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James Irwin on Apollo 15
NASA astronaut James Irwin salutes the American flag during the Apollo 15 mission in 1971. Irwin experienced irregular heart rhythms while on the lunar surface, and suffered at least two serious heart attacks after returning to Earth. He died in 1991. (Credit: NASA)

Is deep-space radiation hazardous to your cardiovascular system? A newly published study focusing on the Apollo astronauts suggests that it is, but the sample size is too small to firm up the connection.

The suggestion of a link comes out of a study published today in Scientific Reports, an open-access journal affiliated with Nature.

A research team led by Florida State University’s Michael Delp looked at the death reports for seven Apollo astronauts who passed away after taking part in lunar missions. They compared those reports with similar mortality statistics for astronauts who stayed in low Earth orbit, as well as astronauts who never got into orbit.

The aim was to find out whether the increased exposure to radiation that astronauts get when they travel beyond Earth’s protective magnetic field might have additional health impacts.

“We know very little about the effects of deep space radiation on human health, particularly on the cardiovascular system,” Delp explained in a news release. “This gives us the first glimpse into its adverse effects on humans.”

Such findings could have an impact on future space missions: NASA is planning to send astronauts beyond Earth orbit starting in the mid-2020s, with the objectives of studying asteroids and eventually visiting Mars and its moons. SpaceX founder Elon Musk, meanwhile, is talking about sending people to Mars by as early as 2025.

If the radiation exposure in deep space presents a serious health problem, serious measures will have to be taken to counter its effects.

Delp and his colleagues found that three of the seven Apollo astronauts they studied – that is, 43 percent – died due to a cardiovascular problem. That raw percentage is four to five times higher than the percentage for the larger pool of Earth-orbiting astronauts (42 subjects) and non-orbiting astronauts (35 subjects).

The researchers suggested that higher radiation exposure had an effect on the thin layer of tissue that lines blood vessels, known as the vascular endothelium. To test that hypothesis, the researchers exposed mice to the levels of radiation that Apollo astronauts would have experienced. After six months – the mouse equivalent of 20 years of human life – the arteries of the mice were found to be impaired in a way that’s known to cause cardiovascular disease in humans.

“What the mouse data show is that deep-space radiation is harmful to vascular health,” Delp said.

However, he and his fellow researchers acknowledge that the Apollo mortality sample size is small. When only seven people are in the sample, one death more or less makes a big difference in the percentages. “Therefore, caution must be used in drawing definitive conclusions regarding specific health risks,” they wrote.

There are a few other caveats: Some of the astronauts may have suffered from pre-existing cardiovascular conditions, for example, and the mice in the experiment received their radiation dose at a faster rate than the astronauts did.

NASA stressed the caveats in a statement obtained by NBC News: “It is not possible to determine whether cosmic ray radiation affected the Apollo astronauts. Limitations of the study include the small number of astronauts in the Apollo program and lifestyle factors that cannot be quantified – such as family genetics and diet.”

It’s already well-known that the deep-space environment can be detrimental to health – due in part to the effect of weightlessness on the heart and other muscles, on the bones, eyes and immune system, plus skin and hair.

Space radiation has long been cited as a cancer risk factor as well as a potential problem for brain functionFuture studies will have to determine how radiation’s effect on the cardiovascular system changes the risk equation.

In addition to Delp, the authors of “Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality: Possible Deep Space Radiation Effects on the Vascular Endothelium” include Jacqueline Charvat, Charles Limoli, Ruth Globus and Payal Ghosh.

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