Exposure and mineralogical correlates of pulmonary fibrosis in chrysotile asbestos workers

Our plain-English summary of Exposure and mineralogical correlates of pulmonary fibrosis in chrysotile asbestos workers, by F.H. Green, R. Harley, V. Vallyathan, et al (Occupational and Environmental Medicine, 1997).

The study

• Authors: F.H. Green, R. Harley, V. Vallyathan, R. Althouse, G. Fick, J. Dement, R. Mitha, F. Pooley
• Journal: Occupational and Environmental Medicine
• Published: 1997;54(8):549-59
• Cited by: 42 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: The relation between lifetime cumulative exposure to asbestos, pathological grade of pulmonary fibrosis, and lung burden of asbestos at death, was explored in a necropsy population of former workers in a chrysotile asbestos textile plant in South Carolina.

Methods: Estimates of cumulative, mean, and peak exposures to asbestos were available for 54 workers. Necropsy records and lung tissue samples were obtained from hospital files. Matched control cases were selected from consecutive necropsies performed at the same hospitals. The extent and severity of pulmonary fibrosis was graded on tissue sections. Mineral fibres in lung tissue were characterised by transmission electron microscopy combined with x ray spectroscopy.

Results: A significant positive correlation (r = 0.67, P < 0.0001) was found between lifetime cumulative exposure to asbestos and total lung burden of asbestos fibres. This relation was also found for the individual types of asbestos associated with the exposure: chrysotile and tremolite. Pulmonary fibrosis was correlated with both cumulative exposure to asbestos (r = 0.60, P < 0.01) and the concentration of asbestos fibres in the lung (r = 0.62, P < 0.0001). The concentration of tremolite fibres in the lung provided a better estimate of lung fibrosis than did the concentration of chrysotile. Asbestosis was usually present in asbestos textile workers with more than 20 fibre-years cumulative exposure. The lengths and aspect ratios of chrysotile asbestos, but not amphibole asbestos, were greater in the lungs of asbestos fibre workers than in the control population. Textile workers with lung cancer had significantly greater cumulative exposures and fibrosis scores than workers without lung cancer.

Conclusions: Both cumulative exposure to asbestos and lung fibre burden are strongly correlated with severity of asbestosis. The data also support the hypothesis that the high prevalence of asbestosis and lung cancer in this population resulted from exposure to long fibres of chrysotile asbestos in the workplace.

In plain English

The authors studied autopsy lung tissue from former workers at a chrysotile asbestos textile plant in Charleston, South Carolina, and matched controls from the same hospitals. Lung scarring (fibrosis) was graded in 54 workers and 34 controls, and lung fibres were analysed in 39 workers and 31 controls. Workers' estimated cumulative exposure correlated with total lung asbestos fibres (r = 0.67) and fibrosis score (r = 0.60). Fibrosis also correlated with lung asbestos fibre burden (r = 0.62); tremolite predicted it better than chrysotile. Asbestosis of grade 2 or more was consistently found in workers with over 20 fibre-years, and also in three workers with 10 to 20 fibre-years. The authors could not determine a threshold exposure. Chrysotile fibres in workers' lungs were longer than controls' (median 1.5 versus 0.5 micrometres). The 10 workers with lung cancer had higher cumulative exposures and fibrosis scores than other workers. The authors concluded that cumulative exposure and lung fibre burden both predicted asbestosis. They also concluded that the dimensions of airborne fibres in textile work most plausibly explained this group's high lung cancer and asbestosis mortality.

