Dr. Beatrice Golomb of the University of California, San Diego School of Medicine, said that veterans from the 1990-91 Persian Gulf conflict have a higher rate of chronic, multi-symptom health problems than either non-deployed personnel or those deployed elsewhere.

The study analyzed evidence regarding the class of chemicals acetylcholinesterase inhibitors and organophosphates, which includes nerve gas chemicals.

Some in the U.S military were exposed to nerve gas -- sarin -- when demolishing Iraqi munitions and pesticides were used aggressively in Gulf regions to control sand flies and other insects, Golomb said. Carbamate pyridostigmine bromide pills given to service members to protect against potential nerve-agent exposure falls into the same category of chemicals, Golomb said.

The study, published in the early online edition of the Proceedings of the National Academy of Sciences, linked exposure to these chemicals with the chronic, multi-symptom health problems in 25 percent to 33 percent of returning Gulf War veterans.

"There is evidence that genetics have something to do with how a body handles exposure to these chemicals," Golomb said in a statement. "Some are genetically less able to withstand these toxins."