Dirty Clothes and Ozone Can Create an Indoor "Pollution Cloud," Study Shows

A single dirty t-shirt can reduce indoor air pollution but at a cost.


There’s good ozone and there’s bad ozone. The good type makes up a protective layer in Earth’s upper atmosphere and shields us from harmful ultraviolet rays. Meanwhile, ground-level ozone is an air pollutant and the main ingredient in smog. At times, this sort of ozone makes its way inside, especially in the summer.

At the moment, the scientific understanding of indoor ozone is relatively limited. As a step toward remedying that, scientists recently evaluated what happens to ozone when it interacts with indoor features — specifically, our clothes.

In May, they published a study in Communications Chemistry explaining that soiled clothes actually minimize indoor ozone levels — but sometimes at a cost to human health.

Human skin oils contain substances like fatty acid, wax esters, and squalene, which is a natural organic compound that’s also found in shark liver oil.

Previous studies have shown that these substances react with ozone — so much so that clothes, which often contain skin oil, can remove significant amounts of ozone indoors.

The scientists note that a single soiled t-shirt can remove between 30 to 70 percent of the ozone circulating near a person. 

Squalene is especially effective at removing ozone because it has a higher reaction rate. However, it can also lead to another problem: In this study, the scientists compared the reaction between ozone and squalene, and between ozone and clothing, with computer modeling. These models simulated indoor spaces with varying ventilation conditions and different levels of air quality.

They discovered that when a squalene-ozone reaction happens ozone is removed, but new, volatile substances are produced, including carbonyls that can irritate skin and lungs. This process is called squalene ozonolysis.

When ozone hits oils on skin and in dirty clothes, it can produce a personal cloud of irritants.

Penn State

“When ozone is depleted through human skin, we become the generators of the primary products, which can cause sensory irritations,” co-author and Penn State assistant professor Donghyun Rim, Ph.D. explained Thursday. “Some people call this higher concentration of pollutants around the human body the personal cloud, or we call it the ‘Pig-Pen Effect.’”

Pig-Pen, the 'Peanuts' character, is known for his own personal cloud.


Where does that leave us? Rim notes that it doesn’t necessarily mean we should rely on only ever having clean clothes around, which means we “might be breathing in more of this ozone, which isn’t good for you either.” But the Rim and colleagues say there are ways that we can take steps to improve indoor air quality and human exposure to squalene ozonolysis.

One is simply relying more on your laundry room: The scientists write that soiled clothing awaiting laundering “could be stored in a room that is not regularly used by people in order to reduce their exposure.” They also emphasize the benefits of having good indoor ventilation.

There’s also the possibility that we can worry less about this reaction if we reduce ozone levels in general. That takes both change on a personal and systematic level. On the personal level, direct steps to reduce ozone include turning off lights when you’re not using them, driving less, not keeping engines idle, and using cleaning products that don’t contain volatile organic compounds.

“The bottom line is that we, humans, spend more than 90 percent of our time in buildings, or indoor environments,” says Rim, “but as far as actual research goes, there are still a lot of unknowns about what’s going on and what types of gases and particles we’re exposed to in indoor environments.”

Multiphase reactions of ozone with human skin oils impact indoor air quality by depleting ozone and forming semi-volatile organic compounds, which can be respiratory and skin irritants. Here we demonstrate the impact of clothing on indoor air composition and human exposure by integrating indoor chemistry modeling over a wide range of different spatial and temporal scales. Constrained by molecular dynamics simulations that provide key kinetic parameters, the kinetic model reproduces experimental measurements and predicts that squalene could persist in clothing for several hours to over a day depending on ozone concentrations. Soiled clothing protects skin from ozone exposure even with high concentrations, but can enhance concentrations of oxidation products to a ppb level depending on air exchange rates. Computational fluid dynamics simulations reveal that primary products have ~1.6–2.0 times higher concentrations in the breathing zone than in bulk room air, while secondary products are distributed more uniformly throughout a room.
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