Updates on exposure estimation for the Québec cohort of chrysotile miners and millers: implications for risk assessment and threshold hypothesis

Our plain-English summary of Updates on exposure estimation for the Québec cohort of chrysotile miners and millers: implications for risk assessment and threshold hypothesis, by B.W. Case, A.A. Korchevskiy, A. Korchevskiy (Frontiers in Public Health, 2025).

The study

• Authors: B.W. Case, A.A. Korchevskiy, A. Korchevskiy
• Journal: Frontiers in Public Health
• Published: 2025;13:1654772
• Cited by: 1 article, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The McGill University cohort of Québec chrysotile miners and millers (total of 11,379 members) has been a major determinant of risk assessment. The U. S. EPA and some other meta-analyses omit this cohort, the largest ever exposed to high concentrations of chrysotile. Preliminary analyses using new data showed average cumulative exposures in the full cohort of 528.5 f/cc-years (95% CI 510.3, 546.7) or 979 f/cc-years (95% CI 945, 1,013), depending on the regression equation used for the modeling. The lowest estimated exposure level observed for any mesothelioma case with diagnostic certainty was 68 f/cc-years. We confirmed that the estimation of potency factor for Québec chrysotile should not be expected to differ significantly from Darnton's estimation for non-textile chrysotile (RM = 0.0014%), if better quality data for correspondence between impinger and fiber count measurements is involved. Among several estimations we made, the average meta-analytical mesothelioma potency factor for all types of chrysotile would be 0.0014% (95% CI 0.0010, 0.0.0018), and for non-textile chrysotile 0.0011% (95% CI 0.0008, 0.0015). Several estimations of a possible threshold that would correspond to non-elevated mesothelioma risk based on the Québec cohort were also made. The minimal cumulative exposure in mesothelioma cases demonstrated values that ranged from 4.8-6 MPCF-years, or 18-50 f/cc-years. Also, based on theoretical models, the threshold can be found at the level of 6.38 MPCF-years (standard deviation 2.6), or in the range from 35.9 f/cc-years (standard deviation of 15.4) to 65.9 f/cc-years (standard deviation of 29.6).

Copyright © 2025 Case, Korchevskiy and Korchevskiy.

In plain English

This paper re-analyzed data from the McGill cohort of Québec chrysotile miners and millers. Old midget impinger dust counts (MPCF) were converted to fiber counts (f/cc) using 621 paired measurements and two equations. After exclusions, 8,486 persons with 30 mesothelioma deaths were analyzed. Estimated average cumulative exposure was 528.5 f/cc-years (95% CI 510.3, 546.7) or 979 f/cc-years (95% CI 945, 1,013), depending on the equation. These gave meta-analytic mesothelioma potency estimates for all chrysotile of 0.0010% to 0.0017%, compared with 0.0014% in Darnton's earlier analysis. The article gives conflicting minimum exposures for mesothelioma cases. Its Table 2 gives 28.6 or 50 f/cc-years, depending on the equation and unchanged by excluding low-confidence diagnoses, and 18 f/cc-years for the original estimates. Its abstract gives 68 f/cc-years for a case with diagnostic certainty. The authors' models gave average threshold estimates of 6.38 MPCF-years (standard deviation 2.6), and of 35.9 f/cc-years (standard deviation 15.4) or 65.9 f/cc-years (standard deviation 29.6), depending on the equation. The authors concluded the new estimates do not significantly change Darnton's potency estimates. They stated their analysis cannot demonstrate that a threshold exists. All diagnoses predated modern immunohistochemical tests.

