Let’s face it: Floating 220 miles above Earth in a sealed space station for months doesn’t sound like the most sanitary venture. After all, even inhabitants of the junkiest dwellings on land have the benefit of occasional fresh air. Earlier this year, astronaut Scott Kelly described the International Space Station’s singular aroma as something akin to a mixture of antiseptic and garbage. There's no positive way to spin that one. But how clean (or dirty) is the International Space Station? New research on the spacecraft’s unique bacterial population contains some reassuring news — and a few icky insights.

In their quest to characterize the cleanliness of the ISS, scientists from NASA’s Jet Propulsion Laboratory got the literal dirt on the station. Astrobiologist Kasthuri Venkateswaran and his colleagues studied a HEPA filter that had been on the craft for 40 months (there are 21 total on the craft at any given time). They also got their hands on two bags of dust from the ISS vacuum cleaner. It might not sound glamorous, but a daily sweep is one of the crew's many tasks.  

After performing a genetic analysis, the team compared its data to debris from NASA cleanrooms on Earth. Aimed at preventing contamination in space, clean environments of this nature have been in use since the Apollo era. Ideally, both the cleanrooms and the space station are designed to be free of the germs and debris of the outside world. But there’s a major difference between the two venues: Only one is used as a home.

The difference was clear in DNA analysis of the different dust samples. Skin bacteria called Actinobacteria were much more prevalent on the ISS — not too surprising for a place that serves as a full-time astronaut apartment. Samples from the vacuum bags, which contained dust that was just a day old, boasted critters like Staphylococcus, which can cause diseases ranging from food poisoning to skin infections. The findings suggest that astronauts pick up more skin microorganisms with their vacuums than with their filters — and that the air being circulated through the ISS is much cleaner than its surfaces.

In fact, the viable microbial population of the vacuum dust collected from the ISS was almost 75 times that of the filter and nearly twice the population found in cleanrooms on Earth. That’s probably due to the fact that people are living on the ISS, but also indicates that the HEPA filter is pretty good at filtering out living organisms long-term.

But there were also some nasty hitchhikers on board that couldn’t be found in the cleanrooms on Earth: Opportunistic pathogens like Propionibacterium. It’s not certain whether these pathogens are actually dangerous to astronauts in the closed environment of the space station, says Venkateswaran— but he wants to find out. Next, his team will study whether the bacteria are harmful and how microgravity affects their growth and prevalence. Not only will the microbiome of the International Space Station serve as a baseline for future studies, but it can help inform better cleaning protocols and research on how to keep future Mars astronauts safe during their long voyage.

To Venkateswaran, the hassle of waiting months for samples and relying on a capsule that just fell from the sky to deliver them is worth it. After all, he laughs, astronauts could culture or sequence some of the pathogens in space, but it might turn the International Space Station into “a flying time bomb.” Getting a bit dirty now could help astronauts avoid disease and even interplanetary contamination in the future.

So is the International Space Station dirty or clean? Venkateswaran puts it into perspective: “The ISS is a unique built environment. People assume it’s filthy, but it’s not. It’s many, many times cleaner than your bathroom at home.”

Erin Blakemore (@heroinebook) is a freelance journalist from Boulder, Colo. She is the author of The Heroine’s Bookshelf (Harper). 

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