Nevertheless, absence of evidence isn’t evidence of absence, said Elizabeth Ward, National Vice President of Intramural Research at the American Cancer Society. Many of the biggest risk factors remain unknown, she said.
The problem with most studies is that they measured levels of chemicals in women later in life, after they were diagnosed with cancer, not during periods when the breast is most susceptible, said Suzanne Fenton, a reproductive toxicologist at the National Institute of Environmental Health Sciences in North Carolina.
“The research doesn’t prove that the link doesn’t exist or that these chemicals are safe for the breast,” Fenton said. “It shows that we may not have been asking the right question.”
The strongest evidence for this link emerged decades ago. Researchers first suspected that hormone-mimicking chemicals may play a role in breast cancer when they discovered that women who took the anti-miscarriage drug diethylstilbestrol (DES) – a potent form of estrogen prescribed for pregnant women from 1938 until 1971 – had about a one-in-six lifetime risk of developing breast cancer. The risk is one in eight for all women. In addition, their daughters, who were exposed to DES in the womb, developed breast cancer at about two times the rate of unexposed women.
Some scientists say timing of exposure may be the single most important factor when evaluating how chemicals may contribute to breast cancer risk.
The breast is a complex tissue that undergoes several important periods of development and remodeling over the course of a woman’s life. During these periods – before birth when the bud of the mammary gland forms, at puberty when breast cells are rapidly growing and dividing and during pregnancy as the mammary gland transitions to lactation – the breast may be especially susceptible to outside chemicals.
When breasts are exposed to hormone-like substances during those sensitive times, it could “influence susceptibility of the tissue to carcinogens or other hormonal stimuli that could increase cancer risk later on,” said Ruthann Rudel, a researcher at the Silent Spring Institute, a nonprofit research group in Massachusetts, and lead author of a 2011 review.
Cohn and colleagues at the Public Health Institute are using the blood samples of more than 2,000 women who enrolled in the Child Health and Development Studies in the 1960s to investigate exposures during two of these critical periods, pregnancy and postpartum. The women were members of the Kaiser Permanente Health Plan in the Oakland, Calif., area who gave birth between 1959 and 1967.
The scientists recently reported that women who had high levels of a certain PCBs in their blood shortly after giving birth were three times more likely to develop breast cancer later in life than women with lower levels. Because PCBs break down very slowly in the body, a woman’s blood levels postpartum may also predict the PCB levels in her blood during earlier periods of her life, such as puberty, Cohn said.
Banned in the United States 35 years ago, the industrial chemicals persist in the environment and accumulate in food webs. Nearly every U.S. resident still has detectable levels in his or her blood.
In a previous study, Cohn and her colleagues demonstrated that age at time of exposure matters for other chemicals, too. In the same group of women, they found that those with high blood levels of the banned pesticide DDT shortly after giving birth were five times more likely to develop breast cancer before age 50 than the women with the lowest blood levels. Other studies measuring DDT exposure later in life found no link.
Cohn can’t say for sure that the associations they observed between breast cancer and PCBs or DDT were not due to some other factor. “No human study can be definitive,” said Cohn, an epidemiologist who has been involved with the study group for 17 years. “It’s impossible to measure every single exposure pertinent to breast cancer."