Airplane and hospital air is cleaner than you might think

• Link to: Northwestern Now Story

EMBARGOED UNTIL 8 P.M. EST (U.S.) ON WEDNESDAY, DECEMBER 3, 2025

• ·     Scientists sampled worn face masks from travelers and health care workers
• ·     Team detected a diverse but mostly harmless mix of bacterial species
• ·     Most airborne bacteria came from human skin, not illness
• ·     Microbial communities from hospitals and airplanes were highly similar

EVANSTON, Ill. --- When it comes to the air in public places, germophobes can breathe a bit easier. According to a new Northwestern University study, the ambient air on airplanes and in hospitals mostly contains harmless microbes typically associated with human skin.

In the first study of its kind, scientists used an unexpected sampling tool — used face masks and an aircraft air filter — to uncover the invisible world of microbes floating in our shared air. Their results revealed that the same types of harmless, human-associated bacteria dominate both airplane and hospital air.

Across all samples, the team detected 407 distinct microbial species, including common skin bacteria and environmental microbes. While a few potentially pathogenic microbes did appear, they were in extremely low abundance and without signs of active infection.

Not only does the study help illuminate what microbes exist in shared air, but it also demonstrates that face masks and air filters can be repurposed as non-invasive, cost-effective tools to monitor confined, high-traffic environments.

The study will be published on Wednesday (Dec. 3) in the journal Microbiome.

“We realized that we could use face masks as a cheap, easy air-sampling device for personal exposures and general exposures,” said Northwestern’s Erica M. Hartmann, who led the study. “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.”

An expert on indoor microbiomes, Hartmann is an associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering.

A second life for used face masks

Hartmann and her team conceived the project in January 2022, amid the COVID-19 pandemic. At the time, travelers became increasingly concerned with how well airplane cabins filtered and circulated air. Hartmann received a grant from the Walder Foundation to collect airplanes’ cabin filters to look for evidence of pathogens.

Although Hartmann did procure an aircraft’s high-efficiency particulate air (HEPA) filter, which had been used for more than 8,000 flight hours, she quickly realized the project might be impractical.

“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.”

Searching for another method that passively traps microbes, Hartmann and her team pivoted to a much cheaper and much less disruptive tool: face masks. For the study, volunteers wore face masks on both domestic and international flights. After landing, they put the masks into sterile bags and sent them to Hartmann’s lab. For comparison, Hartmann also collected face masks that volunteers took on flights but never wore.

To understand how indoor environments differ, Hartmann and her team wanted to examine another high-traffic, enclosed environment with heavily filtered air. The team selected hospitals as the second testbed. After wearing a face mask during a shift, hospital workers submitted their masks to Hartmann’s lab.

“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.”

The sky is clear

After receiving masks from travelers and health care workers, Hartmann’s team collected DNA from the outsides of masks. They found the air in hospitals and on airplanes contains a diverse but mostly harmless mix of microbes, with only minimal traces of potentially pathogenic species.

In both environments, common human-associated bacteria — especially those found on skin and in indoor air — dominated the samples. Although the abundance of each microbe present was slightly different, the microbial communities from hospitals and airplanes were highly similar. The overlap suggests that people themselves — rather than the specific environment — are the main source of airborne microbes in both settings. And those microbes floating around indoor air come from people’s skin, not from illness.

Hartmann’s team also identified a handful of antibiotic resistance genes, linked to major classes of antibiotics. While these genes do not indicate the presence of dangerous microbes in the air, they highlight how widespread antibiotic resistance has become.

Although indoor air might not be as harmful as some people may have feared, Hartmann emphasizes that airborne spread is just one way infections can travel. For many common illnesses, other routes — such as direct contact with an infected individual or interacting with high-touch surfaces — are far more important.

“For this study, we solely looked at what’s in the air,” Hartmann 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 study, “Metagenomic profiling of airborne microbial communities from aircraft filters and face masks,” was supported by the Walder Foundation (award number 21-00661).