Breast cancer: Pollution link remains hazy
What’s the case for environmental pollutants contributing to breast cancer? Circumstantial evidence keeps patients, doctors, advocates and scientist asking this question, but so far no clear relationship between exposure and disease has been shown in people.
Known risks for breast cancer include family history, mutations in the BRCA1 and BRCA2 genes, a woman’s reproductive history — the age at which she gets her first period, the number of children she has and when she enters menopause — and lifestyle factors such as cigarette and alcohol use, diet and exercise. Experts say the development of disease in any particular person is likely spurred by a combination of these and other factors.
When it comes to environmental pollutants, “it’s very difficult to tease these things out, because there’s so many factors you’re looking at,” says Dr. Amber A. Guth, a breast surgeon at New York University School of Medicine in Manhattan.
The notion that chemicals used in manufacturing and agriculture and created as the byproducts of modern life contribute to breast cancer comes from studies of animals. In the lab, researchers have shown that the offspring of pregnant mice and rats exposed to dioxins or bisphenol A at levels comparable to those encountered by humans were more susceptible to experimentally induced mammary tumors as adults compared with control animals that weren’t exposed.
In people, the link, if any, between pollutants and breast cancer is far less clear. For starters, there’s no easy way to track how much exposure a woman has had to a particular chemical or for how long. There’s also the fact that the gap between a potentially toxic exposure and the onset of disease can be measured in years, even decades. And in the real world, people don’t necessarily encounter these chemicals at the same concentrations that are tested in labs.
“It’s really, really hard to make a direct connection with any particular chemical,” says Janet Gray, a psychology professor at Vassar College in Poughkeepsie, N.Y., who used to study how hormones influence animal behavior and now focuses on environmental risks for women’s health.
One of the largest population studies conducted to date was triggered by activist groups that wanted to get to the bottom of what appeared to be breast cancer clusters on New York’s Long Island. State data showed that from 1988 to 1992, Nassau County had 137.8 cases of breast cancer per 100,000 women and Suffolk County had 133 — both well above the statewide rate of 121.8 cases per 100,000 women. Government reports ascribed those higher rates to demographic factors associated with breast cancer risk, especially the high concentration of affluent Jewish women who were older than the norm when they started their families. But breast cancer survivors worried that pesticides and industrial pollutants in their communities were really to blame, and they lobbied legislators to investigate. In 1993, Congress ordered the National Institutes of Health to study the issue in detail.
In one of multiple studies done in response to that mandate, researchers from the University of North Carolina, Columbia University in New York and other institutions examined 1,508 Long Island women newly diagnosed with breast cancer and 1,556 women from the same counties who were of similar age but did not have the disease. The scientists interviewed subjects about their environmental exposures and took blood samples to look for the presence of two major classes of chemicals: organochlorines (including the pesticides DDT and industrial PCBs) and polycyclic aromatic hydrocarbons, or PAHs, commonly found in cigarette smoke and car exhaust.
The reason those chemicals were studied was because they have been linked to mammary tumors in animal studies and are associated with reproductive toxicity in wildlife. (Other studies looked at the presence of chemicals in fat and breast tissue.)
The overall picture that emerged from the various studies in Long Island was that exposure to the chemicals studied did not correlate with rates of breast cancer. However, a few associations did emerge.
For instance, when women were grouped into five categories depending on how much PAH had attached to the DNA in their blood, researchers found that 82% of the women with breast cancer had the highest levels of PAH in their blood, compared with 54% of women without cancer. The difference suggests that PAH exposure may increase cancer risk, though causation can’t be shown in this kind of backward-looking study.
A different retrospective study that analyzed geographic data for 105 women with breast cancer found that women who were diagnosed with breast cancer were nearly three times more likely to live within one mile of an organochlorine-containing hazardous waste site than cancer-free women who were matched according to age and race.
A summary of the results from the Long Island Breast Cancer Study Project was published in the journal Nature Reviews Cancer in 2005.
“It might have hit pay dirt, but it didn’t,” says Deborah Winn, deputy director of the Division of Cancer Control and Population Sciences at the National Cancer Institute in Bethesda, Md., who wrote the review article. “It was terrific science, but no clear connection was found.”
Similarly, Caucasian women in California’s Marin County have historically had higher rates of breast cancer than their counterparts in surrounding urban counties, with 177 cases per 100,000 women in the years 1997 to 2001, according to data from the California Cancer Registry, a program of the state’s public health department. That was 6% higher than the rate for the entire San Francisco Bay Area during that period, a difference that was statistically significant.
Studies on these women were smaller and used time spent in the county as a way to estimate exposure to environmental pollutants, but the bottom line was the same: no measurable contribution to breast cancer risk from environmental exposure. Factors that did correlate to disease included more frequent screening for breast cancer and consuming at least two alcoholic drinks per day. These results were summarized in a 2003 report in Breast Cancer Research.
Both regions are affluent communities, and higher socioeconomic status is associated with higher risk for being diagnosed with breast cancer. The reasons for this are not completely understood, but wealthier women tend to be older, have had fewer children and have their children later in life — all risk factors for breast cancer. They are also more likely to get mammograms on a regular basis, which increases the odds of finding any cancer that develops.
“In Long Island and Marin County, if you adjust for socioeconomic status, the high rates go away,” Winn says.
The failure of these and other studies to make a link between pollutants and breast cancer has not convinced everyone that the two are unrelated. Many researchers and advocates are continuing the hunt.
Gray says her biggest concern is a class of chemicals called endocrine disrupters that includes bisphenol A (found in plastic bottles and the linings of metal food cans), phthalates (found in flexible plastic toys) and parabens (used as preservatives in many personal care products, such as shampoo and shaving cream).
“We’re just now understanding that endocrine disrupters are particularly potent in the very young,” she says, citing animal studies. Disrupting reproductive hormones during development seems to have long-term consequences, including susceptibility to tumor growth.
The National Cancer Institute and the National Institute of Environmental Health Sciences are collaborating on another large population study that aims to identify factors, including chemicals in the environment, that contribute to early puberty in girls, a known risk factor for breast cancer. Winn says it’s a way of looking at one piece of the long trajectory from exposure to cancer. Results are expected soon, she says.
Meanwhile, many doctors want to keep the focus on undisputed risk factors that women can actually influence, such as their weight, diet, exercise routine and habits like smoking and drinking, Guth says. “These are things we can change — for the prevention of breast cancer and prevention of recurrence.”
