Young women who smoke or are exposed to second-hand smoke face an increased risk of breast cancer, suggests an expert panel.
The Ontario Tobacco Research Unit published the report, based on a review by experts researching the latest evidence.
"Even moderate exposure to passive smoking such as living or working with a smoker early in life increases a woman's risk of breast cancer when she's in her 30s, 40s or 50s," said panelist Anthony Miller, a professor and associate director of research at the Dalla Lana School of Public Health at the University of Toronto.
"That's important information that people should know."
An estimated 80 per cent to 90 per cent of women have been exposed to tobacco smoke in adolescence and adulthood, and face an increased risk from the exposure, said Neil Collishaw, chair of the panel.
The relationship between active smoking for women of all ages shows a cause-and-effect relationship, the panel concluded.
But there wasn't enough evidence to draw a link between exposure to second-hand smoke and breast cancer for older, post-menopausal women.
The panel also found that women with a particular genetic makeup are 35 per cent to 50 per cent more likely to develop breast cancer if they smoke. About half of women in North America have this genetic makeup (known as the NAT2 slow acetylation genotype), depending on ethnicity.
The findings show smoking is keeping cancer rates high beyond those suggested by lung cancer statistics, said Dr. Gerald Batist, who heads a cancer treatment and research centre at McGill University in Montreal.
"The fact that breast cancer isn't declining may also be related to that, so it's just another element that's fixable in preventing breast cancer," Batist said.
The panel reviewed evidence at a two-day conference in Toronto in November.
The review was funded by the Public Health Agency of Canada.