The surface of the earth is constantly exposed to cosmic radiation. The effect of this is diminished by the atmosphere, although high energy cosmic rays are unaffected. At high altitudes, and particularly in passenger aircraft, the minimum dose rate experienced by passengers and crew is greater than at ground level. There is evidence to suggest that cosmic radiation is a source of genetic mutation.
Pilots and flight attendants on many airline routes are exposed to more radiation than most workers in nuclear power plants, although the effects of such low doses are uncertain and nearly impossible to measure in the population. The risk is greater at high altitudes and closer to the poles. It is considered to be especially dangerous to pregnant women, especially if they fly frequently or during peaks of the 11-year sunspot cycle. For 100,000 crew members spending 20 years flying 960 hours a year on routes of more than 3 hours, 59 to 61 premature cancer deaths could be expected to occur. For 100,000 passengers flying 480 hours a year on the same route, namely about 9 hours a week, 29 to 39 additional cancer deaths could be expected (over the 22,000 which would otherwise be expected for that group).