Secondhand smoke, active smoking and breast cancer risk: How strong is the epidemiologic evidence?

IARC, the California Environmental Protection Agency and the U.S. Surgeon General's Office have each weighed in on the issue of passive smoking and breast cancer in the last 4 years, with each expert group coming to a different conclusion.

A meta-analysis of the 20 published studies of passive smoking and breast cancer risk presented pooled relative risk estimates for breast cancer calculated for: 1) life-long nonsmokers with regular passive exposure to tobacco smoke; and 2) women who smoked. They were compared to women who were never regularly exposed to tobacco smoke.

The overall premenopausal breast cancer risk associated with passive smoking among lifelong nonsmokers was 1.68 (95% CI 1.33-2.12) based on 14 studies. In the subset of 5 studies (all case-control studies) with more complete exposure assessment (quantitative long-term information on the three major sources of passive smoke exposure: childhood, adult residential and occupational), the pooled risk estimate for exposed non-smokers was 2.19 (95% CI 1.68-2.84). For the 9 studies with less complete passive exposure measures the risk was 1.35 (95%CI, 1.07-1.71) overall, 1.48 (95% CI 1.00-2.21) for 5 case-control and 1.29 (95% CI 0.93-1.81) for 4 cohort studies

For women who had smoked the breast cancer risk estimate was 1.46 (95%CI 1.15-1.85) when compared to women with neither active nor regular passive smoke exposure; 2.08 (95% CI 1.44- 3.01) for more complete and 1.15 (95% CI 0.92-1.43) for less complete passive exposure assessment.

A compelling case can be made for a breast cancer risk associated with tobacco smoke exposure.