The atmospheric shield.
The layers of air in the atmosphere give a protection against cosmic rays which decrease with altitude and dependi on the route taken. For commercial aircraft flying at an altitude of 10km the dose received varies from 0.005 mSv to 0.007 mSv per hour depending on whether the aeroplanes do or do not pass through the Poles. At 18km the dose is approximately doubled. For an astronaut in a space station at 400km of altitude the dose rate reaches 0.040 mSv per hour, and in addition two and a half days in orbit is enough to be exposed to a year of natural radioactivity on earth (2,4 mSv)
SCK.CEN (source L.de Saint-Georges).