Passive smoking and breast cancer in never smokers: prospective study and meta-analysis.

BACKGROUND: Active smoking has little or no effect on women's risk of developing breast cancer, but it has been suggested that passive exposure to tobacco smoke may increase this risk among women who have never smoked. OBJECTIVE: To evaluate the possible relationship between passive smoking and breast cancer risk within the Million Women Study, a large UK prospective study, and to report a meta-analysis of published results. METHODS: In the large prospective study, 224 917 never smokers who completed a questionnaire that asked women whether their parents had smoked and if their current partner smoked were followed up for an average of 3.5 years for incident breast cancer. In the meta-analysis, studies that had recorded exposure information prospectively and retrospectively were considered separately. Main outcome measures Adjusted relative risk of breast cancer in never smokers who were passively exposed to tobacco smoke at various ages compared with never smokers with no such exposure. RESULTS: In the prospective study, 2518 incident invasive breast cancers occurred during follow-up and the adjusted relative risk of breast cancer for passive exposure either as a child or as an adult vs neither exposure was 0.98 (95% CI 0.88-1.09); results were similarly null for childhood exposure (0.98, 0.88-1.08) and adult exposure (1.02, 0.89-1.16) separately. We identified seven other studies with prospectively recorded exposure data; when results of all eight studies were combined (including 5743 never smokers with breast cancer), the aggregate relative risk was 0.99 (0.93-1.05) for any passive exposure. The aggregate findings differed substantially (P = 0.0002) between these 8 studies and 17 other studies with retrospectively recorded information (including 5696 never smokers with breast cancer). CONCLUSIONS: Aggregate results from studies with prospectively reported information show that the incidence of breast cancer is similar in women who did and did not report passive exposure to tobacco smoke either as a child or as an adult. The aggregate findings from the retrospective studies may have been distorted by some women becoming more likely to report past exposures because they knew that they had breast cancer.