Quantitative relationships of exposure to chrysotile asbestos and mesothelioma mortality

Our plain-English summary of Quantitative relationships of exposure to chrysotile asbestos and mesothelioma mortality, by D. Loomis, D.B. Richardson, L. Elliott (American Journal of Industrial Medicine, 2019).

The study

• Authors: D. Loomis, D.B. Richardson, L. Elliott
• Journal: American Journal of Industrial Medicine
• Published: 2019;62(6):471-477
• Cited by: 24 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: While asbestos has long been known to cause mesothelioma, quantitative exposure-response data on the relation of mesothelioma risk and exposure to chrysotile asbestos are sparse.

Methods: Quantitative relationships of mortality from mesothelioma and pleural cancer were investigated in an established cohort of 5397 asbestos textile manufacturing workers in North Carolina, USA. Eligible workers were those employed between 1950 and 1973 with mortality follow-up through 2003. Individual exposure to chrysotile fibres was estimated on the basis of 3420 air samples covering the entire study period linked to work history records. Exposure coefficients adjusted for age, race, and time-related covariates were estimated by Poisson regression.

Results: Positive, statistically significant associations were observed between mortality from all pleural cancer (including mesothelioma) and time since first exposure (TSFE) to asbestos (rate ratio [RR], 1.19; 95% confidence interval [CI], 1.06-1.34 per year), duration of exposure, and cumulative asbestos fibre exposure (RR, 1.15; 95% CI, 1.04-1.28 per 100 f-years/mL; 10-year lag). Analyses of the shape of exposure-response functions suggested a linear relationship with TSFE and a less-than-linear relationship with cumulative exposure. Restricting the analysis to years when mesothelioma was coded as a unique cause of death yielded stronger but less precise associations.

Conclusions: These observations support with quantitative data the conclusion that chrysotile causes mesothelioma and encourage exposure-response analyses of mesothelioma in other cohorts exposed to chrysotile.

© 2019 The Authors. American Journal of Industrial Medicine Published by Wiley Periodicals, Inc.

In plain English

This study analysed deaths among 5397 men and women who worked at least one day between 1950 and 1973 in three North Carolina mills making asbestos yarns and woven goods, followed through 2003. The authors described these workers as exposed to chrysotile. Each worker's fibre exposure was estimated from 3420 air samples linked to job histories. There were four deaths from mesothelioma and four from cancer of the pleura. Deaths from these causes combined rose with years worked (rate ratio 1.10 per year; 95% CI 1.04-1.16), with time since first exposure (1.19 per year; 1.06-1.34) and with cumulative exposure (1.15 per 100 fibre-years/mL; 1.04-1.28; 10-year lag). A smoothed curve suggested the rise was less steep above about 2000 fibre-years/mL. When time since first exposure was in the same model, cumulative exposure was not significantly associated. After 1999, when mesothelioma had its own death code, associations were larger but less precise. The authors noted few deaths and likely undercounting of mesothelioma on older death certificates. They concluded that the findings support the conclusion that chrysotile causes mesothelioma.

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 study (further analysis of an established cohort) using Poisson regression of mortality on exposure measures
• Who was studied: 5397 men and women employed for at least 1 day between 1950 and 1973 in three North Carolina mills that produced asbestos yarns and woven goods from raw fibres. Vital status was followed through 2003, giving 172 860 person-years. A subcohort of 2803 workers still at risk from 1999 to 2003 was also analysed.
• Exposure: Chrysotile asbestos fibres in textile manufacturing. Individual exposure was estimated from 3420 air samples taken from the 1930s to the 1980s and linked to work histories. Exposure measures were duration of exposure, time since first exposure, and cumulative fibre exposure in fibre-years/mL.
• Main results:
  • Over 1950 to 2003 the cohort had four deaths with mesothelioma (ICD-10 C45) and four deaths with cancer of the pleura as the underlying cause. All occurred more than 20 years after beginning employment.
  • In the full cohort, mortality from all pleural cancer (including mesothelioma) was positively associated with exposure duration (RR 1.10; 95% CI 1.04-1.16 per year) and with time since first exposure (RR 1.19; 95% CI 1.06-1.34 per year).
  • In the full cohort, cumulative exposure lagged 10 years gave an RR for pleural cancer mortality of 1.15 (95% CI 1.04-1.28) per 100 f-years/mL. With 20- and 30-year lags the RRs were 1.16 (1.04-1.29) and 1.17 (1.03-1.34).
  • With 40- and 50-year lags, the confidence intervals included 1: RR 1.19 (95% CI 0.89-1.59) and RR 1.14 (95% CI 0.64-2.01) per 100 f-years/mL.
  • When cumulative exposure was added to the model for time since first exposure, there was no significant association with cumulative exposure.
  • Smoothing the cumulative-exposure data suggested a curvilinear (less than exponential) relationship. The relationship appeared to rise less steeply above about 2000 f-years/mL.
  • Fitting the EPA/OSHA absolute risk model to the full cohort gave a coefficient of 0.088 × 10^-8 (95% CI 0.027 × 10^-8 to 0.149 × 10^-8) per f-year/ml.
  • In the subcohort at risk from 1999 to 2003 (4 mesothelioma deaths), the patterns were similar. The RRs for time since first exposure and cumulative exposure were larger but less precise than in the full cohort.
  • In that 1999 to 2003 subcohort, Table 4 gives a mesothelioma mortality RR of 1.28 (95% CI 1.02-1.60) per year of time since first exposure. For cumulative exposure lagged 10 years it gives 1.39 (95% CI 0.96-2.04) per 100 f-years/mL.
• The authors' conclusion: The authors concluded that positive, statistically significant associations of pleural cancer and mesothelioma mortality with cumulative chrysotile exposure, duration of exposure and time since exposure support the conclusion that chrysotile causes mesothelioma and provide quantitative data for risk assessment. They encouraged similar analyses in other chrysotile-exposed cohorts.
• Limitations the authors state:
  • Reduced precision due to the small number of informative deaths.
  • Concerns about the quality of mesothelioma ascertainment on death certificates, particularly before specific ICD codes for mesothelioma existed. Under-ascertainment was the primary concern.
  • The post-1999 subcohort findings should be viewed with caution. It included only 8% of total person-years and favoured workers hired later, followed longer and with lower cumulative exposures.
  • The separate effects of cumulative exposure and time since first exposure are difficult to estimate because of limited power and their mutual dependence on time.
  • Fewer exposure measurements and less detailed work history information were available than for the South Carolina cohort. Greater measurement error may have weakened the associations.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Mesothelioma, Work History, rate ratio, confidence interval, Cumulative Exposure.

Related studies

• 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)
• Lung cancer mortality in North Carolina and South Carolina chrysotile asbestos textile workers — L. Elliott, D. Loomis, et al. (2012)
• Asbestos fibre dimensions and lung cancer mortality among workers exposed to chrysotile — D. Loomis, J. Dement, et al. (2010)
• Lung cancer mortality and fibre exposures among North Carolina asbestos textile workers — D. Loomis, J.M. Dement, et al. (2009)
• Cancer mortality in Chinese chrysotile asbestos miners: exposure-response relationships — X. Wang, E. Yano, et al. (2013)
• Exposure-response relationship between chrysotile exposure and mortality from lung cancer and asbestosis — Q. Deng, X. Wang, et al. (2012)

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