Breast cancer: Hundreds of chemicals identified as potential risk factors

Researchers find nearly 300 chemicals linked to breast cancer-contributing hormones in everyday products, and call for a renewed focus on women's exposure risks.

Researchers have identified almost 300 chemicals in everything from hair dye to pesticides that can increase levels of breast cancer-contributing hormones.


Of those chemicals, 219 had not been previously identified as potential carcinogens, Ruthann Rudel, director of research for the Silent Spring Institute and co-author of the new study, told EHN. The findings come in a study out this week in Environmental Health Perspectives.

While scientists have known for decades that higher levels of estrogen and progesterone are linked to breast cancer, experts say that safety screening to test U.S. consumer products rarely looks at how chemicals affect the production of those hormones.

"The way that chemicals are tested now, they are really missing breast-related effects," said Rudel. "We have to do a much better job checking for these effects when we test chemicals."

Environmental chemicals and hormone production

Breast Cancer screening

A postdoctoral fellow in the National Cancer Institute's Experimental Immunology Branch, pipetting DNA samples into a tube. (Credit: National Cancer Institute)

Scientists have historically used animal studies to understand whether chemicals pose a threat to humans. Animal studies take time and money, though, leading researchers and regulators to increasingly use high-throughput tests to more quickly screen chemicals for hormone disruption and other potentially disease-inducing effects. With these high-throughput tests, researchers expose cells and other molecules to chemicals to see whether they trigger any changes.

Researchers from the Silent Spring Institute, a non-profit that studies the environmental causes of breast cancer, decided to look through 2018 EPA ToxCast safety data on roughly 1,800 chemicals to see how many caused cells to increase estrogen and progesterone.

Laura Vandenberg, associate professor of environmental health sciences at the University of Massachusetts Amherst who was not involved in the study, told EHN that one of the characteristics toxicologists commonly look for is whether a chemical mimics estrogen. To date, though, there has been little focus on whether chemicals could actually cause cells to produce more estrogen or progesterone.

Related: Pesticide DDT linked to increased breast cancer risk generations after exposure

Rudel and a colleague found that 296 chemicals increased one or both of the hormones. Some of the hormone-increasing chemicals include the pesticide atrazine, the fungicide imazalil, and hair dye ingredient 1-4, benzenediamine. Vandenberg said she was disturbed but not shocked to see that so many of these chemicals increase hormone production in cells as "many of these chemicals (like pesticides) were designed to be biologically active."

The Silent Spring researchers then looked through carcinogen and reproductive toxicity databases, like California's Prop 65 list, to see which of these chemicals were already on those lists. The researchers found that roughly a third had been identified as carcinogenic, toxic to development and reproduction, or both. But they also found that many of the chemicals on their list "hadn't been tested or evaluated," for impacts to those systems, Rudel said.

Scientists have known for some time now that estrogen and progesterone contribute to breast cancer because both hormones stimulate breast cell growth, increasing the risk of uncontrolled cell growth and DNA damage. Around 70% of breast cancer cases respond to these hormones, and treatment for this disease commonly involves using drugs that inhibit estrogen.

"What we know is that a woman's own production of estrogen is actually one of the biggest risk factors for breast cancer," Vandenberg said, adding that early puberty and delayed menopause, which give the body more time to produce estrogen, have been linked to higher rates of breast cancer.

Women’s health focus

Women's health

"My worry is that women's health always gets sort of short shrift," University of Massachusetts Amherst researcher Laura Vandenberg told EHN. (Credit: Vonecia Carswell/Unsplash)

Rudel said that their list of hormone-increasing chemicals provides a good starting point for both toxicity and human exposure research. "There hasn't been a super systematic approach to what chemicals should be studied in breast cancer (epidemiological) studies," she added.

The researchers also found that even for chemicals that had been previously scrutinized, past risk often didn't look at or dismissed mammary gland impacts. For example, although a multi-generational toxicology study showed that exposure to dichlorophenol, a chemical in some pesticides and disinfectants, stiffened and whitened breast tissue in all doses, the authors of that study didn't take those effects into consideration when determining what a safe dose of that chemical is.

And a lot of the animal tests used by regulators only look at a couple of the hundreds of ducts in the mammary gland, which is "treated as sufficient—and it's not," Vandenberg said.

"My worry is that women's health always gets sort of short shrift, and there's a little bit of an attitude of breast cancer as a disease of the old, and therefore, it's not a priority for regulatory agencies," she said, stressing that no one at those agencies had actually said that.

How to limit your chemical exposure 

Researchers also don't have a great sense of how we're exposed to many of the chemicals on the list, Rudel added.

Silent Spring has a mobile app, Detox Me, that allows people to scan consumer product barcodes and other features to try to minimize their exposure to toxic chemicals.

Banner photo: Woman performing a breast self-examination. (Credit: National Cancer Institute)

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