Anyone who has carried a Geiger Counter on a flight knows that radiation levels go up with altitude. This is primarily because the atmosphere thins, providing less shielding from cosmic radiation. The magnetic field surrounding the earth also provides some shielding. Outside, in deep space, radiation levels increase significantly. Health data for Apollo Astronauts shows need for more attention to deep space effects on human tissue, and also special considerations in shielding. More on this at nature.com and observor.com.