Airline pilots may be at increased risk of eye damage because of their exposure to cosmic radiation, warn experts.
Frequent flying may push pilots' exposure over safe limits
The Icelandic researchers found commercial pilots were three times more likely than normal to develop cataracts - clouding of the lens of the eye.
Cosmic rays - very energetic particles and radiation which bombard the Earth from outer space - have already been linked to cataracts among astronauts.
The research appears in the journal Archives of Ophthalmology.
The authors looked at 445 men aged 50 and over - 71 of whom had a type of cataract called nuclear cataract and 374 with other or no lens problems. Among the men, 79 were commercial pilots and 366 had never been pilots.
When the researchers compared the rates of cataracts with occupation, they found pilots were far more likely than the other men to have this eyesight problem.
Furthermore, the longer the men had worked as pilots and the more cosmic radiation they had been exposed to, the more likely they were to have developed nuclear cataracts.
The researchers also confirmed that the findings could not be explained by other factors known to increase the risk of cataracts, such as UV light exposure and smoking.
The average person in the UK is exposed to 2.7 millisieverts (mSv) of radiation each year, according to the Health Protection Agency.
In comparison, research has suggested that air crews are exposed to an average of 4.6 mSv of radiation each year, while nuclear workers are exposed to 3.6 mSv.
Airline pilots and astronauts have also been found to have an increased risk of mutations to genes in their blood cells and certain cancers, which, again, have been linked to cosmic radiation.
Cosmic radiation comes from the sun and the galaxies of the Universe.
Like some cancer treatments and medical scans, it is ionising, which means it can displace charged particles from atoms.
This can lead to the disruption of molecules in living cells.
Atomic bomb survivors and cancer patients treated with radiation therapy are also at increased risk of cataracts, studies have shown.
No 'cause and effect'
Dr Larry Benjamin from the Royal College of Ophthalmologists said that cosmic radiation and other wavelengths such as UV light might potentially damage proteins in the lens of the eye.
"It damages cell repair mechanisms and probably also causes the protein in the lens to cross link. Sort of like cooking an egg and the egg goes white. Once that has happened it is irreversible."
However, he said cataracts were easy to treat with surgery.
But Dr Michael Clark from the Health Protection Agency said it was unlikely that cosmic radiation was the cause of the pilots' cataracts.
"Pilots and aircrew receive doses that are two or three times normal background in a year, primarily from cosmic rays.
"This is well within the range of variations in background doses at ground level so we would not expect to see any health effects.
"On current evidence, it seems unlikely that cosmic rays could be the cause of cataracts in pilots, but this finding is bound to stimulate more research in this area."