Lizbeth Lopez-Carrillo, a professor of epidemiology at the National Institute for Public Health in Mexico City, says 6 million Mexican women ages 12-65 who have never-smoked are being exposed to secondhand tobacco smoke.
Lopez-Carrillo and colleagues examined 504 women with confirmed breast cancer and compared them with 504 healthy women of similar age. In direct interviews, they asked the women about their smoking and secondhand smoking exposure at home and at work.
Compared with women who never smoked and had no secondhand smoking exposure, women with passive smoking exposure had a threefold higher risk for breast cancer -- regardless of menopausal status.
Among women who smoked, the study found an increased breast cancer risk, but this association was only significant if women began smoking between puberty and the birth of their first child.
"Active and passive smoke exposure is a modifiable risk factor for breast cancer," Lopez-Carrillo says in a statement. "Reducing not only active smoking, but also passive smoking, will prevent new breast cancer cases in this population -- everyone should avoid secondhand smoke."
The findings were presented at the Third American Association for Cancer Research Conference on the Science of Cancer Health Disparities in Miami.