The risk of lung cancer with increasing time since ceasing exposure to asbestos and quitting smoking

Our plain-English summary of The risk of lung cancer with increasing time since ceasing exposure to asbestos and quitting smoking, by A. Reid, N.H. de Klerk, G.L. Ambrosini, et al (Occupational and Environmental Medicine, 2006).

The study

• Authors: A. Reid, N.H. de Klerk, G.L. Ambrosini, G. Berry, A.W. Musk
• Journal: Occupational and Environmental Medicine
• Published: 2006;63(8):509-12
• Cited by: 40 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: To examine if the risk of lung cancer declines with increasing time since ceasing exposure to asbestos and quitting smoking, and to determine the relative asbestos effect between non-smokers and current smokers.

Methods: A cohort study of 2935 former workers of the crocidolite mine and mill at Wittenoom, who responded to a questionnaire on smoking first issued in 1979 and on whom quantitative estimates of asbestos exposure are known. Conditional logistic regression was used to relate asbestos exposure, smoking category, and risk of lung cancer.

Results: Eighteen per cent of the cohort reported never smoking; 66% of cases and 50% of non-cases were current smokers. Past smokers who ceased smoking within six years of the survey (OR = 22.1, 95% CI 5.6 to 87.0), those who ceased smoking 20 or more years before the survey (OR = 1.9, 95% CI 0.50 to 7.2), and current smokers (<20 cigarettes per day (OR = 6.8, 95% CI 2.0 to 22.7) or >20 cigarettes per day (OR = 13.2, 95% CI 4.1 to 42.5)) had higher risks of lung cancer compared to never smokers after adjusting for asbestos exposure and age. The asbestos effect between non-smokers and current smokers was 1.23 (95% CI 0.35 to 4.32).

Conclusion: Persons exposed to asbestos and tobacco but who subsequently quit, remain at an increased risk for lung cancer up to 20 years after smoking cessation, compared to never smokers. Although the relative risk of lung cancer appears higher in never and ex-smokers than in current smokers, those who both smoke and have been exposed to asbestos have the highest risk; this study emphasises the importance of smoking prevention and smoking cessation programmes within this high risk cohort.

In plain English

The authors studied 2935 former workers of the Wittenoom crocidolite (blue asbestos) mine and mill, Western Australia, who answered smoking questionnaires. They compared 138 workers who developed lung cancer by 2002 with matched workers without it. Adjusted for asbestos exposure, odds ratios versus never smokers were 22.1 (95% CI 5.6 to 87.0) for quitting under six years earlier, 9.3 (2.2 to 40.1) for 6 to 9 years, 8.9 (2.6 to 30.1) for 10 to 19 years, and 1.9 (0.50 to 7.2) for 20 or more years, the last not statistically significant. Excluding people diagnosed within three years of quitting lowered the first to 16.3 (3.5 to 76.1). Current smokers had 6.8 (2.0 to 22.7) under 20 cigarettes a day and 13.2 (4.1 to 42.5) above, a cut-off given as both over 20 and 20 or more. The relative asbestos effect was 1.23 (0.35 to 4.32), including 1, the value under a multiplicative interaction; the authors read 1.23 as less than multiplicative. They concluded former smokers stayed at increased risk up to 20 or more years, noting smoking status may have been misclassified.

