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Indoor air has been found to be dramatically altered by chemical reactions on human skin, ie, the skin oil-ozone interaction, a new study found. Scientists at China's Tsinghua University conducted an experiment on human volunteers to study the influence of hygiene, clothing, temperature, and humidity on body emission of volatile organic compounds (VOCs). Authors in ACS Environmental Au state that ozone concentration is the only VOC emission predictor and that the others do not contribute or contribute minimally.

The experiment was conducted on three individuals who were exposed to controlled conditions under varying conditions. These varied from bathing frequency, changes in indoor clothes style, changes in indoor temperature and humidity. While the population sample is small, the research is quite informative regarding how normal human activity influences environmental parameters to decide the quality of air within the indoor environment.

The skin oil contains ozone-reactive compounds such as squalene and other unsaturated fatty acids. Under the influence of ozone, the nature of chemicals reacts with each other to form a mixture of volatile chemicals such as acetone, decanal, 6-methyl-5-hepten-2-one (6-MHO), and geranyl acetone. This type of emission includes indoor VOC levels, which contribute indirectly to air quality and human health.

While previous work with non-human substrates like cotton fabric, filthy clothes, and pure squalene samples had predicted that humidity and hygiene were the major determinants towards VOC emissions, the human-involving study refutes such a hypothesis. The findings show that more than 90% of variability in human skin emission rates can be explained by ozone.

For body hygiene, three days of not showering had little effect on VOC emissions. This is because skin oils rapidly re-established themselves that replaced skin surface chemistry. Three days of wearing clean clothing did increase total VOC emissions by approximately 25%, which is likely because longer skin contact with clothing results in increased ozone-skin oil interaction.

The type of clothing also contributed significantly. Men who dressed in long-sleeved shirts and pants released about 50% less VOC than men wearing short-sleeved shirts and shorts. More material covering restricted the contact of ozone with skin surfaces and thus decreased the chemical reactions that result in VOC production.

Contrary to this, indoor relative temperature fluctuation and humidity was not found to exert a substantial effect on VOC emissions of human skin. This opposes earlier results with non-human material samples where VOC evaporation would increase with an increase in relative humidity. In the authors' opinion, this may not be the case with dynamic self-adjusting human skin.

The study also compared ozone-present and ozone-free controlled environments. Total primary VOC production, including acetone and geranyl acetone, was greatly enhanced under ozone-present conditions again verifying ozone concentration as the main driving factor for indoor skin-derived VOC production.

In spite of the informative results, the authors also outline some of the limitations. The all-male subject sample of limited size in the study and laboratory unnaturalness are not representative of common indoor conditions with heterogeneous inhabitants and activities. Further, the absence of real-time measurements from the direct skin surface reduces the level of detail in the results.

Yet, the study is part of the growing literature on the indoor environmental condition and human biological system as determinants of air quality. Because indoor air pollution continues to increase in dense or poorly ventilated populations, VOC source identification will be helpful in the creation of healthier indoor environments.
The findings also suggest design of building ventilation systems and indoor control strategies for ozone. Indoor control of ozone levels, e.g., would be a more effective strategy than how to alter the individual's behaviour in the matter of bathing or humidifying a room. This is directed towards environmental control rather than personal hygiene behaviour.

Because follow-up studies depend on these findings, subsequent research would be in a position to study more of the population, use real-time measurements, and study more sophisticated real-world implementations to get the full picture of the influence of human skin chemistry on indoor air.

Source:

Charles Blue, Phys.org, on work published in ACS Environmental Au (2025) – DOI: 10.1021/acsenvironau.5c00073
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