There are numerous contaminants in the air we breathe. Outside, these can be washed away by rain and the oxidation that occurs when UV light from the Sun interacts with ozone and water vapor.
So what occurs inside?
The chemical cleaning that occurs via these hydroxyl (OH) radicals - short-lived reactive species whose duty it is to oxidize other molecules - occurs through a combination of ozone leaking in from the outside and the oxidation fields that we build around ourselves, according to a recent study.
Scientists discovered that levels of OH radicals indoors can be comparable to daytime outdoor levels in several conditions. In other words, we're walking, breathing chemical reaction machines, with consequences for indoor air quality and human health.
"The revelation that we humans are not only a source of reactive chemicals, but also capable of transforming these molecules ourselves was really shocking to us," says Nora Zannoni, an atmospheric scientist at Italy's Institute of Atmospheric Sciences and Climate.
The researchers conducted studies with three distinct groups of four people in a specific climate-controlled box with ozone levels that matched the higher end of what might be found inside. OH levels were recorded with and without ozone, as well as before and after humans entered the room.
A combination of computational fluid dynamics models and actual air observations (part of which used mass spectrometry techniques) revealed that OH radicals were present, plentiful, and developing around humans.
The scientists discovered that our own oxidation fields are created when ozone reacts with the oils and fats on our skin, particularly the unsaturated triterpene squalene molecule, which accounts for around 10% of the lipids that protect and keep our skin supple.
"How much ozone is present, where it infiltrates, and how the ventilation of the indoor area is arranged impact the strength and shape of the oxidation field," Zannoni explains.
We spend approximately 90% of our time inside, and these findings have crucial implications for ensuring that time is spent inhaling air that is as clean and healthful for us as possible - something we're all now acutely aware of as a result of the pandemic.
While we've always known that oxidation processes occur indoors, it appears that in some cases, human-generated reactions are the dominant ones.
Because reactions can produce lung irritants while also eliminating pollutants, it's critical to understand these processes both in isolation and in relation to other indoor compounds that may come from building materials, furniture, and scented goods.
There's still a lot of work to be done: the scientists want to know how humidity levels affect the reactions, for example, and how having more individuals in a room changes the picture.
Furthermore, the oxidation fields that individuals generate may influence our perception of odor.
"We need to reconsider indoor chemistry in inhabited areas because the oxidation field we generate will change many of the chemicals in our immediate surroundings," says atmospheric chemist Jonathan Williams of Germany's Max Planck Institute for Chemistry.
The study was published in the journal Science.