Through rain and chemical oxidation, outdoor chemicals naturally dissipate. 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 highly reactive molecules. But scientists learned that combining ozone, human fats, and oils could also happen indoors.
Scientists Discover Indoor OH Radical
It has been widely accepted that the concentration of OH radicals is significantly lower indoors than outdoors because glass windows primarily block UV rays. And the main oxidant of indoor chemical pollution is the ozone that enters from outside.
But it has been discovered that even the simple presence of people and ozone indoors can produce large amounts of OH radicals. This has been proven by a team of scientists from the USA and Denmark, led by the Max Planck Institute for Chemistry.
The discovery that humans are not only a source of reactive chemicals but are also capable of transforming these chemicals ourselves was very surprising, according to Nora Zannoni, the study's first author. This research was first published in the scientific journal Science. How much ozone is present, where it infiltrates, and how the indoor space is ventilated all affect the strength and structure of the oxidation field. The levels the researchers discovered were even on par with outdoor midday OH concentrations.
OH Radical Reaction
The unsaturated triterpene squalene, in particular, reacts with ozone in the presence of skin lipids and fats to produce the oxidation field. This makes up roughly 10% of the skin lipids that shield and maintain the suppleness of our skin.
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. These squalene degradation products were independently identified and measured using fast gas chromatograph-mass spectrometry equipment and proton transfer reaction mass spectrometry.
Additionally, the total OH reactivity was established concurrently, allowing the OH levels to be experimentally assessed.
OH Radical Formation Experimentation on Human
The trials were conducted at Copenhagen's Technical University of Denmark (DTU). Four test subjects stayed in a specialized chamber with a climate control system under regulated settings. 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 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 to understand 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 generated potent spatial gradients.
Professor Manabu Shiraiwa of UC Irvine, who was in charge of the modeling component of the new research, claimed that his modeling team is the first and currently the only one that can connect chemical processes between the skin and indoor air, from molecular to room scales. The model explains the measurements and why OH is created during 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.
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