Background: Studies comparing ever smokers with never smokers have found little increase in breast cancer risk. However, the five published studies examining passive smoking and breast cancer have all suggested associations with both passive and active smoking, particularly premenopausal risk.
Methods: We analyzed data collected through the Canadian National Enhanced Cancer Surveillance System, from 805 premenopausal and 1512 postmenopausal women with newly diagnosed (incident), histologically confirmed, primary breast cancer and 2438 population controls. The mailed questionnaire included questions on breast cancer risk factors and a lifetime residential and occupational history of exposure to passive smoking.
Results: Among premenopausal women who were never active smokers, regular exposure to passive smoke was associated with an adjusted breast cancer odds ratio (OR) of 2.3 (95% confidence interval [CI] 1.2-4.6). Passive exposure showed a strong dose-response trend (test for trend p = 0.0007) with an OR of 2.9 (95% CI 1.3-6.6) for more than 35 years of passive residential and/or occupational exposure. When premenopausal women who had ever actively smoked were compared with women never regularly exposed to passive or active smoke, the adjusted OR for breast cancer was also 2.3 (95% CI 1.2-4.5). Among postmenopausal women who were never-active smokers, regular exposure to passive smoke was associated with an adjusted breast cancer OR of 1.2 (95% CI 0.8-1.8) and an OR of 1.4 (95% CI 0.9-2.3) for the most highly exposed quartile of women. The adjusted OR for postmenopausal breast cancer risk for ever-active smokers compared with women never regularly exposed to passive or active smoke was 1.5 (95% CI 1.0-2.3). Statistically significant dose-response relationships were observed with increasing years of smoking, increasing pack-years and decreasing years since quitting. Women with 35 or more years of smoking had an adjusted OR of 1.7 (95% CI 1.1-2.7).
Conclusions: Active and passive smoking may be associated with increased breast cancer risk, particularly premenopausal risk.