Environmental causes

 Tobacco

Most studies have not found an increased risk of breast cancer from active tobacco smoking, although a number of studies suggest an increased risk of breast cancer in both active smokers and those exposed to

secondhand smoke compared to women who reported no exposure to secondhand smoke.

Radiation

Women who have received high-dose ionizing radiation to the chest (for example, as treatments for other cancers) have a relative risk of breast cancer between 2.1 to 4.0.

Impact of environmental estrogenic mimics

Although environmental exposures are not generally cited as risk factors for the disease (except for diet, pharmaceuticals and radiation), a substantial and growing body of evidence indicates that exposures to certain toxic chemicals and hormone-mimicking compounds including chemicals used in pesticides, cosmetics and cleaning products contribute to the development of breast cancer. A recent Canadian study concluded that female farm workers are three times more likely to have breast cancer.The increasing prevalence of these substances in the environment may explain the increasing incidence of breast cancer, though direct evidence is sparse.