Boing Boing has a nice summary of the risks associated with radiation found in space and its impact on missions to the ISS or potential travels to Mars. A little background:

Galactic cosmic radiation – also called galactic cosmic rays – is the kind of radiation that researchers are most worried about. It’s made up particles, bits and pieces of atoms that were probably flung off from the aftermath of supernovas. The majority of this radiation, roughly 90%, is made up protons ripped from atoms of hydrogen. These particles travel around the galaxy at almost the speed of light.

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These particles are very heavy and very fast and we don’t experience them here on the ground. They’re the kind of things that get filtered out and broken down by Earth’s defense systems. But HZE ions can cause more damage, and different kinds of damage, than the radiation scientists are really familiar with. We know this because scientists actually compare samples of astronauts’ blood before and after a spaceflight.

According to Maggie Koerth-Baker, we know historical data shows there are increase risks of cataracts in astronauts. There is also good chance of increased risk of cancer but not much is conclusively known. However, the study that started her post suggests there is an increased risk in Alzheimer’s, as well. All of this becomes relevant as we seek to grow beyond our planet. If humanity is so fortunate, it will be yet another complication in it’s next migration.