Both active and “passive” smoking (exposure to secondhand smoke) increase the risk of breast cancer in premenopausal but not postmenopausal women, a study of middle-aged Japanese women suggests.

The investigators think that higher levels of estrogens present in the body of premenopausal women may act jointly with external cancer-causing agents, such as tobacco, to fuel the development of breast cancer.

In the study, Dr. Tomoyuki Hanaoka from the National Cancer Center in Tokyo, Japan, and colleagues studied associations between smoking and breast cancer in close to 22,000 women who were between the ages of 40 and 59 years in 1990.

A total of 180 women developed breast cancer by the end of 1999, they report in the International Journal of Cancer this month.

Among all of the women, 5.7 percent were current smokers, 1.7 percent were ex-smokers, and 92.6 percent had never been active smokers. Sixty-nine percent of these “never-active smokers” reported that they had been exposed to sidestream smoke.

Compared with never-active smokers with no exposure to secondhand smoke, ever-smokers who had yet to enter menopause had a greater than 3-fold elevated risk of developing breast cancer. The elevated risk of developing breast cancer among ever-smokers was not observed in postmenopausal women.

Premenopausal but not postmenopausal women who had never smoked but had been exposed to secondhand smoke had a 2.6-fold increased risk of developing breast cancer.

These results, the authors conclude, show that both active and passive smoking increases the risk of breast cancer in premenopausal women. “Both active and passive smoking are promising targets in the prevention of breast cancer,” they write.

SOURCE: International Journal of Cancer March 10, 2005.