By Devin Reese

We humans stuff ourselves into tight quarters for various reasons. Sometimes we pack by the dozens into airliners bound for Boca. Other times, we’re forced into close proximity as we sit in hospital waiting rooms.

Especially on the heels of a global respiratory disease pandemic, being in such crowded spaces can induce anxiety. But is the air circulating through airplanes and hospitals swimming with airborne pathogens or other toxic ick, as our imaginations would sometimes have us believe?

According to researchers studying the air quality in those two common proxies for human sardine cans, it’s not so bad.

New findings published in Microbiome, showed the presence of diverse, and mostly benign, microbial communities that were remarkably similar between hospitals and airplanes. The few pathogenic microbes, such as Escherichia coli, were in low densities and not indicative of active infections. However, the 23 types of antibiotic-resistant genes that showed up in both samples, linked to major classes of antibiotics like gentamycin and streptomycin, were grim evidence for the rise in antibiotic-resistant bacterial infections.

The 407 distinct microbes detected by a team of researchers led by Northwestern University scientists had largely come from people’s skin. These included the harmless bacteria, Staphylococcus epidermidis and Cutibacterium acnes. While the thought that we’re continually breathing in others’ skin microbes is a bit unsettling, it’s also to be expected.

“Indoor air looks like indoor air, which also looks like human skin,” said Northwestern synthetic biologist and co-author Erica Hartmann in a statement. As Hartmann and her colleagues from the Harvard T.H. Chan School of Public Health and the Argonne National Laboratory stated in the paper, humans naturally carry around 1012 microorganisms on their skin, which become airborne as we shed millions of skin cells per day.

While airplane and hospital air teeming with harmless skin bacteria may be relieving or distressing, depending upon who you ask, the methods the research team used to generate these findings were undeniably resourceful. The scientists gathered data from discarded facemasks used by air travelers and healthcare professionals to learn more about indoor air quality at their study sites.

“We realized that we could use face masks as a cheap, easy air-sampling device for personal exposures and general exposures,” Hartmann added.

The COVID-19 pandemic inspired a spate of research on airborne microbes. But they’re difficult to sample, since air is a medium that’s continually in motion, carrying around minuscule, low-mass microbes such as viruses, bacteria, and fungi. It’s like trying to sample moving needles in a haystack. Instead of directly assessing the airborne microbial community, researchers extracted microbes from face masks that had been worn by people in planes or hospitals to gauge microbial loads in those environments.

Using a “shotgun metagenomics” approach, Hartmann and colleagues extracted DNA from the outside of 22 disposable face masks. Travelers wore masks on domestic and international flights, while hospital workers wore masks for the duration of a shift, then mailed them in sterile bags to Harmann’s lab. Unworn face masks served as study controls.

In addition to better characterizing the microbial milieu in some of the places where humans crowd in together, the research pioneered a way that face masks can be repurposed to study air quality in enclosed environments.