Age of smoking initiation and risk of breast cancer in a sample of Ontario women

Data from two large population-based case-control studies conducted among Ontario women aged 25–75 years were combined for analysis (n = 12,768).Women who had ever smoked and were exposed to passive smoke had a significant increased risk of breast cancer (OR 1.13, 95%CI 1.01–1.25). A significant increased risk was also observed among women who initiated smoking: at age 26 or older (OR 1.26, 95%CI 1.03–1.55); more than five years from menarche (OR 1.26, 95%CI 1.12–1.42); and, after their first live birth (OR 1.25, 95%CI 1.02–1.52).The results suggest that women who initiate smoking at an older age are at an increased risk of breast cancer.Smoking is the largest preventable cause of cancer [1], yet smoking rates among women remain high. In 2007, 18.1% of Canadian women aged 15 and older were current smokers [2], with young adult females aged 20–24 exhibiting the highest prevalence of use (25.6%). Considering that declines in the prevalence of smoking among young adult females have reached a plateau [2], and older ages of smoking onset among young adult women is an emerging trend [3], it appears that smoking will continue to represent a considerable public health burden among Canadian women.Breast cancer, the most common cancer in Canadian women excluding non-melanoma skin cancer, is a leading cause of cancer-related morbidity and mortality in Canada [4]. There is some evidence of a link between tobacco smoke exposure and breast cancer [e.g., [1,5-16]]. Animal models have demonstrated that mammary tissue may have increased susceptibility to carcinogenic exposures during the years from pre-puberty to age at first full-term pregnancy [17,18]. The breast experiences its highest rate of cellular proliferation during this period and there may be a decreased ability of DNA repair mechanisms to correct damage before cell division occurs [6]. This is consistent with the hypothesis that cigarette smoke, either active or passive, in this age period increases the risk of breast c