Asbestos, Asbestosis, Smoking, and Lung Cancer. New Findings from the North American Insulator Cohort

RATIONALE: Asbestos, smoking, and asbestosis increase lung cancer risk in incompletely elucidated ways. Smoking cessation among asbestos-exposed cohorts has been little studied. OBJECTIVES: To measure the contributions of asbestos exposure, asbestosis, smoking, and their interactions to lung cancer risk in an asbestos-exposed cohort and to describe their reduction in lung cancer risk when they stop smoking. METHODS: We examined lung cancer mortality obtained through the National Death Index for 1981 to 2008 for 2,377 male North American insulators for whom chest X-ray, spirometric, occupational, and smoking data were collected in 1981 to 1983 and for 54,243 non-asbestos-exposed blue collar male workers from Cancer Prevention Study II for whom occupational and smoking data were collected in 1982. MEASUREMENTS AND MAIN RESULTS: Lung cancer caused 339 (19%) insulator deaths. Lung cancer mortality was increased by asbestos exposure alone among nonsmokers (rate ratio = 3.6 95% confidence interval (CI), 1.7-7.6), by asbestosis among nonsmokers (rate ratio = 7.40 95% CI, 4.0-13.7), and by smoking without asbestos exposure (rate ratio = 10.3 95% CI, 8.8-12.2). The joint effect of smoking and asbestos alone was additive (rate ratio = 14.4 95% CI, 10.7-19.4) and with asbestosis, supra-additive (rate ratio = 36.8 95% CI, 30.1-45.0). Insulator lung cancer mortality halved within 10 years of smoking cessation and converged with that of never-smokers 30 years after smoking cessation. CONCLUSIONS: Asbestos increases lung cancer mortality among nonsmokers. Asbestosis further increases the lung cancer risk and, considered jointly with smoking, has a supra-additive effect. Insulators benefit greatly by quitting smoking.