Women who are former or current smokers as well as those who have been exposed to decades of second-hand smoke face a higher risk of breast cancer after menopause, according to a new study.

Smokers and women who quit up to 20 years previously are more at risk of the disease compared to other postmenopausal women who have never smoked or have never been around second-hand smoke, researchers said in Wednesday's issue of the British Medical Journal.

Prof. Juhua Luo of the department of community medicine at West Virginia University and her co-authors looked for confirmed cases of invasive breast cancer among nearly 80,000 American women 50 to 79 years old who entered the study between 1993 and 1998.

Over an average followup of 10 years, 3,520 cases of breast cancer were found in the Women's Health Initiative study, which is funded by the U.S. National Heart, Lung and Blood Institute.

Current smokers had 1.16 times the breast cancer risk compared to women who never smoked, the team found. Breast cancer risk was raised by 1.09 times among former smokers after menopause. Twenty years after butting out, the risk appeared drop off.

"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. 

Female smoking and empowerment

On Tuesday, a report published in the World Health Organization's Bulletin said millions of women in developing countries risk an early death by 2030 as rising economic and political status leads more people to smoke.

In countries like Canada and Norway with relatively high female employment, women smoke almost as much as men do.

The analysis of data from 74 countries concluded men are nearly five times more likely to smoke in countries with lower rates of female empowerment in terms of representation in parliament, voting rights and income.

Those countries included China, Indonesia, Pakistan, Saudia Arabia and Uganda.

Geoffrey Fong of the University of Waterloo and his co-authors said the findings point to the need for tobacco control measures like advertising bans, more prominent graphic warnings and higher cigarette taxes to prevent the tobacco industry from targeting women.

The link between increased risk of breast cancer and second-hand smoke was only found for extensive exposure  — more than 10 years recalled during childhood, more than 20 years at home as an adult and more than a decade at work.

Since there was no clear trend for lower doses of second-hand smoke, the researchers cautioned the data is suggestive of a link that needs to be confirmed by other studies.

"Our findings highlight the need for interventions to prevent initiation of smoking, especially at an early age, and to encourage smoking cessation at all ages," the study authors concluded.

Until recently, most scientists agreed there wasn't enough consistent evidence to show whether active smoking can cause breast cancer, the researchers said.

The authors pointed to a 2009 review by a Canadian panel of experts who  examined more than 100 population and toxicology studies. The reviewers concluded that young women in particular need to be aware of the breast cancer risk of second-hand smoke.

The latest findings support the idea that smoking increases breast cancer risk "in particular when the habit starts early in life," Paolo Boffetta from the Mount Sinai School of Medicine in New York said in a journal editorial.

Boffetta also cautioned that the second-hand smoking evidence should be "considered suggestive of an association at best."

The researchers did not collect data on the intensity or frequency of second-hand smoke exposure, which they said may have hampered their ability to detect an effect from increasing doses.

More than 80 per cent of women in Western countries have been exposed to second-hand smoke at home or at work, according to a study cited by Luo's team.

With files from The Canadian Press