While flighterdoc was down in the weeds having more fun than any mortal should, going waay waay to fast for my comfort
, I spent a long time above 35,000 feet. We really never thought about radiation until ALPA (Airline Pilots Association) started soliciting studies on the dangers of ionizing radiation to flight crews (not only from external but internal sources as well.) Most domestic flight crews don't have to worry too much, but the international widebody crews definitely think about it. The B747-400 usually flight plans for a final altitude of above 40000 ft, after step climbs, of course.
From FAA AC No: 120-52
At an altitude of 35,000 feet, for example, the dose equivalent rate from cosmic rays is about 6 microsieverts (0.6 millirem) per hour (O'Brien 1978, as revised) while at 41,000 feet the dose equivalent rate at polar latitudes is about 12 microsieverts (1.2 millirem) per hour (O'Brien 1978, as revised).
Think about a 10 hour transpacific flight: crews really start building up some exposure.
The estimated annual dose equivalents to air carrier crewmembers from galactic radiation range from 0.2 to 9.1 millisieverts (20 to 910 millirem). The recommended annual limit for occupational radiation exposure of an adult is 50 millisieverts (5 rem) (EPA 1987).
Since the level of radiation exposure doubles with each 6,000 foot increase in altitude, the radiation exposures received by crewmembers flying now at altitudes at or near 40,000 feet are substantially higher than those received on equipment used in the 1960s and 70s. (Health Physics Society)
According to Dr. Martha A. Simpson, D.O. the average run of the mill chest x-ray will give you about 8 millirems.
For the average passenger, it shouldn't even cross their minds, but it does tend to creep up into the noggins in a few of the more informed and concerned flight crewmembers who are flying 900 hours a year for 20 or 30 years (both pilots and flight attendants.)
....wheeled and soared and swung high in the sunlit silence.