Fiber burden and patterns of asbestos-related disease in chrysotile miners and millers

Our plain-English summary of Fiber burden and patterns of asbestos-related disease in chrysotile miners and millers, by A. Churg, J.L. Wright, S. Vedal (The American Review of Respiratory Disease, 1993).

The study

• Authors: A. Churg, J.L. Wright, S. Vedal
• Journal: The American Review of Respiratory Disease
• Published: 1993;148(1):25-31
• Cited by: 63 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

To examine how fiber type, fiber concentration, and fiber size correlate with the presence of asbestos-related disease in workers with heavy chrysotile exposure, we used analytic electron microscopy to determine the fiber content of the lungs of 94 long-term chrysotile miners and millers from the region of Thetford Mines, Quebec. Mesothelioma, airway fibrosis, and asbestosis were strongly associated with a high tremolite fiber concentration, whereas pleural plaques and carcinoma of the lung showed no relationship to tremolite burden. Similar patterns were seen for chrysotile concentration, but further analysis suggested that the apparent effect of chrysotile probably was due to the high correlation (r = 0.70) between chrysotile and tremolite concentration rather than to an independent effect of chrysotile. Increased tremolite-chrysotile ratio was marginally associated with the presence of pleural plaques but not with any other disease. Very high correlations (r > 0.90) between the concentrations of fibers longer or shorter than 8 microns prevented assessment of the effects of long compared with short fibers. Pleural plaques were very strongly associated with higher mean tremolite fiber aspect ratios, but no differences in mean fiber size (length, width, aspect ratio, surface area, and mass) were seen for any other disease. Total fiber size measures (total fiber length/g and others) showed differences similar to fiber concentration for mesothelioma, airways fibrosis, and asbestosis, but no one measure was clearly better than another or better than fiber concentration. We conclude that, in this population of heavily exposed chrysotile miners and millers, the presence of airways fibrosis and asbestosis and, probably, mesothelioma reflects high tremolite burden.(ABSTRACT TRUNCATED AT 250 WORDS)

In plain English

This study measured fibers in autopsy lungs of 94 long-term chrysotile miners and millers from Thetford Mines, Quebec. Cases were chosen to include mesothelioma, asbestosis, airway fibrosis, pleural plaques and lung cancer; only six had no asbestos-related disease. The authors used electron microscopy to count and size chrysotile and tremolite fibers. After accounting for other diseases, both fiber types were higher in people with asbestosis or airway fibrosis than in the six without disease. For mesothelioma the link was of borderline significance. The two fiber levels were strongly correlated (r = 0.70). When each fiber type was analyzed controlling for the other, tremolite stayed linked to combined fibrosis and chrysotile did not. Pleural plaques were linked to tremolite fiber shape (higher aspect ratio), not concentration. Tremolite was higher in lung cancer cases at first, but after accounting for other diseases no fiber measure was associated with lung cancer. The authors concluded that tremolite's role was established and chrysotile's was unclear. They noted that a role for quickly cleared chrysotile could not be ruled out and that the no-disease group was small.

