For astronauts living in the International Space Station, one of the most common medical complaints is skin rashes. It could be because things aren’t dirty enough in space.

You might think itchy skin is more likely to be a byproduct of poor hygiene and too many bacteria. But an exhaustive census of microbes living aboard the space station suggests the ailments could instead be caused by too few species of these microscopic creatures, rather than too many. In other words, the space station might be too clean.

“If we really want life to thrive outside Earth, we can’t just take a small branch of the tree of life and launch it into space and hope that it will work out,” said Rodolfo Salido, a bioengineer at the University of California, San Diego (UCSD), who helped lead the new survey of the space station’s microbiome, published last week in Cell.

The research comes amid a growing appreciation of the power of microbial ecosystems to influence our health for good and ill. Some scientists suspect that increases in immune disorders are tied to more people living in urban settings, where they are exposed to a less diverse range of microbes.

The space station is radical version of a human-made environment, one almost entirely cut off from the natural world. It turns out that’s true at the microscopic level as well.

To get a complete picture of what microbes were keeping them company, astronauts living in the section of the space station run jointly by U.S., European, Canadian and Japanese space agencies swabbed more than 800 spots. Then they shipped the swabs to Earth for analysis.

The samples contained just 6% of the spectrum of microbial life. That means the space station has notably less diversity than the 10-16% levels found in homes in other studies, and a small fraction of the nearly 30% found in natural terrestrial environments.

The station provides “an extreme example of an industrialized environment,” the scientists noted.

That’s not to say every room in the space station was identical. In dining and cooking areas, there were more microbes associated with food. In an area containing the toilet, microbes tied to feces and urine were more abundant.

The new study doesn’t directly link this lack of microbial diversity to health problems. Some research has found evidence that exposure to a wider array of microbes can boost people’s immune systems, making them less prone to conditions such as asthma. But other research has found that, at least in mice, microbial diversity doesn’t reduce allergies.  

The authors of the new paper are clear that the space station could benefit from a microbial infusion. “There’s a big difference between exposure to healthy soil from gardening versus stewing in our own filth, which is kind of what happens if we’re in a strictly enclosed environment with no ongoing input of those healthy sources of microbes from the outside,” said Rob Knight, a UCSD microbiologist involved in the new research.

It’s not clear exactly how to remedy the situation. NASA would probably frown at astronauts simply arriving for launch with dirt on their shoes. The researchers suggest closer scrutiny of the high use of disinfectants and the possibility of using probiotics to assist with sanitation, as well as “intentionally fostering diverse microbial communities resembling Earth’s natural exposures instead of relying on highly sanitized spaces.”

At a time of growing interest in sending people to the moon and Mars, now is the time to give more thought to what microscopic companions travel with them.

Salido, et. al. “The International Space Station has a unique and extreme microbial and chemical environment driven by use patterns.” Cell. Feb. 27, 2025.

Top image based on image by Yuri Arcurs | Dreamstime.com