Heading into space can be dangerous for many reasons. But new research examines how it could affect one’s cardiovascular health—and warns astronauts who venture into the deep space against going too far.

New research from a NASA-affiliated team shows how deep space travel can be the cause of cardiovascular disease. They studied data on the 24 Apollo astronauts who flew to the moon in the late ‘60s and early ‘70s. The cardiovascular disease mortality rate among these individuals (43%) was four to five times higher than in astronauts who did not enter space or stayed in lower Earth orbit.

Michael Delp, the lead author on the study and a professor of human sciences at Florida State University, found in further research on mice that the cause of cardiovascular disease in astronauts who went into space may have gotten it due to radiation. The study was published July 28 in the journal Scientific Reports.

When astronauts head to space and stay in the Earth’s magnetosphere, they are shielded from radiation. But once they leave it, as the astronauts from the Apollo mission did, their only protection is the shielding from their spacecraft and spacesuits.

There are concerns in the legitimacy of studying such a small population group. Experts question whether the results can be considered credible since a difference in a single person could drastically alter the statistics.

However, the mouse studies helped to show that radiation exposure could have been the cause of cardiovascular diseases in the Apollo astronauts.

Knowing the radiation affects could help save lives in the future as NASA prepares to send more astronauts to the deep space. Possible solutions could include antioxidants, improved exercise habits and better shielding on spacecraft.