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: Cohort study in which lung cancer cases were compared with non-cases of the same age band and sex using conditional logistic regression
• Who was studied: 2935 former workers (199 women, 2736 men; 42% of the original workforce) of the Australian Blue Asbestos crocidolite mine and mill at Wittenoom, Western Australia, who completed at least one smoking questionnaire between 1979 and 2000. 202 with incomplete smoking information were excluded, leaving 138 lung cancer cases and 2595 non-cases. Lung cancers were counted to September 2002 (Western Australian residents) or September 2000 (others).
• Exposure: Crocidolite only, mined and milled at Wittenoom between 1943 and 1966. Each worker's cumulative exposure was estimated in fibre-years per ml from dust measurements and employment records. Smoking status, amount smoked and time since quitting came from questionnaires.
• Main results:
  • There were 138 incident lung cancer cases in the cohort between 1979 and 2002.
  • Cases had longer and greater cumulative asbestos exposure than non-cases (geometric mean cumulative exposure 9.2 versus 5.5 f/ml-year). In the adjusted model, the odds ratio per log unit of cumulative exposure was 1.17 (95% CI 1.05 to 1.30).
  • Compared with never smokers, the odds ratio was 22.1 (95% CI 5.6 to 87.0) for former smokers who had quit less than six years earlier. This was adjusted for time since first asbestos exposure and cumulative exposure.
  • Compared with never smokers, the odds ratio was 9.3 (95% CI 2.2 to 40.1) for former smokers who quit 6 to 9 years earlier and 8.9 (95% CI 2.6 to 30.1) for those who quit 10 to 19 years earlier.
  • For former smokers who had quit 20 or more years earlier, the odds ratio was 1.9 (95% CI 0.50 to 7.2, p = 0.341). This was not significantly greater than for never smokers.
  • Compared with never smokers, current smokers of fewer than 20 cigarettes a day had an odds ratio of 6.8 (95% CI 2.0 to 22.7). The heavier current smokers had an odds ratio of 13.2 (95% CI 4.1 to 42.5). This group is labelled more than 20 per day in Table 3 but 20 or more a day in the results text.
  • After excluding people diagnosed with lung cancer within three years of quitting (n = 6), the odds ratio for those who had quit six years or less earlier fell to 16.3 (95% CI 3.5 to 76.1).
  • People who stopped smoking at age 50 or older (n = 25) had an odds ratio of 1.41 (95% CI 0.67 to 2.97) compared with those who stopped younger (n = 19). This difference was not statistically significant.
  • The relative asbestos effect was 1.23 (95% CI 0.35 to 4.32). It compared never smokers plus those who quit 20 or more years earlier with current smokers plus those who quit less than 20 years earlier. The modified measure was 1.59 (95% CI 0.12 to 20.50).
  • The article states that under a multiplicative interaction the relative asbestos effect equals 1. The 95% CI of 0.35 to 4.32 includes that value.
  • The authors interpreted the RAE of 1.23 and RAEm of 1.59 as a less-than-multiplicative interaction.
  • The authors stated that although the relative effect of asbestos appeared higher in never and ex-smokers, people who both smoked and were exposed to asbestos had the highest risk.
• The authors' conclusion: The authors concluded that, in this crocidolite-exposed cohort, former smokers remained at increased risk of lung cancer compared with never smokers for up to 20 or more years after quitting.
• Limitations the authors state:
  • About 30% of participants last answered the smoking questionnaire in 1979, and their smoking status was assumed unchanged through 2002.
  • Former smokers who relapsed would inflate the estimated risk for former smokers. The authors suggested this may partly explain the very high risk in recent quitters.
  • Current smokers who later quit but were counted as still smoking could weaken the estimated effect of current smoking.
  • Some people reporting not smoking may in fact have still been smoking because of social stigma.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Wittenoom, Western Australia

All asbestos mines

Topics

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

Medical terms in this study, explained in our Lexicon: Lung Cancer, Cohort Study, Crocidolite, Wittenoom, Asbestos Exposure, Relative Risk.

Related studies

• Mesothelioma risk after 40 years since first exposure to asbestos: a pooled analysis — A. Reid, N.H. de Klerk, et al. (2014)
• Effects of asbestos and smoking on gas diffusion in people exposed to crocidolite — H.S. Alfonso, L. Fritschi, et al. (2005)
• Effects of asbestos and smoking on the levels and rates of change of lung function in a crocidolite exposed cohort in Western Australia — H.S. Alfonso, L. Fritschi, et al. (2004)
• A pilot study of telephone-based smoking cessation intervention in asbestos workers — O.Y. Osinubi, J. Moline, et al. (2003)
• Study of smoking behavior in asbestos workers — O.Y. Osinubi, A.A. Afilaka, et al. (2002)
• Time-dependent effect of intensity of smoking and of occupational exposure to asbestos on the risk of lung cancer: results from the ICARE case-control study — E. Lévêque, A. Lacourt, et al. (2018)

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