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The strategy of keeping spacecraft as clean and sterile as possible to ensure astronauts don’t become ill may be a mistake. Our immune systems may need stimulation from certain kinds of molecules and microbes to stay healthy, say researchers who have been studying the International Space Station (ISS).

“The general notion is to try to have as few microbes as possible, but the question is if that is the best thing to do for long-term space travel,” says at the University of California San Diego. “And it’s also relevant for places on Earth, such as research [stations], hospitals and submarines, where you can spend months or more.”

One reason for sterilising spacecraft is to avoid contaminating planets, such as Mars, with microbes from Earth, but the main concern is that an infection that would be easily treatable on our planet could become a major issue on a spacecraft.

Dorrestein and his team have analysed more than 700 swabs of surfaces onboard the ISS, and found it is indeed squeaky clean in terms of the diversity of the molecules and microbes present.

“The space station is just devoid of a lot of molecules and microbes,” he says. “It is at the extreme end of human living.”

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The team thinks this lack of exposure to the usual wide array of molecules and microbes could be one reason why there are significant changes to the immune system in space. On the space station, astronauts often get rashes, unusual allergies, fungal or bacterial infections, as well as activation of latent viruses such as Epstein-Barr, which was found in one of the samples.

“We don’t have a full understanding of that,” says Dorrestein. “But the way I view this is that your immune system has to be periodically pinged.”

The researchers say we need to find ways to make environments like the space station “dirtier, so that they are more diverse in molecules and microbes, but without introducing any infection-causing pathogens”.

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One way to do this would be to apply bacteria such as Bacillus subtilis to surfaces instead of disinfectants, says Dorrestein. B. subtilis is already widely used for its antifungal activity.

Growing a range of plants could help as well. “We know from other studies that when people are exposed to a lot more plant molecules, they tend to have reduced connections to asthma and allergies,” says Dorrestein. There is a complete absence of these plant molecules on the space station, he says.