INSTITUTE, W.Va. - Union Carbide Corp. announced today it is suspending production of aldicarb oxime at its Institute plant until it determines what caused the poison gas to leak this week, injuring 135 people.

The company defended itself against criticism of emergency notification procedures during the Sunday poison gas leak. But officials of the company conceded it delayed notifying county-emergency services of the gas leak for 20 minutes, while the company's own rescue squad administered care to six injured workers.

"In view of the need for a complete understanding of the exact circumstances which led to the incident, we have decided to temporarily halt all aldicarb solution manufacture at Institute," said spokesman Thad Epps.

Aldicarb oxime is considered about as toxic as methyl isocyanate, according to a company memo, said an aide to U.S. Rep. Henry Waxman. Methyl isocyanate, or MIC, killed more than 2,000 people after it leaked at a Carbide plant in Bhopal , India. However, company officials disputed the assertions in the memo.

The memo said a health specialist for Union Carbide Corp., in December 1983, recommended that the chemical substance involved in the gas leak be classified in a category as among the most dangerous chemicals handled at its Institute, W.Va., plant.

The specialist said the substance is capable of causing irreversible damage to the nervous system and organs, fertility impairment and birth defects.

But Carbide spokesman Tom Sprick responded that "the category has considerable range to it, and all products are not equal in weight. From what I understand, it (aldicarb oxime) is not as dangerous."

Doctors said Monday that those exposed at Institute were expected to recover fully because of lower concentrations than the gas emitted at Bhopal . The company said it would pay medical expenses of those injured in the Sunday leak; 13 remained hospitalized Monday.

County officials said they would investigate the company's emergency response.

"The system didn't work," said Charleston Mayor Mike Roark, who joined mayors from St. Albans, Nitro, Dunbar and other communities in criticizing the warning procedures.

Rep. Bob Wise, D-W.Va., said today that his office interviewed 254 residents Monday evening, and 144 said they were exposed to the gas before alarms sounded.

But Carbide officials disputed the charges. Plant manager Hank Karawan said the plant's alarm was activated within 60 seconds of the leak's being discovered at 9:24 a.m.

Kanawha County emergency officials were not notified until 9:44 a.m. because, Karawan said, "at that time we did not believe the emergency would affect the community because the cloud was hovering over the plant." He said the community warning whistle at the Institute fire station was sounded shortly before 10 a.m.

But Emergency Broadcast System's first report was not broadcast until 10:09, said Bill White, Kanawha Valley emergency services coordinator.

By that time, the stinging, white gas cloud already was settling on Institute, forcing 3 ,100 residents to flee or seal themselves inside their homes.

Thousands stayed indoors for two hours, and more than 300 were checked at an emergency medical center set up two miles away. More than 130 were treated at hospitals for burning eyes, noses, throats and lungs.

MIC is made in the Houston area at only one plant, the La Porte facility of DuPont Co. Spokesman Irv Lipp said the chemical is used in the manufacture of a liquid insecticide but no more than two or three pounds exist at any one time. Lipp said the MIC lasts only long enough to pass through a 2-foot pipe.

DuPont began making the chemical at La Porte after supplies from West Virginia were interrupted, he said. Also, making the chemical as it is used means that it is not necessary to store it. Lipp said the $13 million MIC facility has 31 safety systems, including an automatic shutdown, a scrubber to remove the chemical from the air and an incinerator. During 15 years of operation using MIC, the company had one spill of eight ounces, he said.

Neither MIC nor aldicarb oxime is made nor used at Union Carbide's Texas City plant, said spokesman Tony Shannon.