A cloud of toxic gas leaked from Union Carbide Corp.'s Institute, W.Va., plant yesterday, sending six employes and more than 125 residents to doctors, briefly shutting down nearby highways and leading local officials to warn residents to stay indoors until the fumes dissipated.

Union Carbide spokesman Dick Henderson identified the chemical as aldicarb oxime, a derivative of methyl isocyanate (MIC). MIC is the substance blamed for at least 2,000 deaths in a leak last December at a Union Carbide plant in Bhopal, India.

Henderson said that aldicarb oxime is much less toxic than MIC and that yesterday's leak would have included no more than trace amounts of MIC. Paramedics, who set up a temporary treatment center at the nearby Shawnee Park golf course clubhouse, said they had treated more than 125 persons suffering from breathing problems, burning eyes, nausea and dizziness.

Paramedics administered oxygen and checked blood pressure and other vital signs. The more serious cases were taken to the hospital by ambulance.

Paramedic supervisor Phurman Williams said most of those treated had "shortness of breath, nausea, and a burning sensation in their eyes."

"We've had very few that are very bad, but a whole lot who have these symptoms," he said. "Many of the people are just concerned . . . just want to make sure everything's okay."

Stephanie Waller, nursing supervisor at Thomas Memorial Hospital in South Charleston, said 20 to 30 people had been admitted and that calls were continuing from people who had been exposed to the fumes. So far, she said, the chemical exposure did not seem to be "life-threatening" to any of the persons being treated.

At the Charleston Area Medical Center, an emergency room employe said that so many persons had come in for treatment that no one had time to count them.

Kanawha County officials declared an emergency when the leak was reported by Union Carbide, alerted by an emergency warning system in the plant. In radio and television broadcasts, nearby residents were advised to stay indoors for two hours. Those who were outside were told by police to cover eyes and mouths and seek shelter.

Roads in the narrow Kanawha River Valley, including Interstate 64 and state Highway 25, were shut down up to an hour. Stranded motorists reported a choking and burning sensation from the fumes, which smelled "strong, bitter, kind of like gasoline," according to one driver.

Aldicarb oxime is the major ingredient in the pesticide sold by Union Carbide under the commercial name Temik. Last month California officials ordered grocers and wholesalers to destroy all their watermelons because of suspected Temik residues after 180 people in four western states and Canada became ill. Symptoms of Temik poisoning include nausea, vomiting, diarrhea and blurred vision. In severe cases, the chemical can cause central nervous system damage and death.

Spokesman Henderson said the chemical leak began at about 9:35 a.m. after a valve failed in the pesticide unit. He said the leak lasted for about 15 minutes and created a visible cloud.

Emergency crews reported to the facility immediately, he said, adding that the six employes did not appear to be seriously injured. "The word we have is that they're all okay," he said.

Four workers were taken to Charleston Area Medical Center, and two were treated at Thomas Memorial Hospital in South Charleston. All six were reported in stable or satisfactory condition.

Henderson said that by noon the chemical had dissipated and that "there never was a threat to the community." He said that most of the cloud had drifted back over the plant area where it was not a threat to the public. "We activated our computer tracking, and it showed most of material went back over the plant, southwesterly," he said.

The emergency declaration was terminated shortly before noon.

Sheriff's Lt. I.D. Burdette told reporters that the area of the plant where the leak occurred was flooded with water to contain fumes. Union Carbide's Institute facility is the only place in the United States where MIC, an intermediary chemical used in the manufacture of pesticides, is produced. The MIC unit was shut down for more than three months after the disaster in Bhopal on Dec. 3.

In February, the company notified the Environmental Protection Agency that 190 small chemical leaks have occurred over the past five years at the Institute plant, including 61 of MIC; 107 of phosgene, the chemical that was known as mustard gas during World War I, and 22 leaks that included both MIC and phosgene.