INSTITUTE, W.Va. - A choking cloud of yellow gas used to make a pesticide leaked from a Union Carbide plant and rolled "like a fog" through four cities, slightly injuring at least 125 people before it dissipated.

A chemical derived from methyl isocyanate leaked, trapping thousands of people indoors until the cloud of toxic fumes dissipated two hours later.

Authorities warned 20,000 people living within a 10-mile radius of the plant to stay indoors and turn off ventilation systems.

Aldicarb oxime, the main ingredient in the pesticide Temik, leaked from the plant's MIC unit Sunday morning after a valve failed, said Carbide spokesman Dick Henderson. A company statement blamed the leak on a "pressure buildup" in a storage tank containing about 500 gallons of the chemical.

Henderson said aldicarb oxime is made from MIC - the deadly pesticide ingredient that leaked from a plant in Bhopal , India, last year, killing more than 2,000 people and injuring about 100,000 others - but does not contain any MIC itself.

"At most there might be a few parts per billion because a few molecules didn't react fully, but that would be all," he said.

But Union Carbide spokesman Thad Epps said Sunday that aldicarb oxime is a separate chemical and is not related to MIC. He said there was no MIC in the chemicals that leaked Sunday.

Henderson said the chemical is an "eye and lung irritant" that also causes nausea, and sheriff's Lt. I.D. Burdette said individuals exposed to the fumes reported burning eyes, nausea and shortness of breath. Of the 125 people who sought treatment at hospitals, about 30 were admitted with mild to moderate injuries. The rest were treated and released.

The 200-yard-wide cloud of yellowish gas rolled over the towns of Dunbar, Institute, Nitro and St. Albans before dissipating.

Company officials said they turned on a siren as soon as they knew of the leak, but some residents complained the chemical cloud had already sickened them before they heard a warning.

"It sort of moved in like a fog," said Pat Baciu, a gas station attendant at Dunbar. "It had a sort of sulphur smell to it, just sort of stunk. My eyes got a little bit red and I got a little sick at my stomach."

"I didn't know what it was when I saw that white cloud go up, disperse and spread out," said Crawford Willis, a custodian at nearby West Virginia State College. "I just locked the building, picked up my wife and took off."

Barbara Cyrus, who lives 500 yards from the plant, said the first thing she noticed was a very strong odor seeping into her house.

"I thought maybe it was the cat litter," she said. "But then I opened the door to pick up the paper and it almost knocked me down."

Her husband, Clifford Cyrus, said he immediately thought of leaving the house "but I didn't hear no whistle so I didn't know what to do. About 10 minutes later we heard the whistle and then we headed toward Charleston."

The plant siren indicates residents should seek information on the radio or television. County officials broadcast bulletins telling residents to remain indoors.

Officials said more than 100 people were treated at an emergency center set up at Shawnee Park about two miles away. Many were sent later to hospitals, which reported treating at least 125, said David Seidler, vice chief of emergency services at Charleston Area Medical Center.

About 30 residents were admitted with "mild to moderate injuries caused by irritant gases," Seidler said. "We anticipate recovery will be very quick, a day or two at most."

He said six injured plant workers had "severe eye irritations that required more extensive treatment." All six were reported in stable or satisfactory conditions.

Seidler said the medical center received calls from hospitals as far away as Gallipolis, Ohio, 60 miles to the west, and Montgomery, 30 miles to the east, asking for information on treating patients exposed to the chemical while driving by the plant on Interstate 64.

Three or four people who passed through the area at the time of the leak became stricken in Gallipolis and stopped at a hospital there for treatment, authorities said.

Buses were used to take the injured to hospitals and an emergency medical center was set up at a golf course after the leak was reported.

The company statement said "an immediate investigation will be made" to determine why pressure in the storage tank built to the point that the valve failed. It said the tank contained "approximately 500 gallons of material, the maximum amount which could be released."

"However, much of the material was neutralized through venting to a scrubber and flare," it said.

Henderson insisted that "there never was a threat to the community," but Kanawha County officials declared an emergency when the leak was reported. They advised the 3 ,100 residents of Institute to stay indoors and told those outside to cover their eyes and mouths and seek shelter.

The emergency declaration was terminated shortly before noon Sunday.

"We activated our computer tracking and it showed most of material went back over the plant, southwesterly," Henderson said.

Burdette said the area of the plant where the leak occurred was flooded with water to contain fumes.

Paramedic supervisor Phurman Williams, who helped treat patients at the emergency center, said people were "very disturbed - very. What I've heard is that the warning to the surrounding communities wasn't relayed as quickly as what they thought it should have been."

Henderson disagreed. He said the plant's emergency siren was sounded as soon as the leak occurred. "Most everyone stayed indoors and it's good to see that the system worked," he said.

Henderson said the company would not know how much material leaked until it could conduct a "materials balance" check of substances remaining in the plant's MIC reactor unit.

The Environmental Protection Agency reported last Jan. 23 that safety records at Union Carbide's Institute, W.Va., plant indicated 28 leaks between 1980 and 1984 of methyl isocyanate. The EPA said it had not determined whether any of the gas spread outside the plant.