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Human-generated hydroxyls may affect indoor air quality

Molecules on people’s skin can combine with ozone to produce significant quantities of reactive hydroxyl radicals

by Alla Katsnelson, special to C&EN
September 20, 2022


Four people sitting in a test chamber wearing t-shirts, shorts, and socks and face masks connected to collection hoses.
Credit: Mikal Schlosser/TU Denmark
Volunteers spent 6 hours in a climate-controlled test chamber while researchers sampled the air in the presence and absence of ozone.

We humans spend the vast majority of our time indoors, and we are constantly bombarded with compounds emitted from environmental sources. But we aren’t just passive targets of a chemical barrage. Our bodies generate a surprisingly high level of hydroxyl (OH) radicals when our skin oils react with ozone—and indoors, those hydroxyls go on to oxidize many other chemicals in the cloud of air directly around us (Science 2022, DOI: 10.1126/science.abn0340). How these highly reactive chemicals affect indoor air quality is largely unknown, though some may be toxic, the researchers say.

The new study “opens up this field, now, of oxidation chemistry around the human body,” says Richard L. Corsi, a chemist specializing in indoor environments at the University of California, Davis, who was not involved in the work.

Illustration showing high concentrations of OH radicals surrounding three people sitting around a table in a test chamber.
Credit: Science/UC Irvine
Indoors in the presence of ozone, humans generate a large amount of hydroxyl radicals, which can then react with numerous other molecules.

Traditionally, indoor air quality assessments have focused on chemicals emitted from sources such as air fresheners, cleaning detergents, paint, and furniture. “But in any living space, the one common element is the human being, so it’s good to ask, ‘What do we bring to the party? What are we emitting?’ ” says Jonathan Williams, a chemist at the Max Planck Institute for Chemistry, who co-led the work. “It turns out we are emitting quite a lot.”

To track those emissions, the researchers sampled the air inside a climate-controlled test chamber while sets of 4 volunteers at a time were inside. The chamber contained realistic levels of ozone but was free of emissions from paint, clothes, furniture, or other sources. Williams and his colleagues used gas chromatography/mass spectrometry and proton-transfer-reaction mass spectrometry to measure volatile organic compounds present in the chamber and, separately, how quickly they react with OH radicals. From those calculations they inferred the levels of OH radicals in the chamber over time and in the presence of different amounts of ozone. Then they used computer models to map out the levels in spatial gradients around each person’s body.

In the absence of ozone, little to no hydroxyls were present. The most reactive products in the air came from compounds in the breath, particularly isoprene. But in the presence of ozone, hydroxyls were produced mainly through reactions with compounds near the body’s surface. That’s because ozone reacts with a skin oil called squalene to produce alkenes, including 6-methyl-5-hepten-2-one (6-MHO), which then directly reacts with ozone to produce hydroxyl radicals.

“6-MHO looks like the dominant source of hydroxyl radicals from ozone reactions but also the dominant sink” as the compound primarily reacting with them, Corsi says.

If humans are significantly changing the chemical composition of the air just by their presence, scientists may now have to rethink how to assess indoor air chemistry to account for this effect. “We are surrounded by an oxidation field, if you like, and it has a lot of consequences,” Williams says. “This sofa I’m sitting on—it’s been tested for emissions, but it wasn’t tested with someone sitting on it .”

Williams and his colleagues are looking further at how the generation of hydroxyl levels depends on different factors, such as a person’s age or the temperature of their environment. They are also investigating which chemicals surrounding the body are actually taken up by the lungs, as opposed to breathed back out. “Those are the ones you have to look at in a bit more detail,” he says.


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