Association between cumulative fiber exposure and respiratory outcomes among Libby vermiculite workers

Our plain-English summary of Association between cumulative fiber exposure and respiratory outcomes among Libby vermiculite workers, by T.C. Larson, V.C. Antao, F.J. Bove, et al (Journal of Occupational and Environmental Medicine, 2012).

The study

• Authors: T.C. Larson, V.C. Antao, F.J. Bove, C. Cusack
• Journal: Journal of Occupational and Environmental Medicine
• Published: 2012;54(1):56-63
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objective: To examine the association between cumulative fiber exposure and health outcomes in workers (n = 336) with Libby amphibole exposure.

Methods: Exposure-response relationships were explored by the use of logistic regression, with cumulative fiber exposure modeled in categories and as a continuous variable.

Results: The use of spline functions with lifetime cumulative fiber exposure as a continuous variable showed that the odds of localized pleural thickening were significantly elevated at less than 1 f/cc-y. Odds of parenchymal abnormalities, restrictive spirometry, and chronic bronchitis were also significantly elevated at 108, 166, and 24 f/cc-y, respectively.

Conclusions: The odds of several pulmonary health outcomes are correlated with cumulative exposure to Libby amphibole. That relatively low-lifetime cumulative exposures are associated with localized pleural thickening has implications for the non-cancer-risk assessment for Libby amphibole.

In plain English

This cross-sectional study looked at 336 former workers of the vermiculite operation in Libby, Montana. They took part in a government community health screening held in 2000 and 2001 and could be matched to company records. Each worker's cumulative fiber exposure at the vermiculite operation (all fiber types) was estimated from past air sampling and job histories, in fibers per cubic centimeter-years (f/cc-y). These estimates were compared with chest X-rays, breathing tests and questionnaire answers. Localized pleural thickening was found in 117 workers (35%). Its odds became statistically significantly raised at lifetime exposures below 1 f/cc-y, although the risk declined above 20 f/cc-y. Odds of parenchymal (lung tissue) abnormalities, restrictive breathing-test results and chronic bronchitis became significantly raised at 108, 166 and 24 f/cc-y respectively. Diffuse pleural thickening, shortness of breath and excess cough showed no statistically significant association. The authors concluded that the results provide evidence that Libby amphibole causes adverse lung outcomes. They noted that only 18% of eligible workers took part, that the exposure estimates may contain errors, and that non-work amphibole exposure in Libby may also have misclassified workers' exposure.

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: Cross-sectional study linking community screening data to estimated cumulative fiber exposure, analyzed with logistic regression (exposure in quartiles and as smoothed spline curves)
• Who was studied: 336 former Libby, Montana vermiculite workers who took part in the ATSDR community screening program (2000 to 2001) and could be matched to company records, out of 370 participants who reported working there; 93.2% male, median age 55.6 years
• Exposure: Work at the Libby vermiculite operation. The article describes the amphibole fibers in Libby vermiculite as winchite, richterite, tremolite and actinolite. Exposure was measured as cumulative fiber exposure in f/cc-y. It was estimated from a NIOSH job-exposure matrix based on historical air sampling at the operation, which counted fibers of all types by phase contrast microscopy. ATSDR supplemented the estimates for 1981 to 1993. The estimates covered occupational exposure only, not non-work exposure. The median among subjects was 3.6 f/cc-y.
• Main results:
  • Among the 336 workers, 46% (n = 154) had pleural abnormalities: 117 had localized pleural thickening (35%) and 18 had diffuse pleural thickening (5%)
  • Localized pleural thickening: in the spline analysis, odds were statistically significant (lower 95% confidence limit above 1) at lifetime exposures below 1 f/cc-y
  • Localized pleural thickening, quartile model: odds were significantly elevated only in the third exposure quartile (3.6 to 15.7 f/cc-y; OR 2.3, 95% CI 1.1 to 4.9 in Table 3) and were lower in the fourth. In the spline curve, risk declined above 20 f/cc-y.
  • Diffuse pleural thickening: odds ratios for the upper quartiles ranged from 1.7 to 2.3, with no steady trend and confidence intervals that included 1
  • Parenchymal abnormalities were found in 18 workers (5%). The quartile model showed a non-significant increase for exposure of 15.7 f/cc-y or more (OR 6.8, 95% CI 0.6 to 78.8 in Table 3). In the spline analysis, odds became significantly elevated at 108 f/cc-y.
  • Restrictive spirometry was found in 16% of workers. Odds were not significantly elevated in the highest quartile (OR 1.5, 95% CI 0.5 to 4.9). In the spline analysis, odds became significant at 166 f/cc-y.
  • Chronic bronchitis was reported by 26 workers (8%). Odds were significantly elevated in the third exposure quartile (OR 3.8, 95% CI 1.2 to 12.4). In the spline analysis, odds became significant at 24 f/cc-y.
  • Shortness of breath (66 workers, 20%) and excess cough (49 workers, 15%) showed positive but not statistically significant exposure-response curves
• The authors' conclusion: The authors concluded that the exposure-response curves provide evidence that Libby amphibole is a causative agent of adverse pulmonary outcomes. They noted that the level at which localized pleural thickening odds became significant (below 1 f/cc-y) is far below the 4.5 f/cc-y a worker would accumulate at the current permissible exposure level over 45 years. They said the results have implications for public health policy and non-cancer risk assessment.
• Limitations the authors state:
  • Possible selection bias: only 18% of the eligible worker cohort participated, and participants had lower exposure and worked later in the operation's history
  • More highly exposed workers may already have been diagnosed and chosen not to participate, which may have lowered the prevalence of localized pleural thickening at high exposure levels
  • Exposure estimates may contain errors: they rested on judgments about limited air sampling, with few samples before 1969 and none before 1956
  • Additional exposure misclassification from non-occupational (community) amphibole exposure in Libby
  • Self-reported respiratory symptoms may have been biased

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: Pleural plaques and thickening, Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Libby Vermiculite, Libby amphiboles, Pleural Thickening, Spirometry, Cumulative Exposure.

Related studies

• Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole — T.C. Larson, V.C. Antao, et al. (2010)
• Asbestos-related radiographic findings among household contacts of workers exposed to Libby vermiculite: impact of workers' personal hygiene practices — T.J. Hilbert, A. Franzblau, et al. (2013)
• Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study — P.A. Sullivan (2007)
• 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)
• 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)
• Libby vermiculite exposure and risk of developing asbestos-related lung and pleural diseases — V.C. Antao, T.C. Larson, et al. (2012)

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