Contradicting previous findings, a major new study finds no link between breast cancer and exposure to the pesticide DDT or to the industrial chemicals known as PCBs, Kaiser Permanente researchers in Oakland report today.
Several previous studies involving small numbers of women had suggested such a link, frightening women who live in proximity to farming regions and industrial chemical companies. Last week, a study showed that women living near large chemical companies on Long Island, N.Y., had an increased risk of developing breast cancer after menopause.
Although the new study, which appears in the Journal of the National Cancer Institute, leaves open the possibility that other industrial chemicals may play a role in the disease, it seems to eliminate DDT and PCBs as a suspect.
Epidemiologist Nancy Krieger and her colleagues at Kaiser and the Mt. Sinai School of Medicine in New York City studied 300 women from various ethnic groups, using frozen blood samples obtained when the women enrolled in the Kaiser program in the 1960s, before DDT was banned and the use of PCBs was restricted.
Overall, they found no apparent link between levels of the two chemicals in the women's blood and the incidence of breast cancer. Women with the highest levels of the chemicals were not more likely to have breast cancer and those with the lowest levels were not less likely to have it.
"This is not the definitive study," Krieger said, "but I think it helps set a new standard for future studies."
Epidemiologist Robert Hoover of the National Cancer Institute, which funded the study, complimented its design, but cautioned that the study does not completely rule out the role of the two chemicals. "I wouldn't have written (the conclusions) so strongly negative," he said. "It turns out this is a more complex question than people thought."
Krieger also noted that her results do not absolve DDT and PCBs of the other risks they pose. "It's essential to remember that they have already been shown to be carcinogens and to pose health risks to humans and to other animals," she said. "This study in no way suggests that these chemicals are safe."
The concern about the effects of DDT and polychlorinated biphenyls--PCBs--is based on their tendency to dissolve readily in fat and accumulate in fatty portions of the body, such as the breast.
