LOS ANGELES, Dec. 5 (Xinhua) -- Women who are exposed to secondhand cigarette smoke for a prolonged period of time may face greater risk of breast cancer, a new study suggests.

    The risk also applies to women who never smoke cigarettes themselves, according to the study appearing in the December issue of Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research.

    In the study, researchers at the Northern California Cancer Center's Berkeley office collected detailed information via questionnaire from more than 57,000 women in the California Teachers Study, then followed them for a decade. Detailed questions helped the researchers to determine whether age at exposure, setting of exposure or amount of exposure influenced the risk of developing breast cancer.

    The researchers examined the risk of developing breast cancer among women who had never smoked tobacco products, but who had a history of exposure to secondhand smoke either at home, at work or in social settings. Participants also had no history of breast cancer.

    In the years since completing the questionnaire, 1,754 women developed invasive breast cancer.

    Overall, findings showed no evidence that simple measures of secondhand smoke were associated with breast cancer risk. Risk seemed to be confined to exposures experienced during adulthood ( among women aged 20 or older) and primarily among those who were postmenopausal; early-life exposures (before the age of 20) did not alone appear to increase their risk.

    Women exposed to moderate to high levels for a combination of years and intensity of exposure had a significant dose response so that the risks for developing breast cancer increased as the cumulative exposure levels increased,

    "We were initially surprised not to see much effect individually for exposure in household, workplace or social settings," said Peggy Reynolds, Ph.D., senior research scientist at the center. "It does make sense though, if there is an effect for higher levels of exposure, the sum of exposures across settings would be more important than only partial measures of exposure."

    "The question of whether exposure to side-stream smoke could increase risk of breast cancer is one that is unresolved," said Reynolds. "While no single epidemiological study can answer the question, our findings suggest that cumulative high levels of exposure may contribute to breast cancer, adding to the evidence for a variety of other adverse health outcomes."