It’s been 25 years, and Chad Donovan still wonders which toxic hazard in the Gulf War might have caused the fatigue, stomach problems and rashes he has suffered ever since.

Maybe it was the nerve gas pills, which his unit took in Saudi Arabia while standing in formation so nobody refused.

Maybe one of the “false alarms” after a missile attack really did signal the presence of chemical weapons.

Maybe the mushroom-cloud detonation of unused Iraqi ordnance whooshed toxins into the air.

And then there were the sand fleas, pesticides, burning oil wells, dust storms and uranium-depleted bullets that made the Gulf War one of the most toxic conflicts in history.

Today, researchers at the Minneapolis VA Medical Center are leading a wave of studies to solve the mystery of Gulf War Illness, a cluster of unexplained symptoms reported by 25 to 65 percent of the 700,000 soldiers deployed to the Gulf in 1990 and 1991. They have identified genetic markers that could improve tests and treatment, one of the most significant advances in years, and started a clinical trial on a promising prescription drug.

But 25 years later, many Gulf War vets remain like Donovan — sick, sore and exhausted by the lack of answers.

“Everyone I’m still in contact with has had issues,” said Donovan, who served with the Army’s 82nd Airborne Division that deployed days after Iraq invaded Kuwait. “Insomnia, the achy joints, stomach issues. I still have stomach issues, and nobody knows what’s causing them.”

More than $500 million in research hasn’t found causes or cures for the illness, which the U.S. Department of Veterans Affairs classifies as “unexplained illness” or “chronic multisymptom illness.”

The consequence isn’t just scant treatment of the illness, which typically produces joint pain, chronic fatigue or digestive problems. With limited diagnostic criteria, many veterans can’t prove their illnesses stem from Gulf War service and can’t qualify for VA disability benefits.

And while federal policy is to presume that any Gulf War vets with symptoms qualify for VA coverage, the reality is different. Eighty percent of Gulf War Illness-related claims were denied in 2015, according to federal data presented to Congress this year.

“So many veterans can’t get these presumptive illnesses service-connected,” said James Bunker, who directs the National Gulf War Resource Center in Kansas. “So they’re being denied the care they should be getting.”