An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers

Our plain-English summary of An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers, by L. Stayner, E. Kuempel, S. Gilbert, et al (Occupational and Environmental Medicine, 2008).

The study

• Authors: L. Stayner, E. Kuempel, S. Gilbert, M. Hein, J. Dement
• Journal: Occupational and Environmental Medicine
• Published: 2008;65(9):613-9
• Cited by: 67 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: Evidence from toxicological studies indicates that the risk of respiratory diseases varies with asbestos fibre length and width. However, there is a total lack of epidemiological evidence concerning this question.

Methods: Data were obtained from a cohort mortality study of 3072 workers from an asbestos textile plant which was recently updated for vital status through 2001. A previously developed job exposure matrix based on phase contrast microscopy (PCM) was modified to provide fibre size-specific exposure estimates using data from a re-analysis of samples by transmission electron microscopy (TEM). Cox proportional hazards models were fit using alternative exposure metrics for single and multiple combinations of fibre length and diameter.

Results: TEM-based cumulative exposure estimates were found to provide stronger predictions of asbestosis and lung cancer mortality than PCM-based estimates. Cumulative exposures based on individual fibre size-specific categories were all found to be highly statistically significant predictors of lung cancer and asbestosis. Both lung cancer and asbestosis were most strongly associated with exposure to thin fibres (<0.25 microm). Longer (>10 microm) fibres were found to be the strongest predictors of lung cancer, but an inconsistent pattern with fibre length was observed for asbestosis. Cumulative exposures were highly correlated across all fibre size categories in this cohort (0.28-0.99, p values <0.001), which complicates the interpretation of the study findings.

Conclusions: Asbestos fibre dimension appears to be an important determinant of respiratory disease risk. Current PCM-based methods may underestimate asbestos exposures to the thinnest fibres, which were the strongest predictor of lung cancer or asbestosis mortality in this study. Additional studies are needed of other asbestos cohorts to further elucidate the role of fibre dimension and type.

In plain English

This retrospective cohort mortality study followed 3072 workers from an asbestos textile plant in Charleston, South Carolina, through 2001. The plant mainly used chrysotile asbestos, plus small amounts of crocidolite yarn from the 1950s until 1975. The authors adjusted earlier light-microscopy (phase contrast) exposure estimates using electron-microscopy analysis of 84 archived air samples, giving each worker cumulative exposures by fibre length and width. They tested how well each measure predicted 198 lung cancer deaths and 62 asbestosis cases. Electron-microscopy-based estimates predicted both diseases better than light-microscopy estimates. Every fibre size category was a highly statistically significant predictor. When fibres were compared by width alone, very thin fibres (under 0.25 micrometres) were the strongest predictors of both diseases, although the best-fitting length-and-width combinations were wider. Longer fibres (over 10 micrometres) predicted lung cancer most strongly, while asbestosis showed no clear trend with length. Exposures across size categories were highly correlated (0.28 to 0.99), which the authors called the most serious limitation. They also could not account for cigarette smoking in the analysis. The authors concluded that fibre dimension appears to be an important determinant of respiratory disease risk.

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: Retrospective cohort mortality study using Cox proportional hazards models with fibre size-specific exposure estimates
• Who was studied: 3072 workers (white and non-white men and women) from an asbestos textile plant in Charleston, South Carolina, USA, with vital status follow-up through 31 December 2001. The analysis included 198 lung cancer deaths and 62 asbestosis cases. There were only three mesothelioma deaths, and these were not analysed.
• Exposure: Mainly chrysotile asbestos from Quebec, British Columbia and Zimbabwe, with small amounts of crocidolite yarn used from the 1950s until 1975. Cumulative exposure was estimated from a job exposure matrix built on 5952 phase contrast microscopy samples from 1930 to 1975. This matrix was adjusted using transmission electron microscopy of 84 archived samples from 1965 and 1968, covering 18 840 fibres, of which only two were amphiboles.
• Main results:
  • Of the fibres measured by electron microscopy, 93% were very short (5 micrometres or less) and thin (under 0.25 micrometres). These would not have been counted by traditional phase contrast microscopy.
  • Electron-microscopy-based cumulative exposure fitted the data better than phase-contrast-based exposure. For lung cancer, the fit statistic (minus 2 log likelihood, lower is better) was 2494.2 versus 2498.7. For asbestosis, it was 743.6 versus 750.2.
  • All fibre size-specific cumulative exposure estimates were highly statistically significant predictors of lung cancer or asbestosis mortality.
  • In analyses based on fibre width alone, very thin fibres (under 0.25 micrometres in diameter) were most strongly associated with both lung cancer and asbestosis.
  • Some models combined a specific fibre length and diameter. For lung cancer, the best fit among these came from fibres 20 to 40 micrometres long and 0.25 to 1.0 micrometres in diameter (minus 2 log likelihood 2486.5, chi-square 65.9).
  • For asbestosis, the best fit of any model came from fibres over 40 micrometres long and 1 to 3.0 micrometres in diameter (minus 2 log likelihood 718.5, chi-square 110.6).
  • For lung cancer, among the length-only categories, the strongest relationship was with fibres 20 to 40 micrometres long (chi-square 62.1).
  • For asbestosis, there was no clear trend with fibre length. Among the length-only categories, fibres 10 to 20 micrometres long were the strongest predictors (chi-square 92.3).
  • Short fibres (5 micrometres or less) were compared with longer fibres (over 5 micrometres). For asbestosis, short fibres fitted slightly better. For lung cancer, longer fibres fitted slightly better.
  • Cumulative exposures in the different size categories were correlated from 0.28 to 0.99, all highly statistically significant. Models including several size categories together generally failed because of this correlation.
• The authors' conclusion: The authors concluded that asbestos fibre dimension appears to be an important determinant of respiratory disease risk. They added that current phase contrast methods may underestimate exposure to the thinnest fibres, which were the strongest predictor in this study. They called for studies of other asbestos cohorts.
• Limitations the authors state:
  • The study could not include other lung cancer risk factors, most notably cigarette smoking.
  • Only 84 electron microscopy samples were used to adjust the exposure matrix, and all came from 1965 to 1968. The authors therefore had to assume that fibre sizes stayed constant over the whole study period.
  • There were likely substantial errors in exposure classification.
  • The authors identified the high correlation between the size-specific exposure measures as the most serious limitation. It made it hard to separate the role of each fibre dimension.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Chrysotile, phase contrast microscopy (PCM), Transmission Electron Microscopy (TEM), Cumulative Exposure, Lung Cancer, Asbestos Exposure.

Related studies

• Asbestos fibre dimensions and lung cancer mortality among workers exposed to chrysotile — D. Loomis, J. Dement, et al. (2010)
• Increased lung cancer mortality among chrysotile asbestos textile workers is more strongly associated with exposure to long thin fibres — D. Loomis, J.M. Dement, et al. (2012)
• Development of a fibre size-specific job-exposure matrix for airborne asbestos fibres — J.M. Dement, E.D. Kuempel, et al. (2008)
• An epidemiological study of the respiratory health of workers in the European refractory ceramic fibre industry — H.A. Cowie, P. Wild, et al. (2001)
• Asbestos standards: Impact of currently uncounted chrysotile asbestos fibers on lifetime lung cancer risk — D.B. Richardson, A.P. Keil, et al. (2018)
• Lung cancer mortality in North Carolina and South Carolina chrysotile asbestos textile workers — L. Elliott, D. Loomis, et al. (2012)

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