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 four-fold 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 A. 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."
Elizabeth Ward, senior vice president of intramural research for the American Cancer Society, said the group of mothers and daughters the researchers are studying is a "unique resource" for studying potential associations between maternal blood levels of chemicals and risk to their children.
"What makes this study interesting is its analysis of in-utero exposure," she said. However, she said that the number of breast cancer cases was small -- 103 -- so "the results should be interpreted cautiously."
In an interview, Cohn said the paper is part of a series of studies on chemicals and their effects on hormones or development during gestation. Earlier studies she led have looked at the effects of DDT on the time of pregnancy of the daughters of women exposed (they found it could slow their ability to become pregnant or shorten it depending on level of exposure) and on incidence of testicular cancer among the male children (those exposed to the highest levels had an almost three-fold risk compared with those with lower exposure). She is also studying the effect of other chemicals used for stain control on carpeting and waterproofing for food containers.
"We are looking at a vulnerable period in utero," she said. "In some ways it is not surprising that early in life is a time when some of these chemicals can have a strong effect."
Earlier work by Cohn also supports the idea that timing of the exposure matters. In a 2007 paper, she found that DDT affected breast cancer only for women who were exposed before age 14. The meta-analysis that didn't find any association between DDT and exposure looked at studies of women who were exposed later in life.
DDT is still widely used in other parts of the world, including regions of Africa and Asia, where it is used to control the spread of malaria.
"Our findings don't change the perception of benefits, but they do change the perception of risks," Cohn said. "We are hoping that policymakers will use this information as they continue to debate the use of DDT around the world."