A New Culprit in Air Pollution: Reactions Triggered by Human Skin

The COVID-19 pandemic has sparked newfound interest in indoor air quality. People are now thinking about how virus particles can spread indoors, but the hazards of indoor environments go beyond pandemic-causing pathogens. Air pollution is the world’s largest environmental health threat, according to the World Health Organization (WHO), but a majority of people likely don’t think of their own bodies as part of the problem, especially within their own homes.

Now, an interdisciplinary collaboration between atmospheric chemists and engineers in Germany, Denmark, France, and the US has shown that oil from human skin reacts with ozone to generate potent, free radicals that can further react with most organic compounds present in the indoor environment to in turn produce dangerous pollutants.

This reactive pathway, detailed in a study published yesterday (September 1) in Science, helps explain how the human body directly influences the chemistry of indoor environments. It may also aid research into the adverse health effects of breathing in toxic compounds that are produced in part by us, experts say.

“People are aware of outdoor pollution, but when we come inside, we think, “Oh, now we’re safe.” And it’s not really true,” says study coauthor Jonathan Williams, an atmospheric chemist at the Cyprus Institute and the Max Planck Institute for Chemistry in Germany. “It’s a surprising discovery that ozone reacts on our skin and that releases more chemicals into the atmosphere around us.”

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In the study, Williams and his colleagues asked three different groups of four adult participants to sit in a climate-controlled stainless-steel chamber about the size of a small bedroom on separate days for as long as five hours. Then, using highly sensitive instruments they had developed, the researchers measured the concentration of organic compounds and hydroxyl radicals in the air inside the chamber. They also looked at the chemical composition of the participants’ breath by asking them to breathe into a special mask to make sure that it wasn’t their breath that was altering the room’s air composition.

After obtaining these measurements, the researchers repeated the experiment, this time introducing 35 parts per billion (ppb) of ozone into the chamber, which is comparable to the level of ozone exposure one gets riding on an airplane. Williams says he was surprised to find that the skin oil squalene reacts with ozone to produce compounds that then react with the ozone again to produce powerful oxidants called hydroxyl radicals. These can then react with other molecules found in indoor environments—such as those emitting from furniture, household cleaners, and even a freshly cooked meal, Williams says—to produce toxic compounds.

The hydroxyl radicals are not harmful in and of themselves and are also present in the outdoor environment. They are produced when sunlight breaks down ozone into oxygen atoms, which attack water to make hydroxyl radicals. They are sometimes called “scrubbers,” says Tara Kahan, an atmospheric chemist at the University of Saskatchewan in Canada, who was not involved in the new work, because they go on to react with hydrocarbon pollutants in the air to scrub the atmosphere clean. But their presence indoors is dangerous, because they can also oxidize organic compounds in the air into products that cause respiratory and cardiovascular diseases.

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The mechanism of hydroxyl production revealed in the study helps answer how we influence the chemistry of the air around us, Kahan says, by showing that “even just having humans present can change what’s in the air.”

To validate their findings, Williams’ team collaborated with researchers at University of California, Irvine, and Penn State University. These two groups used the measurements of chemicals in the chamber air to develop and run computational fluid dynamics models and chemical models, which simulated the 3D distribution of chemicals in the air, including hydroxyl radicals that form a reactive field around the human body.

“If you talk about indoor chemistry, what I would have thought before reading this article would be that ozone is the most important oxidant,” says Thomas Berkemeier, an atmospheric chemist at the Max Planck Institute for Chemistry who studies health effects of outdoor air particles and was not involved the study. “It struck me that it was hydroxyl radicals, which makes everything difficult.” While ozone reacts more selectively, explains Berkemeier, hydroxyl radicals are so reactive that it is very challenging to predict all the types of chemicals they produce.

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There are a lot of factors like temperature, moisture, and extent of skin exposure that can influence the generation of these radicals, says Williams. He and his colleagues from around the world are currently taking a deeper look at how humidity in the air can affect the generation of hydroxyl radicals.

These findings, according to Berkemeier, raise questions regarding the potential long-term health effects of oxidation of chemicals in household items such as perfumes, deodorants, and cleaners. Kahan echoes this, and says she looks forward to seeing researcher test at a wide variety of ozone levels in their experiments to see how they affect the production of hydroxyl radicals, particularly because ozone concentration in a house on a sunny day with open windows is only about 20 ppb. With further experiments, scientists can perhaps gauge the amount of adverse oxidation reactions that may take place in a typical home environment, potentially answering some of those health questions.

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