Analysis of occupational asbestos exposure and lung cancer mortality using the g formula

Our plain-English summary of Analysis of occupational asbestos exposure and lung cancer mortality using the g formula, by S.R. Cole, D.B. Richardson, H. Chu, et al (American Journal of Epidemiology, 2013).

The study

• Authors: S.R. Cole, D.B. Richardson, H. Chu, A.I. Naimi
• Journal: American Journal of Epidemiology
• Published: 2013;177(9):989-96
• Cited by: 52 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

We employed the parametric G formula to analyze lung cancer mortality in a cohort of textile manufacturing workers who were occupationally exposed to asbestos in South Carolina. A total of 3,002 adults with a median age of 24 years at enrollment (58% male, 81% Caucasian) were followed for 117,471 person-years between 1940 and 2001, and 195 lung cancer deaths were observed. Chrysotile asbestos exposure was measured in fiber-years per milliliter of air, and annual occupational exposures were estimated on the basis of detailed work histories. Sixteen percent of person-years involved exposure to asbestos, with a median exposure of 3.30 fiber-years/mL among those exposed. Lung cancer mortality by age 90 years under the observed asbestos exposure was 9.44%. In comparison with observed asbestos exposure, if the facility had operated under the current Occupational Safety and Health Administration asbestos exposure standard of <0.1 fibers/mL, we estimate that the cohort would have experienced 24% less lung cancer mortality by age 90 years (mortality ratio = 0.76, 95% confidence interval: 0.62, 0.94). A further reduction in asbestos exposure to a standard of <0.05 fibers/mL was estimated to have resulted in a minimal additional reduction in lung cancer mortality by age 90 years (mortality ratio = 0.75, 95% confidence interval: 0.61, 0.92).

In plain English

This study re-analyzed records of 3,002 workers at a South Carolina textile plant that used chrysotile asbestos. All had worked there at least one month between 1940 and 1965; follow-up ended at death, loss to follow-up, age 90, or the end of 2001. The authors used the parametric G formula, a statistical method designed to deal with the healthy worker survivor bias, to estimate lung cancer mortality had yearly exposure been capped at past and current OSHA limits. In all, 195 workers died of lung cancer. By age 90, lung cancer mortality was 9.44% in the actual data and 9.22% in a simulation of actual exposure, called the natural course. Capping exposure at 0.1 fibers/mL, the current OSHA standard, gave 6.99%, a risk ratio of 0.76 versus the natural course (95% confidence interval 0.62 to 0.94). A cap of 0.05 fibers/mL gave 6.90%, almost no further drop. The authors concluded that, compared with observed exposure (the natural course), the current standard would have yielded 24% less lung cancer mortality by age 90. They noted possible uncontrolled confounding by smoking.

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 occupational cohort study analyzed with the parametric G formula, which simulates lung cancer mortality under hypothetical exposure limits
• Who was studied: 3,072 men and women employed for at least 1 month between 1940 and 1965 at a textile plant in South Carolina. 70 who left work before age 18 were excluded, leaving 3,002 (median age 24 at entry, 58% male, 81% Caucasian). They were followed for 117,471 person-years between 1940 and 2001. Each person's follow-up ended at loss to follow-up, death, age 90, or December 31, 2001.
• Exposure: Chrysotile asbestos, estimated from detailed work histories with a department-, operation- and calendar-time-specific job exposure matrix. Concentrations were in fibers longer than 5 µm per mL of air, and annual exposure was in fiber-years/mL. 16% of person-years involved exposure, with a median of 3.30 fiber-years/mL among exposed person-years. Simulated caps were 5, 2, 0.1 and 0.05 fibers/mL (the 1971, 1976 and current OSHA standards, and a possible standard).
• Main results:
  • In the whole cohort of 3,002, there were 195 lung cancer deaths and 1,703 other deaths, and 297 (10%) were lost to follow-up.
  • Lung cancer mortality by age 90 under the observed exposure was 9.44% in the observed data. It was 9.22% in the simulated natural course, which served as the reference for all risk ratios and risk differences in Table 3.
  • The authors observed a dose response: lower simulated exposure caps gave lower cumulative lung cancer mortality. By age 90, compared with the natural course, the estimates were 8.56% with a cap of 5 (risk ratio 0.93, 95% CI 0.88, 0.99) and 7.45% with a cap of 2 (risk ratio 0.81, 95% CI 0.71, 0.93).
  • Under the current OSHA standard of 0.1 fiber/mL, lung cancer mortality by age 90 was estimated at 6.99%. Compared with the natural course, the risk difference was minus 2.23 (Table 3 gives this in percent) and the risk ratio was 0.76 (95% CI 0.62, 0.94).
  • Under a possible standard of 0.05 fiber/mL, the estimate was 6.90% (risk ratio 0.75, 95% CI 0.61, 0.92, versus the natural course). Compared with the 0.1 standard, the reduction was near-null (risk ratio 0.99, 95% CI 0.95, 1.04).
  • Compared with the natural course, the numbers needed to treat to protect 1 person were 152, 56, 45 and 43 for the caps of 5, 2, 0.1 and 0.05. With the current OSHA standard as reference, the number was 1,111 for the 0.05 cap.
  • In a sensitivity analysis with follow-up ended in 1980 (64 of the 195 lung cancer deaths), the mortality differences compared with the natural course were −0.94%, −2.05%, −2.37% and −2.47% for the caps of 5, 2, 0.1 and 0.05.
• The authors' conclusion: The authors concluded that, compared with the observed asbestos exposure (represented in the analysis by the simulated natural course), the current OSHA standard of 0.1 fiber/mL would have yielded 24% less lung cancer mortality by age 90. They found that a further reduction to 0.05 fiber/mL yielded no discernible additional reduction.
• Limitations the authors state:
  • Possible uncontrolled confounding by smoking.
  • The effect of asbestos exposure before study entry was not estimated, because of possible selection bias.
  • Other time-varying factors, such as time off work or use of protective equipment, could not be studied and may cause bias.
  • The results depend on the statistical models being correctly specified.
  • Exposure among the exposed was slightly higher in the simulated natural course than in the observed data.

Topics

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

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Lung Cancer, Chrysotile, Fiber Year, Occupational Exposure, confidence interval.

Related studies

• Estimating the effect of cumulative occupational asbestos exposure on time to lung cancer mortality: using structural nested failure-time models to account for healthy-worker survivor bias — A.I. Naimi, S.R. Cole, et al. (2014)
• Lung cancer mortality in North Carolina and South Carolina chrysotile asbestos textile workers — L. Elliott, D. Loomis, et al. (2012)
• Accounting for outcome misclassification in estimates of the effect of occupational asbestos exposure on lung cancer death — J.K. Edwards, S.R. Cole, et al. (2014)
• Lung cancer mortality and fibre exposures among North Carolina asbestos textile workers — D. Loomis, J.M. Dement, et al. (2009)
• Assessing the component associations of the healthy worker survivor bias: occupational asbestos exposure and lung cancer mortality — A.I. Naimi, S.R. Cole, et al. (2013)
• Asbestos standards: Impact of currently uncounted chrysotile asbestos fibers on lifetime lung cancer risk — D.B. Richardson, A.P. Keil, et al. (2018)

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