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Atmospheric Chemistry

Flame retardants form potentially toxic derivatives in city air

Risk assessment of organophosphate esters combines laboratory studies, environmental screening, and computer modeling

by Mark Peplow, special to C&EN
December 15, 2021 | A version of this story appeared in Volume 99, Issue 45


A map of cities where air samples contained transformation products of organophosphate ester flame retardants
Credit: Nature
Air samples from 18 cities around the world all contained transformation products of organophosphate ester flame retardants, many of which were predicted to be more toxic and persistent than their parent molecules.

Flame-retardant chemicals found in city air around the world can transform into a soup of derivatives that are predicted to be more toxic and more persistent than their parent compounds.

That’s the conclusion from a study of organophosphate ester flame retardants (OPFRs) that relies on a new framework for assessing the risks of commercial chemicals (Nature 2021, DOI: 10.1038/s41586-021-04134-6). This three-step framework—which combines laboratory studies, environmental screening, and computer modeling—could help regulators to uncover hidden risks from other groups of mass-produced chemicals.

Such molecules are rarely inert in the environment. Through reactions with light, oxygen, and myriad other pathways, a commercial chemical can spawn a range of transformation products that may have very different properties than their parent. Yet these transformation products are rarely factored into the risk assessments that underpin chemicals regulation programs such as the US Toxic Substances Control Act (TSCA), often because so little is known about the derivatives. “If they have that information, many programs will hopefully take that into account when determining how to assess the risk of the original parent chemical,” says John Liggio of Environment and Climate Change Canada, who co-led the new study.

Chemical structure of Tris(2-chloroethyl) phosphate (TCEP).
One of the organophosphate flame retardants whose breakdown products were identified in air samples from cities worldwide.

The three-step framework he and his colleagues developed aims to plug this data gap. First, the researchers use a flow reactor to generate photooxidation products from a commercial chemical. They then identify and quantify those derivatives with high-resolution mass spectrometry. This spectrum serves as a fingerprint to help identify the transformation products in real-world air samples. Finally, the researchers feed all these data into well-established computational models that predict the environmental risks of each compound. “I think the novelty here is to combine these different approaches,” says environmental chemist Marta Venier of Indiana University, who was not involved in the new research. “It’s a very smart idea.”

Liggio’s team applied this framework to study nine OPFRs used in products such as upholstered furniture. OPFRs have become popular replacements for polybrominated diphenyl ethers (PBDEs), which have largely been phased out due to health concerns. But evidence from in vivo, in vitro, and epidemiological studies suggests that some OPFRs may be carcinogenic or neurotoxic, while environmental surveys have found them to be as ubiquitous as the PBDEs they supplanted (Environ. Sci. Technol. Lett. 2019, DOI: 10.1021/acs.estlett.9b00582).

The researchers’ flow reactor experiments showed that the OPFRs could form 186 photooxidation products. When they looked for these compounds in air samples from 18 cities around the world, they identified 10 key transformation products that appeared in every single sample—likely representing the tip of the iceberg, they say.

Computational modeling predicted that the transformation products of chlorinated OPFRs were on average 2.5 times more persistent in the environment than their parents. Depending on the parent molecule, between 24% and 89% of each OPFR’s transformation products were predicted to be more toxic than their progenitor. “I’m not that surprised, unfortunately,” Venier says. “The fact that [OPFRs] degrade into chemicals that are potentially even more toxic—it’s concerning.”

Over many years, such transformation products may contribute to a wider health burden posed by hundreds of other airborne molecules, Venier says. “You have to think about it in the long term, and the fact that we are exposed continuously,” she says. “This is only a very small part of the puzzle—when you add it all up, you’re really living in a chemical soup.”

Air from London and New York contained the highest concentrations of transformation products, totaling around 12 ng per m3, similar to the concentrations of the parent compounds. Liggio acknowledges these are trace amounts, and it’s unclear whether they pose a health risk. But by identifying the most worrisome of the transformation products, researchers can prioritize these molecules for more rigorous toxicology testing in the lab, he says.

Liggio and his colleagues now plan to apply their framework to chemicals introduced as alternatives to the controversial plastic additive bisphenol A.


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