On the first of two maps accompanying an article yesterday about the chemical leak in Institute, W.Va., the lower portion of the river should have been labeled the New River, which joins the Gauley River southeast of Charleston to form the Kanawha. Institute is on the banks of the Kanawha.

Union Carbide officials acknowledged today that they waited for 20 minutes Sunday before notifying local authorities of a dangerous chemical leak but said they thought at the time that the toxic vapor had been confined to the plant area.

The chemical involved, aldicarb oxime, is rated on a toxicity scale in the same class as methyl isocyanate (MIC), according to a 1983 company memo disclosed tonight by an aide to Rep. Henry A. Waxman (D-Calif.). A leak of MIC at a Union Carbide plant in Bhopal, India, last December killed more than 2,000 people.

Union Carbide said that the rating includes chemicals of varying toxicity and that company officials do not consider aldicarb oxime as toxic as MIC. "I don't believe it is as bad as MIC," Dr. Bipin Avashia, the Institute plant physician, said.

However, Dr. Jay Young, an independent consultant on chemical health and safety, disagreed. "Aldicarb is nasty," he told United Press International. "If there had been more leaked, there certainly would have been some deaths."

Neighbors of the Institute plant and local officials responded angrily that the delay in sounding the warning Sunday could have been disastrous if the leak had been more severe. Six company employes and 136 nearby residents were injured. Thirty-one people were hospitalized overnight, and 17 of them remained in the hospital today. One -- Union Carbide worker Stanley Miller, 30, of Charleston -- was listed in serious condition.

Company officials, still trying to recover from the MIC leak in Bhopal, said in their defense that plant workers had their hands full removing the injured employes and that they did not realize at first that the chemical cloud had drifted off the vast Union Carbide property, which occupies a large part of Institute.

But residents told of waking to choking fumes and escaping from the area long before any alarm was sounded. And Rep. Robert E. Wise Jr. (D-W.Va.), generally a supporter of one of his district's largest employers, said the alarm was sounded after local officials were alerted by nearby residents, not as a result of action by Union Carbide.

The incident at Institute was a serious public relations setback for Union Carbide, which has been telling surrounding communities in the wake of the Bhopal disaster that their plants are safe. Critics said today that holes now have been punched in that argument.

Claire Smith, who lives across from the plant's main entrance in one of the neat frame houses on Highway 25, said that she, her husband and two sons did not learn of the leak until a friend telephoned more than two hours later.

Smith, whose whole family suffered respiratory problems, said her window fans had filled the house with chemical fumes. "I'd come home and gone to bed after working the night shift. The [siren] alarm system wasn't strong enough to wake us up. I'm really fearful, and I'm furious at Carbide. Someone should have checked on the nearby residents. If it had been something worse, we'd all be dead."

Union Carbide spokesmen originally said that aldicarb oxime is a derivative of MIC, which is also produced at the Institute plant. But spokesmen said today that aldicarb oxime is combined with MIC to manufacture the highly toxic pesticide aldicarb, sold under the trade name Temik. Aldicarb residues were found in watermelons in California this summer, sickening hundreds of people and forcing the destruction of about 1 million melons.

The Union Carbide plant at Institute is the only one in the United States where MIC is produced. The MIC unit here was shut down after the Bhopal leak Dec. 3; it resumed production months later, after installation of $5 million worth of additional safety equipment that company spokesman Thad Epps said made "a safe unit safer."

Plant manager Hank Karawan said at a news conference today that the vapors that escaped Sunday also contained dichloromethane, carbon monoxide, carbon dioxide and sulfur compounds.

Sources at the Environmental Protection Agency said dichloromethane causes cancer in laboratory animals and is classified as a "probable" human carcinogen. However, cancer studies generally focus on long-term exposures at low levels, and the health effects of a single heavy exposure are not known.

Officials at five area hospitals where residents were treated said most were suffering temporary breathing difficulty, eye irritation, nausea, dizziness and headaches.

"Our physicians said there should not be any long-term effects" from exposure to the gas, said Jack Canfield, spokesman for Charleston Area Medical Center, where 66 people were treated after the leak.

Karawan said the leak occurred at 9:24 a.m. from a 500-gallon storage tank.

"It is now suspected the material overheated when steam entered a jacket on the storage tank," rupturing three gaskets and releasing the chemical into the air, Karawan said.

He said the storage unit has been shut down while company officials investigate the leak and critique the response procedures. Karawan said the shift coordinator notified the Kanawha County Office of Emergency Services at 9:44 a.m., although employes had not then identified the leaked substance.

Wise, who went door-to-door through Institute interviewing residents, said today he was told that some residents began leaving the area around 9 a.m., long before the official time of the leak, because they smelled the leaking chemicals.

He said his preliminary investigation indicates that county personnel learned of the leak from local residents and turned on warning sirens at the Institute Volunteer Fire Department about 9:45 a.m. -- about the time the company began to notify officials of the leak. In addition, Wise said, local paramedics complained that the company delayed in giving authorities the name of the leaking chemical.

Paul Nuchims said he was playing tennis at 9:15 a.m. on the West Virginia State College courts, several hundred yards from the plant, when "I noticed a beautiful white cloud, about 100 feet wide and 200 feet high, over the plant. Then the Union Carbide internal whistles started going off. I heard someone over the public address system in the plant say, 'gas leak.' "

Kanawha County Sheriff Danny Jones complained that the emergency radio broadcasts did not start until 10:03 a.m.

Bill White, director of Kanawha County Emergency Services, said he would have preferred more detailed information from Union Carbide officials during the early stages of the incident. He said Union Carbide employes did not use the two-way radio that links the plant with emergency services personnel.

Epps, regional coordinator for public affairs for Union Carbide, said that using the two-way radio "is not part of normal procedure. The reason it wasn't used was that people were so busy they didn't have time to use it."

Charleston Mayor Mike Roark said he was not notified by Union Carbide that a leak had occurred. Roark said city officials were deluged with calls from residents unsure of the sirens' meaning.

State Attorney General Charlie Brown said, "Neither the people in the community nor the emergency authorities were pleased with the response by Carbide . . . .