A study has found that if the International Space Station (ISS) is too clean, it can actually harm astronauts' health. Not being exposed to a diverse range of microorganisms, like on Earth, can weaken the immune system. This suggests that playing in the dirt as a child and being exposed to various microorganisms is also beneficial for health in space.
Researchers at the University of California, San Diego (UCSD) noted on the 28th that the space station has almost no soil microorganisms that one would normally encounter on Earth, which could make astronauts more vulnerable to immune-related diseases. The findings were published that day in the international journal "Cell."
Astronauts often experience weakened immunity and various skin conditions during long stays at the space station. The researchers analyzed microorganisms collected from 803 locations in the space station to find the cause.
The microorganisms in the space station resembled those found in hospitals. Most of them were derived from human skin. Some were found in the food preparation area, and fecal microorganisms were also detected in the bathrooms.
However, microorganisms derived from soil or water were hardly found. Dr. Rodolfo Salido, the first author of the paper, explained that "environmental microorganisms found in soil or water play a crucial role in maintaining a healthy immune system." It was inferred that immune disorders such as rashes, stomatitis, fungal infections, and shingles experienced by astronauts are related to the lack of diversity of microorganisms.
Microorganisms compete for resources and space. If the microbial ecosystem is not diverse, certain microorganisms can become excessively abundant, leading to health issues. Professor Rob Knight, the corresponding author, stated that "being exposed to microorganisms encountered in healthy soil is completely different from being exposed only to the microorganisms excreted by humans in a confined environment," and added that "an environment like the space station, where beneficial microorganisms from outside do not enter, is akin to astronauts being trapped with their own microorganisms."
The fact that exposure to diverse microorganisms is beneficial for health has been proven by various studies. Belgian scientists published findings in 2015 in "Science" that rural children are less likely to develop allergies or asthma than urban children, due to increased exposure to microorganisms in their daily lives.
The researchers explained that in rural areas, individuals are often exposed to endotoxins from bacteria through livestock manure or feed. Lungs exposed to endotoxins experience very mild inflammation that heals quickly. Such children do not develop allergies or asthma when eventually encountering house dust mites, as if they had received a vaccine. The researchers validated this through experiments with mice and also identified enzymes involved in the immune process.
The researchers at UCSD stated that creating a microbial ecosystem similar to Earth's natural environment is more beneficial for maintaining astronauts' health than making space habitats excessively sterile. To achieve this, bringing animals into space or storing fermented foods in the food storage areas can also be options.
Research Institute Salido emphasized that "for life forms to thrive in space, we should not only send a part of Earth's ecosystem but also beneficial microbial companions with it," adding that "introducing microorganisms suitable for the space environment is necessary to create a sustainable and healthy ecosystem."
However, there are also opinions that a cautious approach is needed when introducing Earth microorganisms to the space station. Odette Laneuville, a professor at the University of Ottawa in Canada, stated, "There is a risk that harmful microorganisms, such as parasites or fungi, may also be introduced."
References
Cell (2025), DOI: https://doi.org/10.1016/j.cell.2025.01.039
Science (2015), DOI: https://doi.org/10.1126/science.aac6623