How a volatile mix of skin oil, hygiene and ozone affects indoor air quality
Charles Blue
contributing writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Skin is the body's first line of defense against an array of environmental hazards. But when our skin's natural oils react with atmospheric ozone, they emit a cocktail of volatile organic compounds (VOCs), which can impact indoor air quality. The interplay between personal and environmental factors that influence this process, however, is not well understood.
To elucidate the details driving these bodily chemical reactions, a team of researchers from Tsinghua University in China subjected three male volunteers to a variety of scenarios involving changes in temperature and humidity, as well as personal hygiene practices and clothing choices.
The results, in ACS Environmental Au, suggest that variations in bathing frequency, indoor air temperature, and humidity, have little to do with the production of VOCs. The principal culprit is the concentration of ozone in the air, which alone accounts for over 90% of the measured variations in human skin emission rates.
Human skin oil is rich in ozone-reactive compounds, including squalene (C30H50) and a variety of unsaturated fatty acids, triglycerides, and wax esters. When exposed to ozone, these chemicals react to produce a variety of volatile compounds, including decanal, acetone, 6-methyl-5-hepten-2-one (6-MHO), and geranyl acetone.
Past research on nonhuman materials, such as cotton fabrics, soiled clothes, and pure squalene samples, found instead that higher relative humidity and the most-soiled clothing were associated with higher VOC levels.
"Results based on nonhuman materials are insightful but may not fully reflect what occurs on the human body," the researchers write in their paper.
Additional human-based research did find that ozone levels were the main driver of VOC emissions, but these past studies did not account for the effects of temperature, humidity, and hygiene.
This most recent study took a more holistic approach by isolating the effect of potentially confounding variables along with human subjects.
The new results found that refraining from changing clothes for three days did increase the total emissions of VOCs by about 25%, possibly because the material was in contact with the skin longer. Refraining from showering for three days, however, showed minimal effect, which the researchers attribute to the body's ability to rapidly replenish skin oil.
Other variables revealed that the more clothes worn the better. Compared to wearing freshly laundered t-shirts and shorts, wearing clothing that covered more of the body decreased VOCs by nearly 50%.
This study reinforces the prior conclusion that ozone concentration is the main factor controlling VOC emissions from skin. However, the researchers point out that the study's small, single-sex sample size (three males); lack of direct, real-time skin measurements; and laboratory settings, which may differ from real-world scenarios, open the door for additional investigations.
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More information: Yuekun Qu et al, Influence of Temperature, Humidity, Personal Hygiene, and Clothing on the Ozonolysis of Skin Oil at the Skin's Surface: Self Regulation, ACS Environmental Au (2025).
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