Inferences on the kinetics of asbestos deposition and clearance among chrysotile miners and millers

Our plain-English summary of Inferences on the kinetics of asbestos deposition and clearance among chrysotile miners and millers, by M.M. Finkelstein, A. Dufresne (American Journal of Industrial Medicine, 1999).

The study

• Authors: M.M. Finkelstein, A. Dufresne
• Journal: American Journal of Industrial Medicine
• Published: 1999;35(4):401-12
• Cited by: 46 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: The health effects of asbestos are intimately related to the fate of inhaled fibers in the lungs. The kinetics of asbestos fibers have been studied primarily in rodents. The objective of this study was to explore the application of these kinetic models to human autopsy data.

Methods: We analyzed the asbestos fiber content of the lungs of 72 Quebec chrysotile miners and millers and 49 control subjects using analytical transmission electron microscopy. Statistical methods included standard multivariate linear regression and locally weighted regression methods.

Results: The lung burdens of asbestos bodies and chrysotile and tremolite fibers were correlated, as were the concentrations of short, medium, and long fibers of each asbestos variety. There were significant associations between the duration of occupational exposure and the burdens of chrysotile and tremolite. The concentration of chrysotile decreased with the time since last exposure but the concentration of tremolite did not. The clearance rate varied inversely with the length of chrysotile fibers. For fibers greater than 10 mu in length the clearance half-time was estimated to be 8 years.

Conclusions: The patterns in our data are compatible with both of the hypotheses suggested from rodent experiments; the existence of a long-term sequestration compartment and overload of clearance mechanisms in this compartment.

In plain English

This study measured asbestos fibers in autopsy lung tissue from 72 chrysotile miners and millers from Quebec's Asbestos Region, autopsied 1973 to 1992. Of these, 36 had asbestosis, 25 had asbestosis with lung cancer and 11 had mesothelioma. Tissue from 49 subjects with no known asbestos exposure was also measured. The authors used electron microscopy and regression to test fiber build-up and clearance models from rodent experiments. Longer exposure was significantly associated with higher lung levels of asbestos bodies, chrysotile and tremolite. Fiber levels fell with time since last exposure, but the decline was significant only for chrysotile. Estimated chrysotile clearance half-times were 3.81 years (95% confidence interval 2.53 to 7.72) for fibers under 5 microns and 7.92 years (95% confidence interval 3.96 to 1500) for fibers over 10 microns. No tremolite clearance estimate was statistically significant. Smoking was not significantly related to fiber levels. The authors concluded the patterns were compatible with a long-term sequestration compartment in the lung and with clearance overload. They noted the data were cross-sectional, came from relatively few people, lacked exposure-intensity data and need validation.

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 autopsy lung tissue fiber content (cross-sectional data), with regression and kinetic modeling to test rodent-derived models of fiber deposition and clearance
• Who was studied: 72 chrysotile miners or millers from the Asbestos Region of Quebec, selected from 1973 to 1992 autopsy records of the Centre Hospitalier Universitaire de Sherbrooke: 36 with asbestosis, 25 with asbestosis with lung neoplasia, 11 with mesothelioma. Also 49 control subjects from the same region with no known asbestos exposure, who died of accidental causes or acute myocardial infarction between 1990 and 1992.
• Exposure: Occupational exposure in chrysotile mining and milling, measured as years of exposure and years since last exposure. Lung tissue was analyzed for chrysotile, tremolite, crocidolite and amosite fibers in three length groups (under 5, 5 to 10, and over 10 microns) and for asbestos bodies. Many workers also had crocidolite or amosite fibers, possibly from a local asbestos products factory.
• Main results:
  • Among workers, geometric mean concentrations of chrysotile and tremolite were several orders of magnitude higher than in controls. For crocidolite they were about 10-fold higher, and for amosite 2 or 3 times higher.
  • In multiple regression among workers, years of exposure was highly significant for asbestos bodies and for every fiber size of chrysotile and tremolite.
  • Fiber content declined with years since last exposure for each fiber type and length, but the decline was statistically significant only for chrysotile.
  • Chrysotile clearance rate varied inversely with fiber length, with clearance half-times ranging from 4 to 8 years.
  • For chrysotile fibers shorter than 5 microns, the estimated clearance half-time was 3.81 years (95% confidence interval 2.53 to 7.72).
  • For chrysotile fibers longer than 10 microns, the estimated clearance half-time was 7.92 years (95% confidence interval 3.96 to 1500).
  • For tremolite, none of the clearance coefficients was statistically significant. The authors said this was compatible with no clearance of tremolite from the long-term compartment.
  • No significant relations were found between lung fiber concentration and smoking.
  • Smoothed curves suggested a plateau in the mean concentration of longer chrysotile fibers between 30 and 40 years of exposure, with concentrations rising again after 40 years. The authors said this was consistent with overload in men with more than 40 years of exposure.
  • Assuming tremolite made up 1% of deposited dust, the model indicated that transfer of tremolite into the sequestration compartment would be about 20 times greater than that of chrysotile. With 3% tremolite, it would be about 6 times greater.
• The authors' conclusion: The authors concluded that the patterns in their data were compatible with both hypotheses from rodent experiments: a long-term sequestration compartment, and overload of clearance mechanisms in this compartment.
• Limitations the authors state:
  • The data were cross-sectional, so changes over time could not be observed in any individual; the analysis relied on an assumption about lung content at the end of exposure.
  • There was no quantitative information on individual exposure intensities.
  • Few subjects had more than 15 years since last exposure.
  • The rise in chrysotile among men with more than 40 years of exposure might be due to selection bias.
  • The transfer-coefficient estimate depends critically on the assumed relative concentrations of fibers in respirable dust.
  • The inferences rest on relatively few individuals and need validation in other datasets.

Topics

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

Medical terms in this study, explained in our Lexicon: Deposition, Chrysotile, Autopsy, Quebec Chrysotile, Analytical Transmission Electron Microscopy (ATEM), Asbestos Bodies, Tremolite, Occupational Exposure.

Related studies

• Fiber burden and patterns of asbestos-related disease in chrysotile miners and millers — A. Churg, J.L. Wright, et al. (1993)
• Mineralogic correlates of fibrosis in chrysotile miners and millers — A. Churg, J.L. Wright, et al. (1989)
• Lung mineral fibers of former miners and millers from Thetford-Mines and asbestos regions: a comparative study of fiber concentration and dimension — A. Nayebzadeh, A. Dufresne, et al. (2001)
• Deposition and clearance of chrysotile asbestos — A. Churg (1994)
• Screening of miners and millers at decreasing levels of asbestos exposure: comparison of chest radiography and thin-section computed tomography — M. Terra-Filho, E. Bagatin, et al. (2015)
• Mineralogical and exposure determinants of pulmonary fibrosis among Québec chrysotile miners and millers — A. Nayebzadeh, B.W. Case, et al. (2006)

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