Personal hygiene habits affect indoor air quality

At home, school, and work, our bodies encounter a host of volatile organic compounds, or VOCs, given off by furniture, carpet, or paint. But people also emanate VOCs in appreciable amounts, influencing indoor air quality. New research finds that human hygiene habits such as skipping showers or slathering on lotion also affect the profile of indoor VOCs (Environ. Sci. Technol. 2024, DOI: 10.1021/acs.est.4c01698).

Compounds in skin oils have been found to react with oxidants such as ozone—an air pollutant that often finds its way inside from outdoors—to produce VOCs. So it may seem obvious that showering or not showering might change the VOC mix. “But actually,” says Nijing Wang of the Max Planck Institute for Chemistry, “nobody has done such a study.”

Wang and her colleagues had four male volunteers sit together for several hours a day in a steel room that had been rigged with sensitive instruments to detect VOCs. They gave the men unscented personal care products to use during the study and asked them to avoid consuming alcohol or heavily seasoned foods, which can influence the amounts and type of VOCs they emitted.

The team measured the volunteers’ VOC emissions when the men had showered the previous evening, to get a benchmark level, and then again on 2 successive days when they hadn’t showered. The researchers took these measurements in both ozone-containing and ozone-free air; in another experiment, the team analyzed the molecules wafting off the participants when they had applied lotion before entering the chamber.

After skipped showers, both with and without ozone in the air, the volunteers’ VOC emission rates rose. After the participants had missed showers for 2 days, their total VOCs in ozone-free air were 43% higher than the benchmark level when the major contributors from breath were not included. The mix included VOCs that noses can discern at relatively low concentrations, such as acetic acid and some carboxylic acids, which could produce a vinegary smell, Wang says. These compounds likely result from bacteria munching away at skin cells and oils.

“This is something that’s important,” says Brandon Boor, an indoor air quality researcher at Purdue University who was not involved with the study. “Not everybody takes a shower every single day.” These VOC emissions could affect olfaction, which is an underappreciated aspect of air quality, he adds.

Skin oil is loaded with squalene, a big molecule with 30 carbons and many double bonds that reacts quickly with ozone. Since skin oil levels stay fairly constant even if a person forgoes washing for a day or two, Wang and colleagues figured that the emission of ozonolysis reaction products would also be steady. But when the volunteers didn’t shower, both the emission rates and the variety of VOCs that were released rose when ozone was present. The uptick was probably due to secondary VOCs—those formed from reactions with ozonolysis products.

There’s not yet much evidence that these ozonolysis products cause negative health effects at the low concentrations generated, says Delphine Farmer, an atmospheric chemist at Colorado State University who wasn’t involved with the work. Some of the compounds detected—4-oxopentanal and 6-methyl-5-hepten-2-one—may cause lung irritation, and long-term toxic effects remain unknown.

Meanwhile, ozone itself is a well-established air toxin, Farmer notes. Reactions that remove it would at least lower ozone exposure, but it’s not clear by how much. Calculations or modeling could show the degree to which such reactions lower indoor levels of ozone.

The researchers measured lower amounts of skin ozonolysis products when participants had applied unscented lotion before they entered the ozone-containing air. The lotion may be acting as a barrier that prevents some of these reactions, Farmer says. But the lotion itself emitted VOCs and contained oils that served as precursors for some VOCs. Many hair products and skin-care products—including fragrances and deodorants—would also contribute to indoor VOCs, Boor says.

Wang recommends showering daily to keep smells from hampering indoor air quality. It’s not clear how the suite of chemicals produced by reactions between ozone, personal care products, and skin affect our health.

Farmer says that because the study looked only at VOCs coming from these four men, it’s missing out on the diversity of compounds that ultimately result from human metabolism, which correlates with gender, age, and race. Still, she says, the study gets to the interesting fundamental question of how our bodies’ metabolic emissions interact with compounds such as oxidants in the air.