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The indoor environment contains multiple sources of chemical compounds. These include continuous emissions from housing materials such as furniture, floors and furnishings, but also periodic intense emissions from human activities such as cooking, smoking, and cleaning.

Outdoor air chemicals can also enter indoor environments through infiltration and ventilation. Ozone (O₃) from outdoors can react with compounds indoors to create a complex chemical cocktail within the indoor living space. Since people spend up to 90% of their time indoors, exposure to this diverse array of chemical compounds over extended periods is cause for concern, particularly as the human-health impacts of many such chemicals remain poorly understood.

On the basis of their findings in 2022, Jonathan Williams's research group from the Max Planck Institute for Chemistry took a closer look at how the human oxidation field might be influenced by personal care products. The study is in the journal Science Advances.

"Given that the human oxidation field influences the chemical composition of air in the breathing zone and close to the skin, it affects our intake of chemicals, which in turn impacts human health. It is therefore of interest to examine how personal care products can influence the strength and spatial extent of the self-generated OH-field," explains Williams.

The experimental measurements made by the Max Planck team were supported by Manabu Shiraiwa and his team at the University of California (Irvine, U.S.) and Donghyun Rim's group from the Pennsylvania State University.

"Our team took a unique approach to simulate concentrations of chemical compounds near humans in the indoor environment," said Shiraiwa. "We developed a state-of-the-art chemical model that can simulate reactions of ozone with human skin and clothing that can lead to the formation of semi-volatile organic compounds."

"We applied a three-dimensional computational fluid dynamics model to simulate the evolution of the oxidation field around human occupants," said Rim. "This integrated modeling approach highlights the impact of personal care products on the human oxidation field."

Personal care products affect the human oxidation field

First, the researchers examined how the application of body lotion impacts the chemistry in the periphery of the persons tested. Then they investigated how perfume applied to the skin affects the chemical composition of the indoor air. In both cases, Williams and his team saw that the OH concentration around the volunteers decreased. This means that it decreased compared to the standard case without cosmetics, where ozone reacts on human skin to form gas phase products that react again in the air with ozone to make OH.

Concerning the perfume, the researchers explain the OH decrease with the primary perfume component, ethanol: it reacts with OH, using it up, as ethanol does not produce OH when reacting with ozone.

"Regarding body lotion, we can explain the decline in two ways. One is that phenoxyethanol—a chemical in the body lotion—reacts with OH but does not generate OH with ozone. This is the same as the perfume with ethanol. The second explanation is simply that the body lotion gets in the way of ozone reacting with squalene on the skin," states atmospheric chemist Williams.

"The application of a fragrance and a lotion together showed that fragrances impact the OH reactivity and concentration over shorter time periods, whereas lotions show more persistent effects, consistent with the rate of emissions of organic compounds from these personal care products," sums up Nora Zannoni, first author of the study. She is currently employed at the Institute of Atmospheric Sciences and Climate in Bologna, Italy.

Implications for indoor chemistry

While thousands of different fragrances and lotions exist on the market, there are some general conclusions valid for any product that the international research team draws, based on their tests.

Following the new findings of this study, an applied fragrance indoors would be expected to suppress the personal human oxidation field. In contrast with fragrances, lotions have more variable compositions. Despite their variable composition, they expect most lotions to suppress the human oxidation field due to a combination of dilution of skin oil constituents and reduced interaction between O₃ and the skin.

Additionally, marketed lotions contain preservatives acting as antimicrobial agents. Widely used is phenoxyethanol, which further contributes to suppressing the human oxidation field by reacting with the OH radicals as experimentally demonstrated in this study.

"If we buy a sofa from a major furniture company, it is tested for harmful emissions before being put on sale. However, when we sit on the sofa, we naturally transform some of these emissions because of the oxidation field we generate. This can create many additional compounds in our breathing zone whose properties are not well known or studied. "Interestingly, body lotion and perfume both seem to dampen down this effect," says Williams.

The experiments were conducted at the Technical University of Denmark (DTU) in Copenhagen in 2021. Four test subjects stayed in a special climate-controlled chamber under standardized conditions. Ozone was added to the chamber air inflow in a quantity that was not harmful to humans but representative of the higher range of indoor levels. The team determined the OH concentrations indirectly by quantifying the individual OH sources and the overall loss rate of OH. The OH field was only generated when ozone was present.

By combining air measurements from within the chamber with model simulations, they calculated the effect of the lotion and fragrance on the human oxidation field.

These findings are part of the project ICHEAR (Indoor Chemical Human Emissions and Reactivity Project) which brought together a group of collaborating international scientists from Denmark (DTU), U.S. (Rutgers University), and Germany (MPI). The modeling was part of the MOCCIE project based at the University of California Irvine and the Pennsylvania State University.