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: Autopsy lung fiber-burden study: analytic electron microscopy of lung tissue, with fiber measures compared across disease groups using t tests and multiple linear regression
• Who was studied: 94 long-term chrysotile miners and millers from the Thetford Mines region, Quebec. They were selected from about 300 autopsy lungs from the Centre Hospitalier de la region de l'Amiante (autopsies 1981 through 1990) to include examples of mesothelioma, asbestosis, airway fibrosis, pleural plaques and lung cancer. Six had no asbestos-related disease.
• Exposure: Heavy occupational exposure to chrysotile ore, which in this region is contaminated with the amphibole tremolite. Retained chrysotile and tremolite fibers were measured: concentration, mean fiber size (length, width, aspect ratio, surface area, mass) and total fiber size measures.
• Main results:
  • Geometric mean tremolite concentrations were 180 in subjects with mesothelioma (15 subjects), 140 with asbestosis (23), 120 with airway fibrosis (33), 75 with pleural plaques (63) and 49 with lung cancer (36). The comparable figure was 9 in the 6 subjects with no asbestos-related disease. Units are fibers per gram dry lung, with a power of ten that is unreadable in the scanned table. All of these differences were significant (p less than 0.05) on the initial t test, lung cancer included.
  • After accounting for the presence of the other diseases, tremolite and chrysotile concentrations were significantly higher in subjects with asbestosis or airway fibrosis. Tremolite was no longer significantly higher in subjects with lung cancer or pleural plaques. The association of mesothelioma with either fiber type was of borderline statistical significance.
  • Tremolite and chrysotile concentrations were strongly correlated (r = 0.70). The authors judged that the apparent effect of chrysotile was probably due to this correlation rather than an independent effect.
  • After controlling for tremolite concentration, no disease was significantly associated with chrysotile concentration. When airway fibrosis and asbestosis were combined into one 'fibrosis' group, tremolite stayed significantly associated with fibrosis after controlling for chrysotile.
  • Concentrations of fibers longer and shorter than 8 micrometers were very highly correlated for both fiber types. This prevented comparison of long versus short fibers.
  • A higher tremolite-to-chrysotile ratio was marginally associated with pleural plaques but not with any other disease.
  • Subjects with pleural plaques had longer mean tremolite fibers (3.0 vs 2.5 micrometers) and higher mean tremolite aspect ratios (19.4 vs 14.5) than those without plaques. Only aspect ratio remained significant after controlling for length.
  • Mean fiber size measures were associated only with pleural plaques. Mean fiber surface area was not associated with any condition. Total fiber size measures showed a pattern similar to fiber concentration.
  • After controlling for the association between lung cancer and the other asbestos-related diseases, no relationship was found between fiber concentration or size and lung cancer.
  • A multiple-regression model limited to the 29 subjects with 40 or more years of exposure also showed no association between lung cancer and tremolite or chrysotile concentration.
• The authors' conclusion: The authors concluded that in this heavily exposed work force, asbestosis, airway fibrosis and probably mesothelioma are primarily generated by high fiber loads. In their view the role of tremolite is established and the role of chrysotile unclear. They added that their data cannot rule out a role for chrysotile fibers that are inhaled and rapidly cleared. Mean fiber size appeared important only for pleural plaques, and no association with lung cancer was shown.
• Limitations the authors state:
  • Only six subjects had no disease, which could have limited the study's power.
  • Long-fiber and short-fiber concentrations were very highly correlated, which confounded attempts to evaluate long versus short fibers.
  • Residual fiber burden cannot show the role of fibers that were briefly present in the lung and then cleared. This is a particular problem for chrysotile.
  • Few mesothelioma subjects lacked airway fibrosis or asbestosis, so conclusions about mesothelioma alone are tentative.
  • The autopsy population is biased toward lung cancer because lung cancer entitles families to apply for compensation.
  • Exposure data included only years of exposure, without exposure levels.

Topics

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

Medical terms in this study, explained in our Lexicon: Fiber Burden, Chrysotile, Thetford Mines, Mesothelioma, Fibrosis, Tremolite, Pleural Plaques, Carcinoma.

Related studies

• Mineralogic correlates of fibrosis in chrysotile miners and millers — A. Churg, J.L. Wright, et al. (1989)
• Fiber burden and patterns of asbestos-related disease in workers with heavy mixed amosite and chrysotile exposure — A. Churg, S. Vedal (1994)
• 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)
• Fiber size and number in workers exposed to processed chrysotile asbestos, chrysotile miners, and the general population — A. Churg, B. Wiggs (1986)
• Inferences on the kinetics of asbestos deposition and clearance among chrysotile miners and millers — M.M. Finkelstein, A. Dufresne (1999)
• Environmental pleural plaques in residents of a Quebec chrysotile mining town — A. Churg, L. DePaoli (1988)

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