Active and passive cigarette smoking and the occurrence of breast cancer.

Using a case-control design, the authors studied female residents of five Massachusetts towns between 1983 and 1986. The objective was to measure the associations between breast cancer occurrence and exposure to active and passive cigarette smoke. Until recently, exposure to tobacco smoke has not been thought to cause breast cancer. Novel perspectives on measuring the association of tobacco smoke with the occurrence of breast cancer and studies of genetically susceptible populations argue for further investigation. In this study, the authors found that ever-active smokers had an odds ratio of 2.0 (95 percent confidence interval (CI) 1.1-3.6) when compared with never-active, never-passive smokers. Women who smoked only before their first pregnancy (odds ratio = 5.6, 95 percent CI 1.5-21) and women who quit smoking 5-15 years before their index year (odds ratio = 3.9, 95 percent CI 1.4-10) were at the highest risk. Passive-only smokers had an odds ratio of 2.0 (95 percent CI 1.1-3.7) when compared with never-active, never-passive smokers. Among those women who were exposed to passive smoke before age 12 years, the odds ratios were 4.5 (95 percent CI 1.2-16) for passive-only smokers and 7.5 (95 percent CI 1.6-36) for ever-active smokers. Women who were first exposed to passive smoke after age 12 years had lower, although still elevated, odds ratios. The pattern of associations between exposure to cigarette smoke and the occurrence of breast cancer comports with a model of breast carcinogenesis.