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 autopsy (necropsy) study of former asbestos textile workers compared with matched autopsy controls, with grading of lung tissue and electron microscopy analysis of lung fibres
• Who was studied: Former workers at an asbestos textile plant in Charleston, South Carolina, drawn from a cohort of 3744 people employed between 1940 and 1965 and followed to 1975. Pathology was graded for 54 workers (44 men, 10 women). There were 34 controls (22 men, 12 women), matched by age, sex, hospital and year of death from autopsies at the same hospitals. Lung fibre analysis covered 39 workers and 31 controls.
• Exposure: Chrysotile asbestos was the only type processed as raw material, mostly from Quebec. A very small amount (less than 1000 kg) of crocidolite yarn was used from the 1950s to about 1975. Exposure was estimated by job and calendar period as lifetime cumulative exposure in fibre-years. The median was 30.2 fibre-years, with a range of 0.1 to 370.0.
• Main results:
  • In the textile workers, lifetime cumulative exposure was correlated with total asbestos fibres in the lung (r = 0.67, P less than 0.0001). Tremolite (r = 0.48) and chrysotile (r = 0.55) lung fibres were each also correlated with cumulative exposure.
  • In the 54 workers, fibrosis score was related to lifetime cumulative exposure (r = 0.60, P less than 0.01). No independent effect of peak exposure, mean exposure, time since last exposure or latency was found.
  • In the workers, fibrosis score was related to total lung asbestos fibre burden (r = 0.62), tremolite (r = 0.64) and chrysotile (r = 0.51). No relation was found with non-asbestos fibres. Tremolite concentration predicted fibrosis better than chrysotile concentration.
  • Chrysotile and tremolite fibres in the lung were much more numerous in the asbestos workers than in the controls (P less than 0.0001). Geometric means, in fibres x 10^6 per gram of dry lung, were: chrysotile 33.45 in workers versus 6.71 in controls; tremolite 3.56 versus 0.26.
  • Median fibrosis scores were higher in workers than in controls at every level (quartile) of cumulative exposure. The difference was significant for all quartiles except the lowest (0.1 to 7.1 fibre-years).
  • Looking at individual cases, asbestosis of grade 2 severity or more was consistently found in workers with lifetime cumulative exposures above 20 fibre-years. It was also found in three cases with exposures of 10 to 20 fibre-years.
  • Chrysotile fibres in workers' lungs had a median length of 1.5 micrometres and a median aspect ratio of 23.1. In controls' lungs the figures were 0.5 micrometres and 7.7. Amphibole fibre lengths and aspect ratios were generally similar in the two groups.
  • According to autopsy reports, lung cancer was recorded in 18.2% of the asbestos workers and 5.8% of the controls. Pleural plaques were reported in 31% of workers and 3% of controls.
  • The 10 workers with lung cancer had a higher geometric mean cumulative exposure than workers without lung cancer: 94.6 versus 21.3 fibre-years (P = 0.03). Their mean fibrosis score was also higher: 7.7 versus 4.7 (P = 0.01). Total lung asbestos fibres were 61.1 versus 41.5 (P = 0.42). Nine of the 10 had asbestosis diagnosed on the original autopsy report, compared with 14 of 45 workers without lung cancer.
• The authors' conclusion: The authors concluded that lung asbestos fibre concentration reflected past workplace exposure. They concluded that both cumulative exposure and lung fibre burden were accurate predictors of asbestosis. They also concluded that the dimensions of airborne fibres in asbestos textile operations were the most plausible explanation for the high mortality from lung cancer and asbestosis in this population.
• Limitations the authors state:
  • Subjects were hospital patients who had autopsies. The authors could not control for selection for autopsy, the lung site sampled, or the method of lung fixation, which could introduce unknown bias.
  • Smoking histories were available for only a few subjects and smoking was not used in the analyses. The authors judged its effect likely to be minor.
  • There were few cases at low and intermediate exposure, so the model could not determine a minimal or threshold exposure level for asbestosis.
  • The number and quality of tissue sections varied, and the lung site of origin of most sections could not be determined.
  • The authors stated that the study does not provide a definitive explanation for the very high lung cancer risk in this population.

Topics

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

Medical terms in this study, explained in our Lexicon: Pulmonary Fibrosis, Chrysotile, Cumulative Exposure, Autopsy, Transmission Electron Microscopy (TEM), Tremolite, Fibrosis, Aspect Ratio (Length-to-diameter ratio).

Related studies

• Mineralogical and exposure determinants of pulmonary fibrosis among Québec chrysotile miners and millers — A. Nayebzadeh, B.W. Case, et al. (2006)
• An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers — L. Stayner, E. Kuempel, et al. (2008)
• Lung cancer mortality in North Carolina and South Carolina chrysotile asbestos textile workers — L. Elliott, D. Loomis, et al. (2012)
• Asbestos standards: Impact of currently uncounted chrysotile asbestos fibers on lifetime lung cancer risk — D.B. Richardson, A.P. Keil, et al. (2018)
• Follow-up study of chrysotile asbestos textile workers: cohort mortality and case-control analyses — J.M. Dement, D.P. Brown, et al. (1994)
• Lung cancer in chrysotile asbestos workers: analyses based on the two-stage clonal expansion model — D.B. Richardson (2009)

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