Airplane and hospital air may be less concerning to travelers and patients than previously thought, according to a new study from Northwestern University. Researchers found that the air in these confined environments contains mostly harmless bacteria commonly associated with human skin.
The study, which will be published in the journal Microbiome, involved collecting used face masks from both airline passengers and health care workers. Scientists also analyzed an aircraft’s high-efficiency particulate air (HEPA) filter that had been used for over 8,000 flight hours. In total, researchers identified 407 distinct microbial species across all samples. While some potentially harmful microbes were detected, they appeared only in very low amounts and without evidence of active infection.
Erica M. Hartmann, associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering, led the research team. “We realized that we could use face masks as a cheap, easy air-sampling device for personal exposures and general exposures,” Hartmann said. “We extracted DNA from those masks and examined the types of bacteria found there. Somewhat unsurprisingly, the bacteria were the types that we would typically associate with indoor air. Indoor air looks like indoor air, which also looks like human skin.”
Hartmann began the project in January 2022 during heightened concern about COVID-19 transmission on airplanes. Initially supported by a grant from the Walder Foundation to analyze airplane cabin filters for pathogens, she discovered logistical challenges: removing HEPA filters is costly and requires taking planes out of service.
“At the time, there was a serious concern about Covid transmission on planes,” Hartmann said. “HEPA filters on planes filter the air with incredibly high efficiency, so we thought it would be a great way to capture everything in the air. But these filters are not like the filters in our cars or homes. They cost thousands of dollars and, in order to remove them, workers have to pull the airplane out of service for maintenance. This obviously costs an incredible amount of money, and that was eye opening.”
To find an alternative sampling method, Hartmann's team collected used face masks worn by travelers during flights as well as by hospital workers during their shifts. Masks taken on flights but never worn were also collected for comparison.
“As a comparison group, we thought about another population of people who were likely wearing masks anyway,” Hartmann said. “We landed on health care providers.”
Analysis revealed similar microbial communities in both airplanes and hospitals: common skin bacteria dominated both settings regardless of location or population studied. This suggests people themselves are the primary source of airborne microbes indoors.
Hartmann's team also detected some antibiotic resistance genes tied to major classes of antibiotics—an indicator not necessarily linked to dangerous microbes present but rather showing how widespread antibiotic resistance is.
While these findings suggest indoor air might not pose as much risk as previously believed regarding airborne illness transmission, Hartmann noted other infection routes remain important: “For this study, we solely looked at what’s in the air,” she said. “Hand hygiene remains an effective way to prevent diseases transmission from surfaces. We were interested in what people are exposed to via air, even if they are washing their hands.”
The research was funded by the Walder Foundation (award number 21-00661).