Lifestyle risks: Being sedentary, exposure to secondhand smoke, and drinking alcohol

Physical activity has a direct effect on breast cancer risk. One Women's Health Initiative study showed that walking as little as one to two and a half hours per week lowered a woman's breast cancer risk by 18 percent. Being physically inactive increases a woman's risk of developing breast cancer.

While smoking is a risk factor for many types of cancer, it doesn't appear to raise the risk of breast cancer significantly. Oddly, though, breathing secondhand smoke has been linked with an elevated risk of breast cancer. Studies have shown the greatest risk from secondhand smoke is for younger, premenopausal women. This is an area researchers are still trying to understand.

Although the effect of heavy alcohol use on the liver is fairly well known, it's less well known that regular drinking increases breast cancer risk as well. Women who have two to five drinks a day have one and a half times the risk of women who don't drink. Having one alcoholic drink a day raises a woman's risk slightly but not a lot.