Localized pleural thickening: smoking and exposure to Libby vermiculite

Our plain-English summary of Localized pleural thickening: smoking and exposure to Libby vermiculite, by K.Y. Christensen, L. Kopylev (Journal of Exposure Science & Environmental Epidemiology, 2012).

The study

• Authors: K.Y. Christensen, L. Kopylev
• Journal: Journal of Exposure Science & Environmental Epidemiology
• Published: 2012;22(4):320-3
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

There is limited research on the combined effects of smoking and asbestos exposure on risk of localized pleural thickening (LPT). This analysis uses data from the Marysville cohort of workers occupationally exposed to Libby amphibole asbestos (LAA). Workers were interviewed to obtain work and health history, including ever/never smoking and chest X-rays. Cumulative exposure estimates were developed on the basis of fiber measurements from the plant and work history. Benchmark concentration (BMC) methodology was used to evaluate the exposure-response relationship for exposure to LAA and a 10% increased risk of LPT, considering potential confounders and statistical model forms. There were 12 LPT cases among 118 workers in the selected study population. The mean exposure was 0.42 (SD=0.77) fibers/cc-year, and the prevalence of smoking history was 75.0% among cases and 51.9% among non-cases. When controlling for LAA exposure, smoking history was of borderline statistical significance (P-value=0.099), and its inclusion improved model fit, as measured by Akaike's Information Criterion. A comparison of BMC estimates was made to gauge the potential effect of smoking status. The BMC was 0.36 fibers/cc-year, overall. The BMC for non-smokers was approximately three times as high (1.02 fibers/cc-year) as that for the full cohort, whereas the BMC for smokers was about 1/2 that of the full cohort (0.17 fibers/cc-year).

In plain English

This study reanalyzed data on workers at a Marysville, Ohio, plant that used Libby vermiculite, contaminated with Libby amphibole asbestos, to make fertilizer. The authors asked whether smoking changed the link between each worker's estimated cumulative exposure and localized pleural thickening seen on chest X-rays. They studied 118 workers examined in 2002 to 2005 who began work in 1972 or later and reported no previous occupational asbestos exposure. Of these, 12 had localized pleural thickening. Smoking was recorded only as ever or never. Mean cumulative exposure was 0.42 fibers/cc-year. Ever-smoking was reported by 75.0% of cases and 51.9% of non-cases. After accounting for exposure, smoking was of borderline statistical significance (P-value 0.099). A formal test of interaction was not statistically significant. The exposure linked to a 10% extra risk was estimated at 0.36 fibers/cc-year overall, 0.17 for smokers and 1.02 for non-smokers. The authors concluded that smoking may modify the effect of this exposure. They noted the small number of workers, the crude smoking measure, and that only three cases were non-smokers.

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: Analysis of existing worker cohort data (chest X-rays and interviews from a 2002 to 2005 examination), using benchmark concentration exposure-response modeling
• Who was studied: 118 workers at a Marysville, Ohio, USA plant that used Libby vermiculite to make fertilizer. They were drawn from the 280 workers in the 2002 to 2005 examination, began work in 1972 or later, and reported no previous occupational asbestos exposure. There were 12 cases of localized pleural thickening and 106 unaffected workers.
• Exposure: Libby amphibole asbestos from Libby vermiculite, estimated as cumulative exposure. Exposure ranged from 0.001 to 5.51 fibers/cc-year (mean 0.42, SD 0.77). Smoking was also studied, recorded as ever or never.
• Main results:
  • Smoking history was reported by 75.0% of localized pleural thickening cases and 51.9% of non-cases.
  • Controlling for asbestos exposure, smoking history was of borderline statistical significance (P-value 0.099).
  • A formal test of interaction between exposure and smoking was not statistically significant.
  • Time from first exposure, age at X-ray, gender and BMI were not significantly associated with risk of localized pleural thickening after controlling for exposure.
  • For the full study group, the best-fitting model (exposure lagged 10 years) gave a benchmark concentration of 0.36 fibers/cc-year.
  • The exposure level associated with a 10% extra risk of localized pleural thickening was estimated at 0.17 fibers/cc-year for smokers and 1.02 fibers/cc-year for non-smokers.
• The authors' conclusion: The authors concluded that the difference across smoking groups indicates smoking may be an effect modifier of the association between Libby amphibole asbestos exposure and localized pleural thickening.
• Limitations the authors state:
  • The lack of a significant interaction may reflect the limited sample size and the relatively crude measure of smoking.
  • The smoking result was only of borderline significance, possibly because few individuals were analyzed.
  • Only three cases were non-smokers.
  • Differing outcome definitions under different versions of the ILO guidelines limit comparison with earlier studies.
  • BMI was missing for some workers. This matters because fat pads can be mistaken for pleural thickening on X-rays.
  • There were more analytical exposure measurements for later years than for earlier years.

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: Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Pleural Thickening, Libby Vermiculite, Asbestos Exposure, Libby amphiboles, Chest X-Ray, Cumulative Exposure, Work History, Statistical Significance.

Related studies

• Low levels of exposure to libby amphibole asbestos and localized pleural thickening — K.Y. Christensen, T.F. Bateson, et al. (2013)
• 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)
• Mesothelioma associated with commercial use of vermiculite containing Libby amphibole — K.K. Dunning, S. Adjei, et al. (2012)
• 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)

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