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Breast-Cancer Risk Doubles For Women Who Smoke, Study Finds

Jun 3, 2005 |

Women who smoke, or who have long-term exposure to second-hand smoke, have more than double the risk of developing breast cancer in their childbearing years, according to a new Canadian study.

The research, published in the International Journal of Cancer, provides strong evidence that smokers are much more likely to develop premenopausal breast cancer a link that has long been subject to dispute.

More significantly, the study shows that women who never smoked, but were exposed to tobacco smoke in the home (as children or adults) and in the workplace, have virtually the same risk.

"Essentially, we see a doubling of risk," Kenneth Johnson, a research scientist at the Public Health Agency of Canada, said in an interview.

"We see similar rates of premenopausal breast cancer for active and passive smoking," Dr. Johnson said.

While a woman's risk of developing breast cancer increases sharply after menopause, about 20 per cent of breast-cancer cases are diagnosed in premenopausal women.

It is unclear why second-hand smoke would increase risk as much as smoking, but the speculation is that the toxins in cigarette smoke -- and there are more than 4,000 chemicals in the smoke -- influence levels of the hormone estrogen.

There is already much evidence that smoking and exposure to second-hand smoke affect the reproductive system. Women who smoke have a much higher rate of miscarriage and they enter menopause two to four years earlier than non-smokers.

The new research is a meta-analysis, a compilation of previous studies. Dr. Johnson analyzed data from 20 studies, but he noted that the quality varied widely.

In the five studies with the best data including information about smoking and exposure to second-hand smoke over time the link between breast cancer and exposure was most clear. In studies where there was poor exposure data, the link was far less clear.

Dr. Johnson, who is an epidemiologist, said one of the most important aspects of the new research is that it is the first to look at differences between studies.

"If you don't collect data, you could miss 20 years of exposure," he said. Practically speaking, that means in many studies, participants listed as non-smokers may actually have been passive smokers.

This, in turn, likely blurred the fact that there is a link between exposure to tobacco smoke and breast cancer, Dr. Johnson said.

"It's a paradigm shift to see this and understand it," he said.

In earlier research on women aged 45 to 75, Dr. Johnson found that almost 50 per cent had smoked at some point, and that 80 per cent had long-term exposure to second-hand smoke.

"You have to realize that, until recently, we had epidemic levels of exposure to second-hand smoke," he said.

The International Agency for Research on Cancer, the cancer research arm of the World Health Organization, declared last year that second-hand smoke is a carcinogen.

The IARC said that second-hand smoke increases the risk of more than one dozen types of cancer, but it said there was insufficient evidence to link second-hand smoke, or even active smoking, to breast cancer.

In 2005, an estimated 21,600 women and 150 men will be diagnosed with breast cancer, and about 5,300 women and 45 men will die of breast cancer, according to the Canadian Cancer Society.

While there are clear genetic and reproductive risk factors, and much-explored links to exercise and alcohol use, more than half of breast cancer risk remains unexplained, according to background in the new research.

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