It’s not news that smoking is bad for you, whether you’re the smoker or just near one. And research has linked the malodorous habit not just to lung cancer, but other cancers, as well.

Recently, one of the largest-ever studies of its kind clearly linked passive smoking and increased breast cancer risk. The study showed that women who breathed a lot of secondhand smoke during their lifetime but who themselves never lit up appear to be at greater risk of developing breast cancer later.

Leslie Bernstein (©2007 Philip Channing)

“Research now shows that women who started smoking as teenagers, during a critical window of breast development, have greater breast cancer risk than nonsmoking women,” said Leslie Bernstein, Ph.D., director of City of Hope’s Division of Cancer Etiology, “but links between exposure to secondhand smoking and breast cancer risk have been tenuous.

“We need to clarify this issue, particularly since breast cancer is the leading cancer among women.”

The team turned to the California Teachers Study for more answers.

The study follows the medical history, lifestyle and health of nearly 133,500 women who have worked in the California public school system as teachers and administrators.

As part of the study, researchers asked women about exposure to cigarette smoke in their homes, workplaces and social situations during their lifetimes. Scientists also asked how long each exposure lasted and the intensity of the smoke for each exposure.

The researchers then narrowed their focus to the women who never smoked and who had no history of breast cancer, and they followed them for more than 10 years.

They found that postmenopausal women who never smoked but had high intensities of secondhand smoke exposure after age 20 had greater risk of breast cancer than similar women with little exposure. Among these women, those exposed to the thickest secondhand smoke for the longest time had a 26 percent higher risk.

The California Teachers Study provides some of the most extensive data and is one of the largest studies ever conducted on secondhand smoke and breast cancer risk.

Although federal and international agencies have classified secondhand smoke as a carcinogen, researchers are unsure how it may specifically increase breast cancer risk or whether it may increase breast cancer risk among some women more than others.

The National Cancer Institute has more information on breast cancer risk factors on its Web site.