A new study that uses blood samples collected more than 50 years ago finds that women who were exposed to the pesticide DDT in the womb have a four-fold increase in breast cancer risk today.

DDT was banned in the United States more than 40 years ago, a decade after Rachel Carson first raised the alarm about it in her book, Silent Spring. The chemical has long been suspected in the human breast cancer epidemic, but research published in the 1990s could not demonstrate a credible link.

A 2007 study by Dr. Barbara Cohn, director of Child Health and Development Studies at the Public Health Institute in Berkeley, California, cited pre-puberty exposure to DDT as a risk for breast cancer in adulthood, using indirect epidemiological evidence.

Now, Dr. Cohn and her team have taken the research a step further. They have hard evidence, Cohn says, linking DDT exposure in utero to elevated rates of breast cancer when women reach their 50s.

Cohn and her team examined vials of blood that had been collected from more than 15,000 San Francisco Bay area women after they gave birth in the 1960s. The vials have been frozen for more than five decades and offered a unique opportunity for Cohn and other researchers.

“This is the very first study I'm aware of in which we have actually been able to measure in utero exposure to an environmental chemical — in this case DDT — and look forward 54 years at the risk of breast cancer for the women who were exposed in utero,” Cohn explains.

The study found that women exposed to high levels of DDT before birth had four times the chance of breast cancer before the age of 52 than women who were not.

Cohn suspects there are particular periods in life when breast tissue is most vulnerable to outside chemicals that might alter its development and be related to breast cancer. One of these periods is in utero, when the breast is forming.

Other vulnerable times are during the “hormone storm” of puberty, during pregnancy, when the breast undergoes more changes as it prepares for lactation, and in the perimenopausal period. All of these periods have one thing in common: They are times of intense hormonal activity.

“Chemicals that can interfere with these hormonal times are often called endocrine disrupters, and they may perturb the natural and normal signaling that is going on at these critical periods,” Cohn explains. “They may alter the tissue, the cells, and even perhaps the instructions about how DNA and genes are turned on and off, so that, later in life — even much later — the tissue may have a higher risk of becoming cancerous.”

Because opportunities for human studies with a 54-year follow-up are so rare, the next step in the research will have to be done in the laboratory, Cohn says, where experimentalists can try to reproduce the timing of exposure and the doses that are analogous to what she found in her study.

“Human studies can never prove causation,” Cohn says. “We don't do experiments on humans. So it's always possible that there are some other chemicals or other exposures that underlie the DDT connection we've seen. Animal studies are required to determine more carefully whether there's causation.”

Nevertheless, Cohn believes her study is very much a proof-of-concept. “It's very unlikely,” she says, “that we'll ever be able to study the 80,000 chemicals that are out there, some of which may have the potential to increase risk for breast cancer and other health problems, as well.”

“Since we can’t do that,” she concludes, “perhaps what the study is saying is that, because there is this potential for these chemicals to do harm, we have to turn to the public policy arena to discuss what level of risk society is willing to take, and to examine carefully both the benefits and the risks of using certain classes of chemicals in our modern life.”

This article is based on an interview that aired on PRI's Living on Earth with Steve Curwood