Study finds link between pregnant women exposed to DDT and children's cancer risk

WASHINGTON - Banned by the United States in 1972, the insecticide DDT is best known as the impetus for the modern environmental movement. Since Rachel Carson's bestseller Silent Spring sounded the alarm about the poisonous effects of the chemical on wildlife, the environment and human health, numerous studies have linked it to birth defects, miscarriage and reduced fertility.

Its role in cancer has been less clear. The Environmental Protection Agency classifies DDT as a "probable" carcinogen. Roughly three dozen studies have been published about DDT and breast cancer risk for women who lived during its peak use in the 1950s, but a 2014 meta-analysis of that research found that there was no significant association between exposure and breast-cancer risk.

They may have been looking at the wrong generation of women.

A new study published Tuesday in the Journal of Clinical Endocrinology and Metabolism found a startling link between pregnant women exposed to DDT and the breast-cancer risk to their daughters.

The study tracked the daughters of women who were part of a study at the Kaiser Foundation Health Plan from 1959 to 1967 near the city of Oakland, Calif. During that time, DDT was widely used and accumulated in the fat of animals that we eat and was found in milk, butter, cheese and other products in the food supply. It was also in a number of consumer products, including some wallpaper.

During that period, the participants gave birth to 9,300 daughters. Every mother had some measurable level of DDT in her blood. Researchers determined the level of exposure to DDT in utero by analyzing stored blood samples that were taken from the mothers during pregnancy or shortly after they delivered their babies.

By using state records and surveying the daughters, who are now in their late 40s and early 50s, they were able to figure out which ones developed breast cancer.

The researchers found that elevated levels of DDT in the mother's blood were associated with almost a fourfold increase in her daughter's risk of breast cancer and that this was independent of the mother's history of breast cancer. They also determined that those with higher levels of exposure were diagnosed with more advanced breast cancer.

About 83 percent of those who got breast cancer had estrogen-receptor positive breast cancer and were more likely to develop HER2-positive breast cancer, in which a genetic mutation produces an excess of a protein. In previous studies, DDT has been found to interfere with the function of estrogen and, separately, to activate the HER2 protein, which may explain the link.

Barbara Cohn, one of the study's authors and the director of Child Health and Development Studies at the Public Health Institute in Berkeley, Calif., said the 54-year study is "the first to provide direct evidence that chemical exposures for pregnant women may have lifelong consequences for their daughters' breast-cancer risk."