This new large prospective study examined the association between long-term passive smoke exposure and risk of breast cancer among 57,523 women in the California Teachers Study who had never been smokers and were free of any history of breast cancer. The women were followed for 10 years, during which time 1,754 of the women were diagnosed with invasive breast cancer. For women exposed during adulthood, risk of breast cancer was found to be slightly elevated at the highest level of lifetime exposure, primarily among postmenopausal women.

Link to breast cancer is not strong

While the detrimental effects of smoking and secondhand smoke on cardiovascular health and some cancers (among them lung, head and neck, stomach and bladder) are well documented, the link between smoking and breast cancer has been found to be weak or undetectable in numerous studies. However, alcohol consumption appears to heighten the risk associated with smoking (and, possibly, secondhand smoke). Since the U.S. has eliminated most workplace and public smoking, the main sources of secondhand smoke currently are likely to be in vehicles and in the home. We would encourage smokers living with women who have been diagnosed with cancer or who are at high risk of it to take steps to eliminate secondhand smoke exposure for them.