How much radiation do pilots, flight attendants and passengers receive per flight?
For the vast majority of people, even frequent fliers, it's not enough to create any health risks, experts say. Yet for some, notably those predisposed to cancer, even the tiny amount seeping into an airline cabin could spell trouble.
It's enough of a concern that some pilots take precautions to minimise their exposure.
"This is just another risk we encounter every day while we're flying," said Mike Holland, of the Allied Pilots Association, which represents American Airline pilots.
Mostly, planes are infiltrated by cosmic radiation, stemming from exploding stars outside the solar system. They also are zapped with solar radiation, particularly when the surface of the sun flares up. Airline fuselages do not protect against either.
Overall, however, the amount "is really inconsequential," said Dr Edward Dauer, director of radiology at Florida Medical Center in Fort Lauderdale, adding that medical CT scans result in a much higher dose.
He said laptops and mobile phones don't increase exposure because they work on electro-magnetic energy.
On the other hand, much depends on the total amount of radiation a person has been subjected to, as its effects are cumulative, said Dauer, who is also a research associate professor of biomedical engineering and radiology at the University of Miami.
"Even one X-ray, by itself, has the potential to cause a cancer," he said. "The more exposures you have, the more chance you have of developing a problem."
Passengers are most exposed on long-haul flights at more northerly latitudes. That is because the Earth's atmosphere is thickest over the equator and thinnest over the poles.
During a seven-hour flight from New York to London, travellers receive about the same dose of radiation as a chest X-ray; from New York to Tokyo, two chest X-rays.
As for the body-imaging scanners at security checkpoints: studies have found their low level of radiation is extremely safe, even for pregnant women and children.
Radiation is energy transmitted in the form of rays, waves or particles, commonly produced by cosmic particles, the sun and man-made devices, such as X-ray machines and nuclear reactors.
The amount people absorb is commonly measured in millisieverts, reflecting extremely small levels. US residents on average receive about 3 three millisieverts per year from natural sources, such as the ground and the sun. A chest X-ray generally emits up to 0.06 millisieverts.
The main risks of too much radiation: Cancer, passing on genetic defects to future generations and damage to the fetuses of pregnant women.
But the radiation seeping into airliners is far from dangerous, experts say.
According to the Health Physics Society, a scientific organisation that specialises in radiation safety, a traveller would have to spend more than 5000 hours per year on an airliner, or five times as many hours as pilots are allowed to fly, before they would be endangered.
The average passenger is exposed to about 0.01 millisieverts per year.
A 14-hour trip from New York to Tokyo produces about 0.1 millisieverts, less than a quarter of the radiation needed for a mammogram.
There are limits to how much radiation a person should receive - whether in the air or in their everyday lives.
The Health Physics Society recommends no more than 50 millisieverts in one year or no more than 100 millisieverts over a lifetime from natural sources of radiation.
For frequent fliers, pilots and flight attendants, the overall risk is "almost nonexistent," even if they fly for 30 years, the Health Physics Society said.
Mike Holland, of the Allied Pilots Association, said "various studies" show that as a group, pilots have a higher rate of cancer than the general population. Yet he can't confirm that's a result of radiation exposure.
For that reason, he urges pilots who might be sensitive to radiation to request flying shorter hops, which have less exposure. He also recommends they fly at lower altitudes when possible. The reason: Airliners are most exposed to radiation at higher altitudes, where the atmosphere is thinner.
Citing their level of exposure, pilots argued they should be allowed to bypass the body-imaging scanners at checkpoints. In November, the TSA acquiesced, requiring pilots pass a background check and verify their employment status instead.
"You're looking at the cumulative effects over a 30-year career. That's a lot of radiation while flying," said Captain Sam Mayer, an American Airlines captain based in Miami.