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: Re-analysis of existing cohort data, in three parts: re-estimation of historical exposure (converting midget impinger dust counts to fiber counts), an updated meta-analysis of chrysotile mesothelioma potency, and statistical modeling and simulation of possible thresholds
• Who was studied: Québec chrysotile miners and millers in the McGill cohort, born 1891 to 1920. The authors used an archived dataset of 9,244 cohort members. They excluded factory workers with amphibole exposure and short-term workers with other asbestos exposure. That left a sub-cohort of 8,486 persons with 30 mesothelioma deaths, 27 of which were at least 'moderately' confirmed pathologically.
• Exposure: Chrysotile dust in Québec mines and mills, originally measured by midget impinger in MPCF. The authors converted it to phase-contrast microscopy counts of fibers longer than 5 μm (f/cc), using 621 paired measurements reported by Dagbert. Exposure was expressed as cumulative exposure in MPCF-years or f/cc-years.
• Main results:
  • Across all 621 paired datapoints, the regression linking fiber counts to impinger counts (Equation 4) was weak but statistically significant (R = 0.46, R2 = 0.21). After averaging groups of 30 datapoints (Equation 5), R was 0.91 and R2 was 0.82.
  • Average cumulative exposure for the cohort was estimated at 528.5 f/cc-years (95% CI 510.3, 546.7) using Equation 4 and 979 f/cc-years (95% CI 945, 1,013) using Equation 5. Previously published estimates were 600 f/cc-years.
  • In the simulation, Equation 4 underestimated observed average exposure. Equation 5 came closer: the ratio of observed to predicted exposure was not less than 0.8, except possibly for outliers.
  • Using Equation 4 and adding a Russian cohort, the meta-analytic potency factor for all types of chrysotile was 0.0017%, compared with Darnton's 0.0014%.
  • Using Equation 5, the potency factor for chrysotile was 0.0010%, compared with 0.0014% in Darnton. For non-textile cohorts it was 0.0008%.
  • Averaged across the authors' estimations, the mesothelioma potency factor for all types of chrysotile was 0.0014% (95% CI 0.0010 to an upper limit printed as '0.0.0018'). For non-textile chrysotile it was 0.0011% (95% CI 0.0008, 0.0015).
  • In Table 2, the minimal exposure associated with mesothelioma using Equation 4 was 4.8 MPCF, or 28.6 f/cc-years. This was the same with all 30 cases and with the 27 cases left after low-confidence diagnoses were excluded. The original Liddell, Berman and Crump scenario (33 cases) gave 6 MPCF, or 18 f/cc-years.
  • In Table 2, the minimal exposure associated with mesothelioma using Equation 5 was 4.8 MPCF, or 50 f/cc-years. This was the same with all 30 cases and with the 27 cases left after low-confidence diagnoses were excluded.
  • The abstract printed with the article gives a different figure from Table 2: the lowest estimated exposure for any mesothelioma case with diagnostic certainty was 68 f/cc-years. This figure does not appear in the body of the article.
  • Based on the authors' theoretical linear-log models, the average threshold value was 6.38 MPCF-years (standard deviation 2.6). In fiber units it was 35.9 f/cc-years (standard deviation 15.4) or 65.9 f/cc-years (standard deviation 29.6), depending on the conversion equation.
  • In random selections of cohort members, no mesothelioma cases were found for groups with average cumulative exposure of 113 MPCF-years by impinger, 309 f/cc-years by Equation 4, or 577 f/cc-years by Equation 5.
  • Lung fiber data were available for 15 mesothelioma cases. From these, the authors estimated that chrysotile lung burden would not be expected to be elevated below 178 to 307 f/cc-years. They roughly estimated a tremolite asbestos threshold of about 5 f/cc-years.
• The authors' conclusion: The authors concluded that more robust exposure estimates for the Québec cohort are not expected to significantly change Darnton's meta-analytic potency estimates for chrysotile, even when the Russian cohort is added. They also concluded that excluding the Québec cohort from meta-analysis would bias risk analysis.
• Limitations the authors state:
  • The analysis cannot demonstrate that a threshold exists.
  • The range of threshold estimates is wide.
  • Tremolite, an amphibole, occurs in the Québec mines. There is debate over whether tremolite rather than chrysotile is responsible for mesothelioma in the cohort.
  • The proportion of fibers among the total dust particles counted by midget impinger remains unknown.
  • Reconstructing cumulative exposure in fiber units for each individual is difficult, because parallel measurements are limited and confounders are present.
  • All cases were diagnosed before immunohistochemical tests became available. The authors excluded 3 of the 30 cases because their diagnostic probability was low.
  • Only limited lung fiber burden data exist for Québec chrysotile workers.
  • The modeled relationship between mesothelioma mortality and cumulative exposure depends on how exposure categories are chosen.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Cumulative Exposure, Mesothelioma, Midget Impinger.

Related studies

• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: development from 1904 and mortality to 1992 — F.D. Liddell, A.D. McDonald, et al. (1997)
• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88 — J.C. McDonald, F.D. Liddell, et al. (1993)
• Exposure to chrysotile mining dust and digestive cancer mortality in a Chinese miner/miller cohort — S. Lin, X. Wang, et al. (2014)
• Cancer mortality in chrysotile miners and millers, Russian Federation: main results (Asbest Chrysotile Cohort-Study) — J. Schüz, E. Kovalevskiy, et al. (2024)
• The combination of effects on lung cancer of cigarette smoking and exposure in quebec chrysotile miners and millers — F.D. Liddell, B.G. Armstrong (2002)
• Mortality in a Chinese chrysotile miner cohort — X. Wang, S. Lin, et al. (2012)

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