Study Finds Exposure to DDT and PCBs Does Not Increase Breast Cancer Risk

American Cancer Society
Wednesday, 29 October 1997

A new study on environmental contaminants and breast cancer risk in the October 30, 1997 issue of The New England Journal of Medicine suggests that exposure to organochlorines in pesticides and industrial chemicals such as DDT and PCBs does not increase the risk of breast cancer.

Researchers from the Harvard Medical School and Mount Sinai Hospital, New York, measured blood levels of the organochlorine pollutants DDE and PCBs in women with breast cancer and compared the levels with those measured in a matched control group of women in whom breast cancer did not develop. The women studied were from the Nurses' Health Study, which includes 121,700 women from 11 states. After extensive analysis, the authors did not observe any evidence of an increased risk of breast cancer among women with relatively high levels of plasma DDE or PCBs. In addition, the researchers found that women with high levels of both DDE and PCBs were not at higher risk than women with the lowest levels of these compounds.

The authors say that they although they cannot exclude the possibility that exposure in utero or during childhood could increase the risk of breast cancer decades later, because DDT and PCBs were introduced into the environment largely in the 1940s and 1950s, exposure very early in life could not have accounted for most of the increase in the incidence of breast cancer over the past several decades, which has been greatest in postmenopausal women who were already adults when the compounds were being most widely used. They add that the absence of an association with DDE and PCBs does not rule out the possibility that other pesticides and environmental contaminants may be associated with breast cancer.

In an accompanying editorial, Stephen H. Safe, DPhil,Texas A&M University, comments that "the results of Hunter et al. along with those of other recent studies, should reassure the public that weakly estrogenic organochlorine compounds such as PCBs, DDT and DDE are not a cause of breast cancer." According to Dr. Safe, high levels of exposure to DDT have not previously been associated with an increased risk of breast cancer, and there is no increase in the risk of breast cancer among women who are exposed to relatively high levels of PCBs at work. Moreover, he writes, the incidence of breast cancer has increased in industrialized countries over the past 20 years, but the environmental levels of most organochlorine contaminants have decreased as a consequence of strict regulations regarding their use and disposal.

"Several well-done epidemiologic studies have now been reported which find no appreciable increase in risk of breast cancer in relation to past exposure to organochlorine chemicals, notably DDT and PCB," says Clark Heath, Jr., MD, vice president of Epidemiology and Surveillance Research for the American Cancer Society. "This new study reaffirms that conclusion. Our knowledge about causes of breast cancer, however, remains quite incomplete, so continued research is essential, especially with respect to other forms of potential harmful environmental exposures, estrogenic or otherwise," says Dr. Heath.

The study was supported by the National Institutes of Health and the American Cancer Society.

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