Radiation has always been considered as a real threat to astronauts. For this reason, space crews who are working in the International Space Station (ISS) were legally classed as radiation workers. The prolonged exposure to cosmic rays may produce in increased risk for cancer, impaired immune system, and it can even affect the brain and nervous system.  

NASA's Human Research Program, Charles Limoli (UCI team leader and professor of radiation oncology) used the NASA Space Radiation Laboratory at New York's Brookhaven National Laboratory to approximate cosmic rays using an earthbound particle accelerator. The team used fully ionized oxygen and titanium nuclei to approximate the effect of lighter particles of much higher energies, in place of the real cosmic rays that are in space. This research was published in the Nature Research Journal Scientific Reports, according to New Atlas. 

Symptoms of dementia could be developed due to cosmic radiation. Astronauts in deep space can experience this, according to scientists at the UC Irvine. Rodent tests has resulted to cases of "space brain" with an indication of a long-term neurological damage, cognitive impairment, and diminished judgment after being exposed to high-energy particles.

Several patients who are undergoing radiation therapy, as a treatment for brain tumors, are already suffering from extreme neurological symptoms - problems related to cognition and memory, according to the journal published in Scientific Reports.

Radiation can tell whether how many visits to the ISS astronauts can only make in an entire lifetime. Missions which take a lot longer, including missions to Mars, galactic cosmic rays become a major hazard. These rays, which are highly-charged particles from outside of the Solar System, can penetrate through spacecraft hulls as if no astronauts are loaded in it. Heavier versions of these particles, like the nuclei of oxygen and carbon atoms can directly or indirectly bring a major damage to a live tissue.