TORONTO - Postmenopausal women who previously or currently smoke are at greater risk of developing breast cancer compared to their counterparts who never lit up, a new study suggests.

U.S. researchers drew on data from the large-scale Women's Health Initiative Observational Study, which aims to address major causes of illness and death among postmenopausal women. Researchers looked at what, if any, links exist between smoking -- both directly and from second-hand exposure -- and breast cancer.

In the study, published in this week's British Medical Journal, data were analyzed from nearly 80,000 women aged 50 to 79 across 40 clinical centres in the U.S. between 1993 and 1998.

The sample excluded women with a personal history of cancer, except for non-melanoma skin cancer. However, those with a family history of the disease were included.

Study participants were questioned about their smoking status. Those who were current or former smokers were asked the age at which they first started smoking and the number of cigarettes smoked a day. Former smokers were asked the age at which they quit.

Questions concerning second-hand smoking were linked to whether individuals lived in smoking households during childhood and/or adulthood and whether they had worked in smoking environments.

During 10 years of followup, a total of 3,250 cases of invasive breast cancer were identified.

Findings revealed that women who smoked had a 16 per cent increased risk of developing breast cancer following menopause. Meanwhile, the heightened breast cancer risk for former smokers was nine per cent.

"We were able to see the risk elevated in women who smoked as few as five to 15 cigarettes per day; but the more women smoked and the longer women smoked, the higher the risks, and it's really pretty consistent," said study co-author Dr. Karen Margolis, senior clinical investigator with HealthPartners Research Foundation in Minneapolis.

"If you smoked for 30 years or more, clearly your risk is elevated. Women who started as teenagers as opposed to starting at a later age had a more elevated risk, and also that if you quit smoking your risk goes progressively down, but it does not get back down to the rate of a non-smoker until after you've stopped for more than 20 years."

Margolis said that up until about 10 years ago, the data were fairly mixed concerning the link between smoking and breast cancer. Some studies showed an association, others didn't. Since 2002, the evidence has been fairly consistent from large well-conducted studies, she noted.

She also pointed to a Canadian expert panel studying tobacco smoke and breast cancer risk, which in 2009 warned women, particularly young women, to be aware of the risks that come with smoking and exposure to second-hand smoke. The panel had sifted through more than 100 studies on the topic before its report was released.

As for those exposed to second-hand smoke, Margolis said researchers found the group most heavily exposed appeared to be at an increased risk. The group had greater than 10 years of exposure during childhood, greater than 20 years at home as an adult and greater than 10 years in the workplace.

But they didn't find an increased risk with any lesser degree of exposure, she said.

"We looked at lots of different ways that that could be added up and we did not find anything else. So for that reason we're being a little bit cautious in saying we looked at a lot of different things and it's possible that our findings are just due to chance."

In an accompanying editorial, Paolo Boffetta from the Mount Sinai School of Medicine in New York wrote that the study's analysis supports the hypothesis that smoking increases breast cancer risk, "in particular when the habit starts early in life."

However, the professor said the data should be placed in the context of overall evidence and he pointed to other studies that have found no increased risk.

"In the case of second-hand smoke, the evidence can still be considered suggestive of an association at best," he wrote.

Last year, an estimated 5,300 Canadian women died from breast cancer, says the Canadian Cancer Society.

Gillian Bromfield, the society's senior manager of cancer control policy, said whether or not smoking is linked to breast cancer, it is known to be associated with other types of the disease.

She pointed specifically to lung cancer, which kills more Canadians than breast, colorectal and prostate cancers combined.

Bromfield said smoking is associated with 30 per cent of cancer deaths.

"I think the important thing to realize here is whether or not smoking increases the risk of breast cancer or causes breast cancer, it has so many detrimental effects and impacts on so many types of cancer, it's something to be avoided."