In the early 2000s, researchers tested breast milk samples from U.S. mothers and found high levels of toxic compounds used as a common flame retardant in household items.

The compounds, polybrominated diphenyl ethers (PBDEs), were gradually phased out after a link was found with certain health risks. It sounds like a public health success story, but new research suggests it may not be quite that simple.

This summer, scientists detected a new set of similar flame retardants in the breast milk of 50 U.S. women.

Brominated flame retardants — the class of compounds that includes PBDEs and these new compounds — were first developed in the 1970s to prevent burning in household electronics and appliances. Because they're used in so many different products, we come in contact with these compounds in our daily lives, says Dr. Sheela Sathyanarayana, a professor of pediatrics and environmental health at the University of Washington and Seattle Children's Research Institute who is one of the authors of this study.

People are exposed to these flame retardants through dust and air. "Although it seems strange, we actually ingest some amount of dust every day," says Sathyanarayana, "just from touching things or dust landing on our food or in our water."

They're also extremely persistent chemicals, meaning once you're exposed to them, you'd likely have them in your body for years since they don't break down easily.

PBDEs were the most well-known and widely used compounds in this class of flame retardants — until scientists found them in breast milk and started raising the alarm about their potential impact on human health.

A 2015 report from the U.S. Department of Health and Human Services says that PBDE exposure in utero and infancy (via breast milk) may have an effect on hormones in the body, including thyroid hormones, which play a key role in nervous system development. And evidence suggests exposure might be linked to developmental issues in kids.

"PBDEs have been associated with adverse neurodevelopmental health outcomes in children when exposed in utero," says Sathyanarayana.

The Environmental Protection Agency eventually banned PBDEs from commercial use in 2009 after coming to an agreement with manufacturers to phase out most uses of PBDEs by the end of 2013.

But regulation of toxic chemicals tends to ban chemicals one at a time, rather than an entire class of similar compounds, says Sathyanarayana, so companies started using substitutes that were very similar in structure and behavior to PBDEs.

In this new study, published this summer in Environmental Pollution, researchers analyzed the breast milk of 50 U.S. mothers in the Seattle area and detected a total of 25 flame retardants, including 16 replacement chemicals and nine phased-out PBDEs.

Of those replacement chemicals, a type of flame retardant known as bromophenols was found in 88% of samples. Bromophenols are similar in structure to PBDEs as well as the thyroid hormone and preliminary research shows that, like PBDEs, they can affect thyroid function.

This study marks the first investigation into exposures in breast milk since 2012, so while the sample size is relatively small, "this is an interesting start that I hope will spur more research," says Sue Fenton, a reproductive endocrinologist at NIH's National Institute of Environmental Health Sciences.

The PBDEs that were still detected "were much lower in concentration than previous years," says Sathyanarayana. Given how long PBDEs last in the environment and in the body, "it's not too surprising that they were still found in breast milk," says Deborah Bennett, an exposure scientist and professor of environmental health for the School of Medicine at University of California, Davis.

Researchers say it's too early to know whether we should be worried about these new bromophenols. "There's not much information on developmental health effects of several of the bromophenols and [brominated flame retardants] considered as 'replacements,' " says Fenton. "There should be more studies initiated to better understand the roles of these emerging contaminants."

Sathyanarayana says she thinks they'll find similar trends as they did with PBDEs, but echoes that there's not as much research out yet.

The study looked at predominantly white, well-educated women in the Seattle area, so Bennett is interested in seeing if levels of flame retardant compounds in lower-income populations are higher. "Oftentimes exposures are higher among low-income populations."

While researchers work to better understand the health effects of these new replacement flame retardants, states like New York and Washington, as well as the European Union, have passed stricter regulations and bans on the use of the whole class of brominated flame retardants in electronic products. Sathyanarayana says that broader chemical class bans would be more beneficial than individual chemical bans.

"If you regulate individual chemicals," says Sathyanarayana, "what ends up happening is that there are what have been called 'regrettable substitutes' — substitutes that are put into the marker that they think are safer, but then we find that they're just as pervasive and may have just the same amount of toxicity."

Bennett stresses that while "we would like women to be able to have breast milk that doesn't have any contaminants in it, breast milk remains the best for children."

Bec Roldan (they/them) is the 2023 AAAS Mass Media Science and Engineering Fellow at NPR. They are a 5th year Ph.D. candidate and science journalist.