Based on the largest study to date, the School of Public Health and Brigham and Women's Hospital researchers report no increased breast cancer risk from exposure to environmental estrogens, specifically, DDT and PCBs. Women with high blood levels of these chemicals were not at higher risk of breast cancer than women with low blood levels. The study is reported in this week's New England Journal of Medicine.

Among the most abundant contaminants are pesticide 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT) and certain polychlorinated biphenyls (PCBs). These so-called environmental estrogens have been shown to have adverse impacts on wildlife. The work is from the Harvard Nurses' Health Study in which more than 32,000 women gave a blood sample in 1989-90. Researchers obtained information about blood levels of these chemicals in 240 women who subsequently developed breast cancer and compared them to a control group of 240 women without the disease.

"Comparisons between countries lead us to believe that environmental and lifestyle factors account for differences in the incidence of breast cancer," says David Hunter, associate professor of epidemiology at the School of Public Health and director of the Harvard Center for Cancer Prevention. "Because environmental pollutants have been suggested as a potentially harmful source of exposure to estrogen-like chemicals, we sought to examine whether hormonally active organochlorine chemicals explained some degree of breast cancer risk in the U.S. Smaller studies to date have been somewhat contradictory about whether there is a relationship between these chemicals and breast cancer risk. We observed no evidence of a positive association between organochlorine exposure and breast cancer risk."

Hunter continues, "Because these compounds are fat-soluble they accumulate in the body and can still be found in virtually all people living in North America, despite the fact that production of these chemicals ceased in the 1970s. The levels of these chemicals we measured most likely reflect exposure over several decades of life. There are very good reasons to keep these compounds out of our environment due to their demonstrated effects on wildlife and other human health effects. However, based on the weight of the evidence to date, they do not appear to explain the high and increasing rates of breast cancer in the U.S."

The Harvard Nurses' Health Study is an ongoing prospective study of women, ages 34 to 59 at enrollment in 1976. The study is directed by Frank Speizer, professor of environmental science at the School of Public Health and Edward H. Kass Professor of Medicine at Brigham and Women's Hospital and the Medical School. Subjects are subsequently followed every two years, answering questionnaires concerning their diet, lifestyle, and health. Organochlorine levels were measured in the laboratory of Mary Wolff at the Mt. Sinai School of Medicine in New York City. This study was funded by the National Cancer Institute and the National Institute of Environmental Health Sciences.