Environmental tobacco smoke and lung cancer mortality in the American Cancer Society’s Cancer Prevention Study II Cancer Causes and Control. Vol 8. 1997 Victor M. Cardenas, Michael J. Thun, Harland Austin, Cathy A. Lally, W. Scott Clark, Raymond S. Greenberg, and Clark W. Heath, Jr. (Received 8 April 1996; accepted in revised form 5 August 1996) Environmental tobacco smoke (ETS) has been classified as a human lung carcinogen by the United States Environmental Protection Agency (EPA), based both on the chemical similarity of sidestream and mainstream smoke and on slightly higher lung cancer risk in never-smokers whose spouses smoke compared with those married to nonsmokers. We evaluated the relation between ETS and lung cancer prospectively in the US, among 114,286 female and 19,549 male never-smokers, married to smokers, compared with about 77,000 female and 77,000 male never-smokers whose spouses did not smoke. Multivariate analyses, based on 247 lung cancer deaths, controlled for age, race, diet, and occupation. Dose-response analyses were restricted to 92,222 women whose husbands provided complete information on cigarette smoking and date of marriage. Lung cancer death rates, adjusted for other factors, were 20 percent higher among women whose husbands ever smoked during the current marriage than among those married to never-smokers (relative risk [RR] = 1.2, 95 percent confidence interval [CI] = 0.8-1.6). For never-smoking men whose wives smoked, the RR was 1.1 (CI = 0.6-1.8). Risk among women was similar or higher when the husband continued to smoke (RR = 1.2, CI = 0.8-1.8), or smoked 40 or more cigarettes per day (RR = 1.9, CI = 1.0-3.6), but did not increase with years of marriage to a smoker. Most CIs included the null. Although generally not statistically significant, these results agree with the EPA summary estimate that spousal smoking increases lung cancer risk by about 20 percent in never-smoking women. Even large prospective studies have limited statistical power to measure precisely the risk from ETS. Cancer Causes and Control 1997, 8, 57-64 Key words: Lung cancer, environmental tobacco smoke, nonsmokers, United States. Introduction Environmental tobacco smoke (ETS) is a common air pollutant to which many people are exposed. The United States Environmental Protection Agency (EPA) has clas- sified ETS as a known (Group A) human lung carcinogen, 1 based on the numerous carcinogens found in both ETS and mainstream tobacco smoke 2 and on the higher lung cancer risk seen in never-smokers married to smokers in 24 of 30 published epidemiologic studies. 1 From its pooled analysis of 11 published US studies, the EPA estimated that never-smoking women married to smokers had 19 Cancer Causes and Control, 1997, 8, pp. 57-64 Drs Cardenas, Austin, and Clark are with the Epidemiology Division, Rollins School of Public Health, Emory University, Atlanta, GA, USA. Dr Thun, Ms Lally, and Dr Heath, Jr. are with the Department of Epidemiology and Surveillance Research, American Cancer Society, Atlanta, GA. Dr Greenburg was formerly with the Rollins School of Public Health, Emory University, and is now with the Medical University of South Carolina, Charleston, SC. Address correspondence to Dr Thun, Department of Epidemiology and Surveillance Research, American Cancer Society, 1599 Clifton Road, NE, Atlanta, GA 30329-4251, USA. © 1997 Rapid Science Publishers Cancer Causes and Control. Vol 8. 1997 57