The study is published in the journal BMJ.
Adding to a decade’s worth of research supporting that link, the study shows that both smokers and former smokers are at significant risk of developing breast cancer.
“Since 2002, important studies with huge numbers of women have been showing a strong link,” says study co-researcher Karen Margolis, MD, a senior clinical investigator at HealthPartners Research Foundation in Minneapolis. “This adds to the weight of the evidence.”
The study, led by Juhua Luo, PhD, of West Virginia University, drew upon the Women’s Health Initiative Observational Study, which was conducted between 1993 and 1998. Researchers analyzed data on nearly 80,000 women ages 50 to 79.
The participants came from diverse backgrounds and from all over the U.S., says Margolis. During the 10 years that the participants were followed, 3,520 of them were diagnosed with invasive breast cancer.
Smoking Habits and Breast Cancer Risk
Each study participant identified herself as a smoker, former smoker, or lifelong nonsmoker. The smokers and former smokers also answered questions about their smoking history: how old they were when they started to smoke, how many cigarettes a day they smoked, and how long it had been since they quit, if they were not still smoking. The nonsmokers were asked about the degree to which they had been exposed to secondhand smoke over the course of their lives.
Margolis and co-researchers found that compared to nonsmokers, smokers are 16% more likely to be diagnosed with breast cancer after menopause. And the longer a woman smokes, the higher those numbers climb. Among women who began smoking at an early age, before their first pregnancy, the risk of breast cancer was as high as 21%.
For former smokers, the risk is 9% greater than for lifelong nonsmokers. And while quitting reduces the risk, the researchers report that it can take up to twenty years before their risk matches that of women who never smoked.
More Research Needed
Not everyone is so certain, however.
Paolo Boffetta, MD, deputy director of the Tisch Cancer Institute at Mount Sinai School of Medicine in New York City, says that while there is growing evidence of a link between smoking and breast cancer, it is still relatively new and needs further study.
“I think it is a bit too early to say,” says Boffetta, who wrote an editorial that accompanies the Luo study. “I’m a little bit cautious, but the evidence is becoming more and more convincing ... it will emerge more strongly in five or 10 years.”
Secondhand Smoke and Breast Cancer Risk
The researchers also looked for a link between breast cancer and secondhand smoke. They found that for women who had the most extensive exposure to secondhand smoke -- in the home or the workplace -- their breast cancer risk was 32% higher than normal.
Margolis is quick to point out that a lot more study is needed to confirm that finding. For one thing, only 12% of the women in the study said that they had not been exposed to secondhand smoke, so the comparison group that the study used was quite small.
“Secondhand smoke was almost ubiquitous,” Margolis says. “Women aged 50-79 are almost universally exposed.”
Though no link has been conclusively established, Margolis does say that it is biologically plausible that heavy secondhand smoke exposure could cause breast cancer.
“The risk is likely to be elevated,” Margolis says. “You see the same type of damage to DNA in their breast cancer tissue in women who smoke and women exposed to secondhand smoke.”
“There are certain women for whom this message will be enough to convince them,” says Margolis. “You can’t change your age, your genes, your family history, but you can add smoking to the list of risk factors.”