But this does not mean women should stop cooking with canola or cower indoors for fear of getting breast cancer, experts say. Although some scientists would like to see levels of many of the listed contaminants reduced in the environment, the list is more of a starting point for researchers to investigate further.
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Scientists at the institute (which is an organization investigating environmental effects on women's health) combed through five databases that keep track of cancer studies. Any chemical that showed an increase in mammary tumors in at least one animal study was included as a chemical of interest. In all, the scientists found 216.
"The authors identified interesting compounds that need to be studied further. Are they ready for prime time and regulation? I don't think so. But people should study them," says breast cancer researcher Michael Gould of the University of Wisconsin- Madison.
In the database, the authors reported how carcinogenic the chemical had been classified by two organizations: the International Agency for Research on Cancer and the U.S. Environmental Protection Agency. These classifications were based on all cancers, not just breast cancer. They also noted the strength of the studies.
"If we could see the evidence was weak, we put that in the summary," says lead author Ruthann Rudel, a toxicologist at the Silent Spring Institute. "We didn't want to exclude studies because then we would have had to come up with criteria for how strong the evidence was."
Two other databases also were compiled by researchers from a variety of institutions. One looked at which environmental contaminants contribute to breast cancer in people. The best evidence ranked the persistent environmental contaminants PCBs (polychlorinated biphenyls) and car exhaust PAHs (polycyclic aromatic hydrocarbons) at the top of the list.
A third compiled all evidence on how diet and exercise affect breast cancer risk in women.
The animal carcinogen database is not meant as a guideline for the general public to avoid certain chemicals, the authors say. But some researchers think the public might take it that way — and place more stock in it than they should.
"It's alarming," says epidemiologist Leslie Bernstein of USC. "I'd rather not see [the chemical database] highlighted because we don't have that much information. I don't want women to think if they spray their lawns, they're going to get cancer. Because we don't know."
A lot of the chemicals are not known to cause cancer in humans. But that, Gould says, is the point — researchers can start with the list and investigate further. Gould says the authors gave a "balanced" presentation of the available evidence. "They didn't overhype it," he says.
Because the authors tried to be inclusive, some chemicals stand out as more dangerous than others. And some induced a benign form of cancer — one that doesn't threaten life — and wouldn't be as interesting to pursue, Gould says.
It is important to remember that rodent studies don't always extrapolate to humans, Bernstein says. In fact, different strains of rodents can yield different results in studies, says breast cancer researcher Chris Portier of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C. For example, one rat strain might be more likely than another to get tumors as it ages.
Also, almost any researcher will point out that the way chemicals are tested in animals — by exposing the rodents to high levels they'd never encounter in real life — brings up uncertainty.
Portier says that to consider a compound carcinogenic in humans, scientists need strong animal data and at least some human epidemiological data. About 80% of the time, he says, animal data and human data will agree that a chemical is either carcinogenic or not.
The 216 chemicals are not the only or even the most important breast cancer worry. Gould says about 30% of the risk of getting breast cancer is probably because of a person's genetic background, and not only the most well-known genes, BRCA1 and BRCA2. Small variations in other genes and the combinations of genes women carry appear to influence whether a chemical will induce breast cancer, Bernstein says.
Other significant risk factors include lack of exercise and lifetime exposure to sex hormones such as estrogen, but Bernstein says it's hard to say how much each factor contributes. "It's important to emphasize," she says, "these chemicals aren't the only focus when we're trying to understand breast cancer."