Tobacco Smoke: Active and Passive Exposures
CATEGORY*: IARC known carcinogen, NTP known carcinogen, Endocrine disruptor
FOUND IN: Cigarette smoke, secondhand smoke
THE GIST: Smoking cigarettes is dangerous, addictive and will probably shorten your lifespan. Cigarettes contain hundreds of chemicals, including three known human carcinogens. Research indicates that both passive and active smoking are linked to breast cancer incidence.
State of the Evidence on Tobacco Smoke: Active and Passive Exposures
Tobacco smoke contains polycyclic aromatic hydrocarbons (PAHs), which may explain a potential link between increased breast cancer risk and both active and passive smoking. Tobacco smoke also contains hundreds of other chemicals (Cal-EPA, 2005), including three known human carcinogens (polonium-210, a radioactive element; benzene; and vinyl chloride) as well as toluene, 1,3-butadiene and nicotine-derived nitrosamine ketone (NNK), all of which are known to cause mammary tumors in animals. NNK is a tobacco-specific carcinogen that has been shown to increase tumor cell proliferation and the transformation of healthy breast epithelial cells into cancer cells (Chen 2007; Mei 2003; Siriwardhana 2008).
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A large study of California teachers revealed an increased risk of breast cancer among smokers, particularly those who began smoking during adolescence, who smoked at least five years before their first full-term pregnancy, or who were longtime or heavy smokers (Reynolds, 2004). Several earlier studies also suggest that women who begin smoking cigarettes as adolescents face increased risks of breast cancer (Band, 2002; Calle, 1994; Gram, 2005; Johnson, 2000; Marcus, 2000).
Results from the Canadian National Breast Screening Study indicated that increased incidence of breast cancer was associated with longer duration of smoking, number of cigarettes smoked per day, and cumulative exposure to cigarette smoke (Cui, 2006). Similar results were recorded in recent reports from two large prospective studies: the Nurses Health Study (Xue, 2011) and the Women’s Health Initiative study (Luo, 2011), which involved approximately 110,000 and 80,000 participants, respectively.
Although some studies have reported that beginning smoking before a first full-term pregnancy (independent of age of onset of smoking) may make a woman increasingly susceptible to later diagnosis with breast cancer (e.g., Cui, 2006; Xue, 2011), a recent meta-analysis of 23 relevant research papers did not find a statistically significant relationship (Deroo, 2011).
Researchers at Japan’s National Cancer Center recently reported the results of a study involving 21,000 women ages 40 to 59. They found that both active and passive smoking increase the risk of breast cancer in pre-menopausal women (Hanaoka, 2005). A multi-agency Canadian expert panel report also concluded that active smoking was causally linked to the development of breast cancer and that secondhand smoke (passive smoking) was also independently associated with increased risks for the disease, especially in pre-menopausal women (Johnson, 2011). Other major studies, including the Women’s Health Initiative, support the finding of a link between passive smoking lasting more than 10 years and increased risk for breast cancer (Luo, 2011). Similarly, a recent report by the Institute of Medicine found that decreasing exposures to tobacco smoke was one of the most direct means by which women might decrease their risk for developing breast cancer (IOM, 2012).
Until recently, more evidence linked secondhand smoke than active smoking to breast cancer risk. Current evidence suggests that both exposures increase breast cancer risk by about the same amount, even though women who are exposed to secondhand smoke receive a much lower dose of carcinogens than do active smokers (Ambrosone, 1996; Morabia, 1996). One possible explanation for this is that smoking damages the ovaries, thereby lowering estrogen levels in pre-menopausal women (Dechanet, 2011). Researchers hypothesize that the lower level of estrogen decreases breast cancer risk, while at the same time carcinogens in cigarette smoke increase a smoker’s risk of breast cancer.
But in a recent report examining the association between tobacco smoking and sex hormone levels in post-menopausal women, whose ovaries are no longer the major source of their of circulating hormones , smoking was related to higher levels of testosterone (a source of estrogen when metabolized), estradiol and other steroid hormones (Brand, 2011). The increased levels of circulating estradiol were only statistically significant for women who were considerably overweight. By itself, obesity is a known risk factor for postmenopausal breast cancer. Adipose tissue is the main site of estrogen synthesis in men and postmenopausal women and increased adipose tissue can thus contribute to increased circulating estrogens. Greater activation of breast fat cell metabolic pathways by tobacco-containing chemicals may enhance the development of breast cancer (Wang, 2012).
*For chemicals that have been shown to be carcinogens, we provide classifications from two authoritative bodies: the International Agency for Research on Cancer (IARC, an international body) and the National Toxicology Program (NTP, a division of the U.S. Department of Health and Human Services). We have categorized endocrine-disrupting compounds where the body of peer-reviewed research indicates a strong foundation for doing so.