Low-level fiber-induced radiographic changes caused by Libby vermiculite: a 25-year follow-up study

Our plain-English summary of Low-level fiber-induced radiographic changes caused by Libby vermiculite: a 25-year follow-up study, by A.M. Rohs, J.E. Lockey, K.K. Dunning, et al (American Journal of Respiratory and Critical Care Medicine, 2008).

The study

• Authors: A.M. Rohs, J.E. Lockey, K.K. Dunning, R. Shukla, H. Fan, T. Hilbert, E. Borton, J. Wiot, C. Meyer, R.T. Shipley, G.K. Lemasters, V. Kapil
• Journal: American Journal of Respiratory and Critical Care Medicine
• Published: 2008;177(6):630-7
• Cited by: 48 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Rationale: From 1921 to 1990, vermiculite ore from Libby, Montana, was shipped worldwide for commercial and residential use. A 1980 study of a manufacturing facility using Libby vermiculite was the first to demonstrate a small but significant prevalence of pleural chest radiographic changes associated with amphibole fibers contained in the ore.

Objectives: This follow-up study of the original cohort evaluated the extent of radiographic changes and cumulative fiber exposure (CFE) 25 years after cessation of exposure.

Methods: From the original cohort of 513 workers, 431 (84%) were living and available for participation and exposure reconstruction. Of these, 280 (65%) completed both chest radiographs and interviews. Primary outcomes were pleural and/or interstitial changes.

Measurements And Main Results: Pleural and interstitial changes were demonstrated in 80 (28.7%) and 8 (2.9%) participants, respectively. Of those participants with low lifetime CFE of less than 2.21 fiber/cc-years, 42 (20%) had pleural changes. A significant (P < 0.001) exposure-response relationship of pleural changes with CFE was demonstrated, ranging from 7.1 to 54.3% from the lowest to highest exposure quartile. Removal of individuals with commercial asbestos exposure did not alter this trend.

Conclusions: This study indicates that exposure within an industrial process to Libby vermiculite ore is associated with pleural thickening at low lifetime CFE levels. The propensity of the Libby amphibole fibers to dramatically increase the prevalence of pleural changes 25 years after cessation of exposure at low CFE levels is a concern in view of the wide national distribution of this ore for commercial and residential use.

In plain English

This study followed up the 512 workers studied in 1980 at a manufacturing facility that expanded vermiculite ore from Libby, Montana, plus one more worker found in the original records. The ore contained amphibole fibers that the authors describe as winchite, richterite and tremolite. In 2004 and 2005, about 25 years after exposure ended, 280 living workers completed interviews and had readable chest X-rays. Three radiologists read each X-ray independently. Each worker's cumulative fiber exposure was estimated from past air measurements. Pleural changes (localized pleural thickening, or plaques, and diffuse pleural thickening) were found in 80 of the 280 participants (28.7%). Interstitial changes were found in 8 (2.9%). Pleural changes became more common as exposure rose, from 7.1% in the lowest exposure quarter to 54.3% in the highest. This trend remained after removing workers who reported commercial asbestos exposure. Among workers hired after 1973, a similar trend was not statistically significant. The authors concluded that the amphibole fibers cause pleural thickening at lifetime exposures below 2.21 fiber/cc-years. They noted that nonparticipation, limited early air measurements and unrecorded overtime could have affected their exposure and prevalence estimates.

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: 25-year follow-up of an occupational cohort, with chest radiographs and interviews in 2004–2005 and reconstructed cumulative fiber exposure
• Who was studied: The eligible cohort was 513 workers at a facility expanding Libby vermiculite: the 512 participants of the 1980 study plus 1 additional worker identified from the original records. Of these, 82 had died and 431 were presumed living. 280 completed interviews and had readable chest radiographs. Data were collected 2004–2005.
• Exposure: Amphibole fibers in Libby vermiculite ore (described as winchite, richterite and tremolite), used at the facility from 1963 to 1980. Exposure was expressed as cumulative fiber exposure (CFE) in fiber/cc-years; participants' CFE ranged from 0.01 to 19.03.
• Main results:
  • Among the 280 participants, 80 (28.7%) had pleural changes.
  • Among the 280 participants, 8 (2.9%) had interstitial changes, compared with one (0.2%) participant in the original 1980 study.
  • Among participants, pleural changes increased by exposure quartile: 7.1% in the lowest quartile (0.01–0.28 fiber/cc-years) and 54.3% in the highest (2.21–19.03 fiber/cc-years). The crude odds ratio for the highest quartile was 15.44 (95% CI 5.55–42.98). The trend was statistically significant (p less than 0.001).
  • When the 151 living nonparticipants were included and assumed to have normal X-rays, overall prevalence was 18.6% of 431. It ranged from 3.7% in the lowest quartile to 38.3% in the highest.
  • Among the 252 participants reporting no commercial asbestos exposure, 69 (27.4%) had pleural changes. Prevalence by quartile was 6.4%, 19.1%, 33.3% and 50.8%, and the trend was significant.
  • In the 94 participants (10 cases) and 75 living nonparticipants hired after 1973, a similar trend was seen but was not statistically significant (p = 0.09). The authors attributed this to fewer cases.
  • Pleural changes were found in 37.6% of participants hired on or before 1973, compared with 10.6% of those hired after 1973.
  • The 8 participants with interstitial changes had a mean CFE of 11.86 fiber/cc-years. This was significantly higher than in the 198 participants with no radiographic change.
• The authors' conclusion: The authors concluded that exposure to the amphibole fibers in Libby vermiculite within an industrial process causes pleural thickening at low lifetime CFE levels below 2.21 fiber/cc-years. The abstract uses the wording 'is associated with'. They stated that this level is below the 4.5 fiber/cc-years a worker would accumulate at the current OSHA permissible exposure level over a 45-year working life.
• Limitations the authors state:
  • Possible participation bias: workers hired on or before 1973 were more likely to take part.
  • Possible misclassification of exposure because industrial hygiene data were limited. There were no exposure data before 1972, and personal monitoring did not begin until 1976.
  • Extensive overtime reported by workers was not included in the exposure reconstruction, which may have underestimated exposure.
  • Prevalence may be underestimated, particularly in older workers, because of nonparticipation due to illness or death.
  • Readings of chest radiographs may vary between and within readers.

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: Libby Vermiculite, vermiculite, Amphibole Group, Exposure Reconstruction, Chest X-Ray, Interstitial, Asbestos Exposure, Pleural Thickening.

Related studies

• 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)
• 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)
• Radiographic abnormalities and exposure to asbestos-contaminated vermiculite in the community of Libby, Montana, USA — L.A. Peipins, M. Lewin, et al. (2003)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part III. Radiographic findings — H.E. Amandus, R. Althouse, et al. (1987)
• Vermiculite, respiratory disease, and asbestos exposure in Libby, Montana: update of a cohort mortality study — P.A. Sullivan (2007)

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