Young women who smoke - as well as those who are routinely exposed to second-hand smoke - face a significantly higher risk of developing breast cancer later in life, according to a new study.
"Individual women have, on average, a one-in-seven chance of developing breast cancer. If they smoke, they will increase that risk to one-in-four or one-in-five," Anthony Miller, associate director of the Dalla Lana School of Public Health at the University of Toronto, said in an interview.
"The risk is about the same for passive smoke, particularly for a girl growing up in a house where there is a smoker," he said.
Dr. Miller is a member of an expert panel that reviewed the extensive scientific evidence on the link between smoking and breast cancer.
They produced a densely scientific, 75-page report that features a number of clear conclusions, including:
Smoking increases the risk of breast cancer in both premenopausal and postmenopausal women by 50 to 70 per cent.
(A woman's risk of breast cancer rises sharply at menopause. About 70 per cent of breast cancer cases occur after 50.)
Exposure to second-hand smoke increases the risk of breast cancer in premenopausal women by 60 to 70 per cent.
It is unclear if postmenopausal women exposed to second-hand smoke are at increased risk.
Carriers of the so-called breast cancer genes, BRCA1 and BRCA2, more than double their already high risk of breast cancer if they smoke.
Genetics play an important role in determining a woman's risk. For example, those with the NAT2 slow acetylator genotype (about half of women) are far more susceptible to damage from tobacco smoke.
The more a woman smokes and the longer she smokes (or is exposed to tobacco smoke), the more her risk of breast cancer increases.
There is insufficient data available to determine what percentage of breast cancer is caused by tobacco smoke.
Girls who smoke or are exposed to tobacco smoke during puberty, when their breast tissue is growing, seem to be at increased risk of developing cancer prior to menopause, which tends to be more deadly.
"It sends a shiver down my spine when I see girls smoking," Neil Collishaw, research director of Physicians for a Smoke-Free Canada, said in an interview. "Growing tissue is particularly vulnerable to cancer-causing agents."
Dr. Collishaw, who chaired the expert panel, said the report is important because it provides much-needed information in a field where research has evolved, and it is important to get the findings to the public.
"Until recently, the evidence about the link between breast cancer and tobacco smoke, although voluminous, was inconclusive. But the panel's careful analysis of all the available evidence, particularly recent evidence, led us to conclude there is persuasive evidence of risk," he said.
According to the report, there are at least 20 known mammary carcinogens in tobacco smoke. The expert panel noted that the most harmful carcinogens seem to be more abundant in "sidestream smoke" - the tobacco smoke that is not inhaled by smokers.
As a result, while it may seem counterintuitive, "it is possible that carcinogens are more prevalent in passive smoke" and that explains why exposure to second-hand smoke poses about the same risk as smoking itself, Dr. Miller said.
About 19 per cent of Canadians (roughly five million) are current smokers, according to Statistics Canada.
Approximately 15 per cent of teenagers (age 15-19) and 24 per cent of young adults age (20-24) are smokers.
