Policymakers take note: Even in low doses, chemicals common in the environment can work together to cause cancer.

Cancer cells. (Photo: pulmonary_pathology/Flickr)

Researchers used to think carcinogenic chemicals in the environment caused about six percent of all cancers, but that dramatically underestimates the problem, according to a new report from the Environmental Working Group. The real issue, the report suggests, is that, even in low doses, mixtures of toxic chemicals can work together to target a set of "hallmarks" necessary for cancer to fester—and regulators need to start taking that into account.

"It's much more complex than flipping a switch," says William Goodson, a senior clinical research scientist at California Pacific Medical Center Research Institute in San Francisco. Goodson is lead author of a recent review that showed how mixtures of chemicals can work together to overcome our bodies' cancer defenses, even if no single chemical in the mix could lead to cancer on its own.

In fact, Goodson says, there are 10 key steps—or "hallmarks"—common to all cancers. Those steps include a cancer developing its own blood vessels and mechanisms for evading the immune system. Cancers "don't need to get each step from the same chemical," he says; the mixture of chemicals is often the key.

Goodson, Lowe, and more than 170 researchers identified 50 different chemicals that contributed to cancer at low doses, even if they wouldn't necessarily lead to cancer on their own.

Though it might seem obvious in retrospect, that's an entirely new way of thinking about cancer, says Leroy Lowe, president and co-founder of Getting to Know Cancer and second author of the review paper. Most research focuses on whether a single chemical might cause cancer, but "we've never concerned ourselves with those other chemicals that act at a lower level," Lowe says.

Inspired by that new perspective, Goodson, Lowe, and more than 170 researchers worldwide examined the effects of 85 different chemicals—everything from iron to bisphenol-A—and identified 50 that contributed to cancer at low doses, even if they wouldn't necessarily lead to cancer on their own.

What's more, a mixture of 23 of those cancer-enabling chemicals are present in detectable levels in our bodies, says EWG scientist Curt DellaValle. That conclusion was based on a review of data from the National Health and Nutrition Examination Survey, part of the Centers for Disease Control and Prevention's National Biomonitoring Program. This means, according to DellaValle, that chemicals "could be interacting within our bodies" even if we're not being exposed to each one at the same time.

"Historically, the view is that only six percent of cancers are caused ... by chemicals in the environment," says Cancer Prevention and Research Institute of Texas Chief Scientific Officer Margaret Kripke, who was not involved with either study. Along with Goodson and Lowe's review, the EWG report "sets that on its heels." If low-dose chemical interactions can cause cancer, even our definition of carcinogenic chemicals may need to change, she says.

Though the findings pose a "daunting challenge" for researchers, according to EWG's DellaValle, there is precedent for studying complex chemical interactions that lead to cancer—namely, research on prescription drug interactions. "That model exists," he says.

Eventually, regulation will need to change. Current policies "underestimate the complexity of cancer," Goodson says.

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