Breast Cancer and second hand smoke: the state of the evidence
Michael J. Thun, MD, MS, Epidemiology and Surveillance Research, American Cancer Society, 1599 Clifton Road NE, Atlanta, GA 30329
Second hand smoke or environmental tobacco smoke (ETS) is an established cause of lung cancer and coronary heart disease, and in children, respiratory disease, low birth weight, middle ear infections, and sudden infant death syndrome. Whether ETS also causes breast cancer remains controversial, however, despite documentation that tobacco smoke contains chemicals known to induce mammary tumors in animals, and despite a report from the California Environmental Protection Agency (CalEPA) designating the evidence for breast cancer as “consistent with causation” for premenopausal women. Limitations of the current evidence are: a) the lack of association between active smoking and breast cancer risk in a meta-analysis of 53 studies by Oxford University; b) lack of direct evidence supporting the hypothesis that there is a non-linear relationship between tobacco smoke and breast cancer risk; c) reliance on case-control studies (9 of 12 of which show some association) rather than cohort studies (only 2 of 7 support an association); d) the absence of information in most positive studies on potential confounders such as mammography, post-menopausal hormones, and physical activity; and e) reliance on a highly selected and potentially biased control group in case-control studies. Despite these limitations, the health benefits of smoke-free laws are incontrovertible. Our immediate priorities should be to support vigorous grass roots efforts to promulgate smoke free laws and to strengthen the evidence regarding tobacco smoke and breast cancer in order to obtain scientific, as well as political, consensus on this important issue.