A new study found that the International Space Station (ISS) might actually be overly sterile. While having fewer microbes might initially sound like a good thing, it could actually be leading to health challenges astronauts often face in orbit. The research, led by the University of California, San Diego (UC San Diego), discovered that the ISS has far fewer microbes than typical indoor spaces on Earth and that the bacteria on the station are primarily from the astronauts themselves.

According to the researchers, bringing in more beneficial natural microbes may help those on board maintain a healthier immune system and reduce issues like skin rashes and inflammation.

When Bacteria is Missing

In the most comprehensive sampling effort of its kind, astronauts aboard the ISS swabbed over 800 surfaces throughout nine modules in the U.S. Orbital Segment. These samples were sent back to Earth where researchers performed genetic sequencing to identify every possible bacterium. They also used a technique called “untargeted metabolomics” to analyze chemical traces on surfaces from cleaning agents to food residues and even human metabolites.

“Future built environments, including space stations, could benefit from intentionally fostering diverse microbial communities that better mimic the natural microbial exposures experienced on Earth, rather than relying on highly sanitized spaces,” says co-first author Rodolfo Salido at UC San Diego.

Long-standing research suggests that exposure to a broader range of microbes can encourage a healthier immune response, potentially lowering risks of autoimmune disorders or chronic inflammation. Astronauts often report experiencing immune dysfunction, rashes, and allergic reactions while on missions, and the study’s authors think the near-complete lack of natural microbes in the space environment might be a key factor.

While prior studies of the ISS focused on relatively small areas, this new survey collected around 100 times more samples. No corner of the space station was left unturned. The researchers then constructed 3D maps of the station’s interior, revealing which bacteria and chemicals appeared in each module. They found that more than half of all bacteria originated from human skin, matching the way microbes spread in any indoor space. However, the big difference was just how limited the diversity of these microbes was compared to samples from buildings on Earth.

An Over-Sterilized Environment?

The team discovered traces of cleaning chemicals and disinfectants throughout the ISS, a predictable finding for a tightly controlled space environment. But they also noticed a notable pattern: Areas with more abundant disinfectant residues often had higher bacterial diversity. Most of these microbes are not necessarily harmful. They include species commonly found on people’s bodies—like staphylococci—and are spread as astronauts go about their daily tasks.

“We noticed that the abundance of disinfectant on the surface of the International Space Station is highly correlated with the microbiome diversity at different locations on the space station,” says co-first author Haoqi “Nina” Zhao of UC San Diego.

Notably, the ISS lacked many environmental bacteria usually found in soil and water on Earth—organisms that studies suggest can play a vital role in helping calibrate our immune systems. The result is that the ISS, while extremely clean, is so dominated by human-associated bacteria that some researchers wonder if it’s missing a protective layer of broader microbial diversity.

The researchers compared the ISS microbial makeup to thousands of building samples on Earth—homes, offices, hospitals, even research facilities that simulate space-like environments. It turns out the ISS’s microbiome is most similar to highly urbanized, isolated structures such as hospitals or closed habitats, rather than rural homes or more open-air buildings. This places the ISS at what researchers called the “extreme end” of an indoor-environment spectrum.

Could a Little “Dirt” Help?

Introducing more “friendly” soil microbes—or even safe amounts of soil itself—might help create a more balanced microbiome aboard spacecraft. These strategies do not mean compromising hygiene or encouraging pathogens but reintroducing beneficial organisms humans naturally encounter on Earth.

“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,” says Salido. “We need to start thinking about what other beneficial companions we should be sending with these astronauts to help them develop ecosystems that will be sustainable and beneficial for all.”

In the future, the researchers hope to refine their analyses to detect potentially pathogenic microbes and signals of human health from environmental metabolites. They say these methods could also help improve the health of people living and working in similarly sterile environments on our home planet.

The findings appeared in the journal Cell.