Pilots are exposed to cosmic radiation
Pilots and other staff who spend a career at high altitude may have suffered genetic damage from exposure to "cosmic radiation".
US researchers checked DNA from pilots and found more damage in those who spent longer in the air.
At a jet's cruising height, passengers and crew are exposed to more radiation from space.
But experts said the Occupational and Environmental Medicine study did not prove that raised the risk of illness.
The potential threat from cosmic radiation, particles emitted from the sun, and other stars, has caused concern for some years, even though there is no strong evidence that it can cause harm.
While it constantly bombards the Earth, the atmosphere forms a protective barrier for those nearer ground level.
There have been studies which suggest that cabin crews and pilots are more prone to certain cancers than the general population but other explanations, such as disruption to the body clock caused by jetlag, have been suggested.
The study, by researchers from Ohio and Maryland, looked at blood samples from 83 airline pilots and compared them with samples from 50 carefully matched volunteers from other professions.
They then looked for "chromosome translocations" - signs of damage to the DNA.
These have been linked with cancer, although there is no certainty that someone with this kind of damage will go on to get cancer.
They found that while, overall, there was no difference between the non-pilots and pilots, there were significant differences in DNA damage from pilot to pilot, depending on how much time they spent flying.
The frequency of chromosome translocation among those who had flown the most was more than twice that of those who had flown the least.
The researchers said that pilots with the greatest experience may have been exposed to "biologically significant" doses of radiation.
A spokesman for the British Airline Pilots Association said that other studies had suggested that DNA damage might disappear within months of stopping flying.
She said: "This is interesting, but we don't believe that it proves that this type of radiation can cause health problems."
Dr Mike Clark from the Health Protection Agency, agreed, saying that he would have expected the DNA damage in the pilots to be greater than that in the other volunteers if cosmic radiation was the cause.
He said: "The main finding is that there is no difference in the chromosome aberration results for pilots and a carefully selected control group.
"The increase linked to pilots' flight years should be looked at further, but it could be due to factors unrelated to radiation exposure, as the authors acknowledge."