The association between exposure to environmental tobacco smoke and breast cancer: A review by the California Environmental Protection Agency

Background The California Environmental Protection Agency (Cal/EPA) recently completed a health effects assessment of exposure to environmental tobacco smoke (ETS) which resulted in California listing ETS as a Toxic Air Contaminant in January 2006. As part of the assessment, studies on the association between exposure to ETS and breast cancer were reviewed. Methods Twenty-six published reports (including 3 meta-analyses) evaluating the association between ETS exposure and breast cancer were reviewed. A weight-of-evidence approach was applied to evaluate the data and draw conclusions about the association between breast cancer and ETS exposure. Results The published data indicate an association between ETS and breast cancer in younger primarily premenopausal women. Thirteen of 14 studies (10 case–control and four cohort) that allowed analysis by menopausal status reported elevated risk estimates for breast cancer in premenopausal women, seven of which were statistically significant. Our meta-analyses indicated elevated summary relative risks ranging from OR 1.68 (95% C.I. 1.31, 2.15) for all 14 studies to 2.20 (95% C.I. 1.69, 2.87) for those with the best exposure assessment. Conclusions Cal/EPA concluded that regular ETS exposure is causally related to breast cancer diagnosed in younger, primarily premenopausal women and that the association is not likely explained by bias or confounding.