Laboratory research may bear out a definitive link. In lab animals, scientists can test the effects of various levels and mixtures of chemicals, which would be unethical in humans. “The work we do in humans helps frame the type of questions to be answered by animal studies,” Cohn said. Such collaboration, she said, “is critical to advancing our knowledge.”
These questions involve knowing more about how hormonally active chemicals interact with developing breast tissue.
“A chemical that has weak effects later in life may have very different effects during earlier periods of development when the mammary gland is most sensitive,” said Dr. Hugh Taylor, professor of obstetrics and gynecology at the Yale School of Medicine.
Taylor and his colleagues found that in mice, BPA, an estrogen-like chemical, can have the same effect in a developing fetus as the drug DES, a more potent estrogen. Both “turned up” the expression of genes in the developing mammary gland that are known to play a role in tumor formation. “You are essentially changing the software so that things are programmed to read differently,” Taylor said.
Most cancers aren’t one single piece of damage, but a collection of injuries to a cell or a tissue over a lifetime, making it hard to pin a cause on any one agent, Taylor said. Yet, if endocrine disruptors give you a predisposition for tumor growth, “you’re starting life with one strike against you,” he said.
BPA, used to make hard plastics, liners of food cans and some paper receipts, is found in most human bodies.
Studies in mice and rats suggest that exposure to BPA and other endocrine disruptors in the womb not only alters the structure of the breast, but the way that the tissues communicate with one another and receive hormonal signals from other parts of the body.
“BPA sets the thermostat in a more sensitive way so the mammary gland has more sensitivity to estrogen, and the breast tissue now exhibits an exaggerated response to the hormone. It sees a little bit of estrogen and now thinks it is a lot,” said Dr. Ana Soto, a cancer researcher at Tufts University School of Medicine in Massachusetts. And the body can’t tell the difference between synthetic estrogen mimics and natural estrogens.
BPA and other chemicals also may play a role in breast density – a known risk factor for breast cancer. A preliminary study by University of Wisconsin researchers found that women with higher blood levels of BPA had denser breast tissue than women with low levels.
With so much uncertainty about environmental risk factors, these issues remain largely absent from major breast cancer awareness campaigns.
“Despite billions spent in the name of breast cancer, we still don’t know enough about the causes,” said Karuna Jaggar, executive director of Breast Cancer Action, a San Francisco-based advocacy organization that itself is the watchdog of the breast cancer movement.
While decades of research have failed to turn up strong, environmental risk factors, Cohn is optimistic that scientists are now on the right track. “The science is playing catch up. We have learned from what we didn’t learn,” she said.