Travel by aircraft has its downsides but one rarely mentioned characteristic provides very interesting scientific data that is not communicated to passengers. Flying 10km or more above the ground aircraft pressurise the cabin to keep passengers alive in the unbreathably low pressure atmosphere, but the lack of atmosphere also greatly increases ionising radiation exposure which the aircraft provides little protection from.
While not a common instrument personal dosimeters are available to measure radiation exposure. For typical people the majority of the annual “background” radiation dose (which they are exposed to even if avoiding sources) comes from internal exposure of the body breathing air and eating food. The dosimeter used only measures the external exposures from x-ray, beta, and gamma radiation which would be expected to contribute on average 2uSv per day and 600uSv per year from the total annual “background” dose of 2,000uSv per year. Around Melbourne the measured external dose averages 2.0uSv per day with little variation right in line with world averages.
Controllable sources of radiation are carefully monitored to limit the exposure of people, with the worlds bodies settling on a current standard of less than 1,000uSv additional per year for the public, and for Australian “radiation workers” less than 20,000uSv additional per year. This corresponds with an “acceptably low” increased risk of cancer and little or no other health effects. Interestingly, airline crews are “radiation workers” due to their high occupational exposure and the Australian regulator provides its typically frank discussion here:
Which puts airline flights 20 to 50 times higher in radiation exposure than staying on the ground.
So does this match the real world? Data from 2 international flights along the same latitude, with similar central seating position, and flying into the sun at the same time would tend to agree. A narrow body plane (A320) full of passengers recorded an excess exposure of less than 1uSv per hour, while a largely empty wide body plane (A380) logged 2uSv per hour. Both are below the published data above by not measuring the entire range of ionising radiation but still include interesting results. The A380 flies at higher altitudes and on this particular route was 20% higher (41,000 feet compared to 34,000 feet), which should increase the dose rate by approximately 50%. A further explanation that could be contributing to the large difference in rates is the number of passengers on the flight, just as penguins huddle to minimise their exposure to wind humans are rather effective radiation shields.
So for those flyers wishing to minimise their radiation exposure the tips are to select routes operating at low altitudes (and close to the equator) and to pick a seat in the middle of other passengers. Although; the literature is still trying to find a correlation between airline crews exposure and health effects without any strong links at their rates, so the linear no threshold assumption is being re-evaluated and avoiding these rates of radiation might not be important at all.