A study offers the strongest evidence yet that lingering traces of the banned chemicals DDT and PCBs do not trigger breast cancer.
DDT and PCBs are often cited by those who argue that toxins in the environment are responsible for the steady increase in breast cancer over the past half-century.
Both DDT, a pesticide, and PCBs, which were widely used in industrial products as an insulator, have been banned in the United States since the 1970s.
However, these chemicals persist in the environment and build up in people's bodies. Since they may mimic the harmful effects of the female hormone estrogen, some experts wonder if they could increase the risk of breast cancer.
At least three small studies have supported this link, including one published four years ago in the Journal of the National Cancer Institute. That study, based on 58 cancer cases, found that women with elevated levels of DDT in their bodies had four times the usual risk of breast cancer.
Since then, one small study and two larger ones have found no link between breast cancer and DDT or PCBs. The latest of these was published in today's issue of the New England Journal of medicine.
"The overwhelming weight of the evidence now is that exposure to these particular chemicals is not associated with risk of breast cancer," said Dr. David J. Hunter, who directed the analysis.
The research, part of the long-running Nurses' Health Study, looked at blood samples donated in 1989 and 1990 by 240 women who were later diagnosed with breast cancer.
The blood was checked for DDE, the form DDT takes in the body, and PCBs. For comparison, the researchers also checked the blood of women who were similar but didn't have cancer.
The study found virtually no difference in levels of either DDE or PCBs between the two groups. Also, there was no sign that women with high buildups of the chemicals in their bodies faced a higher risk of breast cancer than did those with low accumulations.
Dr. Mary S. Wolff of Mount Sinai Medical Center in New York was a co-author of this research and also lead author of the study four years ago that came to the opposite conclusion. She said she is not ready to dismiss concerns about DDT and PCBs.
"The effect is not universal and it's not as large as we first suspected. However, I don't think the picture is yet complete."
Wolff said at least 30 other studies are going on, and the role of PCBs and DDT will not be clear until more of them are finished.
However, in an editorial in the Journal, Dr. Stephen H. Safe of Texas A&M University said he thinks that the evidence is already clear and that it should reassure the public that these chemicals don't cause breast cancer.