WEDNESDAY, May 16 (HealthScout) -- Exposure to DDT and PCBs doesn't appear to increase a woman's risk of breast cancer, according to a bird's-eye-view report released today.
A review of five previous studies that looked for such a connection shows that neither DDT, a potent pesticide, nor polychlorinated biphenyls cause breast tumors in women. The compounds are classified by the U.S. government as probable carcinogens and have been banned in this country since the 1970s.
DDT and PCBs, a class of roughly 200 chemicals once used as lubricants and for other industrial purposes, don't break down easily in the environment and can accumulate in fatty tissue. So while they haven't been used in the United States for more than two decades, women exposed to the chemicals may still have traces of them in their blood or breast milk.
The compounds affect cells similarly to the hormone estrogen, which can promote cancerous changes in breast tissue.
The National Cancer Institute and the National Institute of Environmental Health Sciences funded five studies in 1993 to probe a possible link between DDT and PCBs and the unexpectedly high cancer rate among women living in the Northeast.
No solid connection was found. Yet some scientists and women's health advocates argued that the investigations were too small to detect the effect of the chemicals, especially if they looked at the substances individually.
The latest work, led by Francine Laden, an epidemiologist at Brigham and Women's Hospital in Boston, is a meta-analysis that combines the earlier studies into a single, larger inquiry. The method, which is common in epidemiology, allowed Laden's group to look for the effects of exposure to DDT (through its breakdown product, DDE) and PCBs in about 3,000 women, 1,400 of whom had been diagnosed with breast cancer.
"By combing the studies, we increased our sample size and increased our power to say that these null results are really convincingly null," says Laden, whose study appears in the current issue of the Journal of the National Cancer Institute.
Not eliminated as suspects
As in the smaller studies, the researchers found no link between increasing blood levels of the chemicals and a woman's odds of breast cancer. Nor did they find a connection between the chemicals and breast cancer in various sub-groups of women broken down by race, weight and breast-feeding habits.
Even so, Gwen Collman, an epidemiologist at the National Institute of Environmental Health Sciences, in Research Triangle Park, N.C., and a co-author of the latest report, says it doesn't eliminate the possibility that DDT and PCBs might cause cancer in some circumstances.
The five studies "only look at one aspect: Women who have [breast cancer] today and the historical levels of two chemicals. It doesn't really get at questions of early-life exposure to these chemicals or others," Collman says. "Breast cancer is a long, latent disease and there may be some things that go on in utero or early childhood. Those questions still remain unanswered to me."
But Dr. Gilbert Ross, medical director of the American Council on Science and Health, a non-profit group in New York City, says the proposed link is an unlikely one. "There really haven't been any good studies that have shown any relationship between these chemicals and cancer," says Ross.
What To Do
For more on environmental health hazards, try the National Institute of Environmental Health Sciences. To learn more about exposure to PCBs, check out Health Canada, a service of the Canadian government.
Try other HealthScout articles about DDT and PCBs.
If you have breast cancer and are interested in clinical trials, visit Veritas Medicine.
