People generate their own oxidation field and change the indoor air chemistry around them

90% of people's lives are often spent indoors, either at work, home, or when traveling. Residents of these enclosed spaces are exposed to a wide range of chemicals from numerous sources, including indoor infiltration of outdoor pollutants, gaseous emissions from furnishings and building materials, and byproducts of our own activities like cooking and cleaning. Additionally, through our breath and skin, we are powerful mobile emission sources of chemicals that enter indoor air.

However, how do the chemicals return? This occurs to some extent naturally on its own, when it rains, and through chemical oxidation in the atmosphere outside. In great part, hydroxyl (OH) radicals are in charge of this chemical cleanup. These highly reactive chemicals, also known as the detergents of the atmosphere, are largely created when sunlight reacts with ozone and water vapor to make these very reactive molecules.

Direct sunlight and rain have obviously much less of an impact on the air within. Since glass windows primarily block UV radiation, it has been widely believed that indoor OH radical concentrations are significantly lower than outside concentrations and that ozone from the outside is the main oxidant of indoor airborne chemical contaminants.

Skin oils and ozone produce OH radicals.

However, it has since been found that the mere presence of people and ozone indoors can result in significant quantities of OH radical production. This has been revealed by a team lead by the Max Planck Institute for Chemistry in partnership with experts from the USA and Denmark.

The study's first author, Nora Zannoni, who is currently employed at the Institute of Atmospheric Sciences and Climate in Bologna, Italy, says, "The discovery that we humans are not only a source of reactive chemicals, but we are also able to transform these chemicals ourselves, was very surprising to us." The scientist from Jonathan Williams' team continues, "How much ozone is present, where it infiltrates, and how the ventilation of the interior area is arranged impact the strength and shape of the oxidation field." The levels the researchers discovered were even on par with outdoor midday OH concentrations.

The unsaturated triterpene squalene, which makes up around 10% of the skin lipids that keep our skin protected and supple, and other oils and fats on our skin react with ozone to create the oxidation field. The reaction releases a variety of double-bonded gas phase compounds, which further interact with ozone in the air to produce significant amounts of OH radicals. Using fast gas chromatograph-mass spectrometry equipment and proton transfer reaction mass spectrometry, these squalene degradation products were independently identified and measured. Additionally, the total OH reactivity was established concurrently, allowing the OH levels to be experimentally assessed.

At Copenhagen's Technical University of Denmark (DTU), the experiments were carried out. Four test subjects stayed in a specialized chamber with a climate control system under regulated circumstances. A quantity of ozone that was not dangerous to people but was typical of larger indoor levels was introduced to the chamber's air intake. The researchers measured the OH levels both with and without ozone before and during the volunteers' stay.

Results from a thorough multiphase chemical kinetic model from the University of California, Irvine and a computational fluid dynamics model from Pennsylvania State University, both in the USA, were combined in order to comprehend how the human-generated OH field during the experiments appeared in space and time. The modeling team looked at how the human-generated OH field altered under various ventilation and ozone conditions, beyond those evaluated in the lab, after validating the models against the experimental results. It was evident from the findings that the OH radicals were present, numerous, and generating potent spatial gradients.

Manabu Shiraiwa, a professor at UC Irvine who oversaw the modeling portion of the new work, said, "Our modeling team is the first and presently the only group that can combine chemical processes between the skin and indoor air, from molecular sizes to room scales." The measurements are explained by the model, explaining why OH is produced in the skin reaction.

Shiraiwa stated that there are still unresolved issues, such as how humidity affects the team's analysis of the reactions. He asserted that the study "opens up a new route for indoor air research."

Adapt testing procedures for construction materials and furniture

Because the oxidation field we produce will change many of the chemicals around, we need to reconsider indoor chemistry in occupied places. More species than ozone can be oxidized by OH, which results in a variety of products that are directly in our breathing zone and have unknowable health effects. According to project director Jonathan Williams, "This oxidation field will also affect the chemical signals we emit and receive" and "may help explain the recent observation that human sense of smell is generally more sensitive to compounds that react faster with OH,"

The new discovery has health ramifications as well: Many materials and furnishings are currently evaluated individually for chemical emissions prior to being given the all-clear for sale. However, atmospheric chemist Williams advises that testing should also be carried out with people and ozone present. This is because oxidation activities can produce minute particles and respiratory irritants like 4-oxopentanal (4-OPA) and other oxygenated species produced by OH radicals close to the respiratory tract. These may be harmful, especially to young children and the elderly.

These results are a result of the ICHEAR (Indoor Chemical Human Emissions and Reactivity Project), which brought together a team of international researchers from Germany, Denmark, and the United States (MPI). The modeling was done as a part of the MOCCIE project, which is run by the Pennsylvania State University and the University of California, Irvine. The A. P. Sloan foundation provided money to finance both initiatives.

Max Planck Institute for Chemistry