screened by ATSDR in 2000 to 2001 and 736 screened by the Montana Department of Public Health and Human Services in 2003 to 2008. To be eligible, a person had to have lived, worked, or played in Libby for at least 6 months before the end of 1990. Follow-up ran from screening to December 31, 2016, for a total of 125,792 person-years.
• Exposure: Libby amphibole asbestos (LAA) in vermiculite from the Libby mining and milling operation. Participants were placed in five exposure groups, from presumed most to least exposed: W.R. Grace & Co. vermiculite workers, secondary contract workers, other asbestos occupations, household contacts of W.R. Grace workers, and residents not in another group.
• Main results:
  • Whole cohort: 1,429 of 8,043 participants died (17.8%).
  • Whole cohort: deaths from all causes were lower than expected (SMR 0.86, 95% CI 0.82 to 0.91), and so were deaths from all cancers (SMR 0.85, 95% CI 0.77 to 0.94).
  • Whole cohort: asbestosis deaths were elevated, with 62 observed against 0.48 expected (SMR 128.15, 95% CI 98.25 to 164.29).
  • Whole cohort: mesothelioma deaths were elevated, with 15 observed against 2.41 expected (SMR 6.21, 95% CI 3.48 to 10.25).
  • Whole cohort: lung cancer deaths were not statistically different from expected, with 123 observed against 137.73 expected (SMR 0.89, 95% CI 0.74 to 1.07).
  • W.R. Grace workers: lung cancer deaths were elevated compared with US rates (SMR 1.77, 95% CI 1.09 to 2.70).
  • W.R. Grace workers compared with residents: the lung cancer rate ratio was 2.83 (95% CI 1.10 to 7.25).
  • Residents without occupational exposure: mesothelioma deaths were elevated compared with US rates (SMR 4.25, 95% CI 1.16 to 10.89).
  • W.R. Grace workers compared with residents: the mesothelioma rate ratio was 15.12 (95% CI 3.32 to 68.90).
  • All exposure groups: asbestosis SMRs were greater than 80 in every group.
  • Compared with residents: the asbestosis rate ratio was elevated for W.R. Grace workers (2.42, 95% CI 1.11 to 5.26) and for household contacts (2.58, 95% CI 1.01 to 6.58).
  • Whole cohort, multiple cause of death: deaths involving three systemic autoimmune diseases combined (rheumatoid arthritis, scleroderma and systemic lupus erythematosus) were elevated, with 16 observed against 3.73 expected (SMR 4.30, 95% CI 2.19 to 6.40).
  • Participants with parenchymal abnormalities on chest radiograph, compared with those without: the lung cancer rate ratio was 3.94 (95% CI 2.10 to 7.38).
  • Participants with pleural abnormalities on chest radiograph, compared with those without: the lung cancer rate ratio was 1.73 (95% CI 1.15 to 2.61).
  • Smokers compared with non-smokers: the lung cancer rate ratio was 37.95 (95% CI 11.91 to 120.94). There were only three lung cancer deaths among non-smokers.
  • Whole cohort: deaths from motor vehicle driver injuries were elevated (SMR 2.90, 95% CI 1.75 to 4.53).
• The authors' conclusion: The authors concluded that, in addition to asbestos-related mortality in the occupational exposure groups, this cohort showed elevated mortality from some asbestos-related causes in non-occupational exposure groups.
• Limitations the authors state:
  • The exposure categories were crude and did not take account of how long or how intensely each person was exposed.
  • The lack of quantitative exposure data may have led to underestimates of associations.
  • There was potential diagnostic bias: local death certifiers may have been especially aware of asbestos-related disease.
  • Causes of death on death certificates may be less reliable for chronic non-cancer diseases.
  • The reference group (residents) itself had elevated mesothelioma and asbestosis SMRs, so the rate ratios for the other groups were lower than they would have been if residents had been unexposed.
  • The study could not separate asbestos's contribution to lung cancer mortality.
  • The lower-than-expected overall mortality may reflect participation bias, since people who join screening may be healthier or more health-conscious.

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: standardized mortality ratio (SMR), Occupational Exposure, Asbestos Exposure, Lung Cancer, vermiculite, Mesothelioma, Non-Occupational Exposure.

Related studies

• Response to 'Follow-Up of the Libby, Montana Screening Cohort: A 17-year Mortality Study: Likely Underestimation of Nonmalignant Asbestos-Related Disease' — T.C. Larson, L. Williamson, et al. (2020)
• Follow-Up of the Libby, Montana Screening Cohort: A 17-Year Mortality Study: Likely Underestimation of Nonmalignant Asbestos-Related Disease — A. Miller, G.M. Loewen, et al. (2020)
• Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study — P.A. Sullivan (2007)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Low-level fiber-induced radiographic changes caused by Libby vermiculite: a 25-year follow-up study — A.M. Rohs, J.E. Lockey, et al. (2008)
• 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)

Browse more research in Journal of Occupational and Environmental Medicine, or return to medical research.