The largest study to date shows that women who are exposed to the chemicals DDT and PCBs have no increased risk of developing breast cancer.

The study, by researchers at the Harvard School of Public Health and Brigham and Women's Hospital, found that women with high levels of these chemicals in their blood were not at higher risk of breast cancer than women with low blood levels.

In the study, published in today's New England Journal of Medicine, the researchers looked at the blood of 240 women with breast cancer and 240 who did not develop breast cancer.