There is evidence linking chronic health problems suffered by Gulf War veterans to exposure to pesticides and nerve agents, US research has found.
A third of veterans of the 1991 war experienced fatigue, muscle or joint pain, sleeping problems, rashes and breathing troubles, the research found.
A US Congress-appointed committee on Gulf War illnesses analysed more than 100 studies in the research.
It found evidence linking the problems to a particular class of chemicals.
These were an anti-nerve gas agent given to troops, pesticides used to control sand-flies, and the nerve-gas sarin that troops may have been exposed to during the demolition of a weapons depot.
Dr Beatrice Golomb, the committee's chief scientist, said that genetic variants make some people more susceptible to such chemicals.
When exposed, these people ran a higher risk of illness, she said.
"Convergent evidence now strongly links a class of chemicals - acetyl cholinesterase inhibitors - to illness in Gulf War veterans," Dr Golomb told Reuters.
Dr Golomb said a lot of attention had been given to psychological factors in illness among Gulf War veterans.
But unlike the most recent conflict in Iraq, the ground conflict during the 1991 Gulf War lasted only a few days, she added.
"Psychological stressors are inadequate to account for the excess illness seen," said Dr Golomb, of the University of California, San Diego.
The study was published in the Proceedings of the National Academy of Sciences.