A valve on a chemical tank blew off and released toxic gas Wednesday, sending nearly 100 people to hospitals with breathing problems and nausea.
Doctors said the effects of arsine gas, a form of arsenic, can be hidden for up to 24 hours.
"Reaction to the gas may be delayed, and anyone who feels they have been exposed should seek medical attention," said Dr. William Banner, medical director of Oklahoma State Poison Control.
Arsine in high concentrations has a garlic or fishy smell. The poison center said, however, that exposure to the gas can occur without the smell being noticed.
Ambulances, city buses and private cars took 95 people to hospitals, said Chris Metcalf, spokesman for the city's ambulance service.
Banner said a handful of the 24 patients taken to St. Francis Hospital showed symptoms of arsine exposure by evening. The patients were admitted for observation because of the delay in symptoms. The 12 patients at Claremore Regional Hospital would be held for at least 12 hours, spokeswoman Audrey Webber said.
The gas release occurred at Solkatronic Chemical, a manufacturer of computer parts.