INSTITUTE, W.Va. - Union Carbide defended itself against criticism of its emergency notification procedures during a poison gas leak that injured 135 people. But company officials conceded that workers at first thought the leak was not a problem.

The chemical involved in Sunday's leak, 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.

Doctors said Monday that those exposed at Institute were expected to recover fully because of lower concentrations than the gas emitted at Bhopal .

Carbide said it would investigate the cause of Sunday's leak, while 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.

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, "at that time we did not believe the emergency would affect the community because the cloud was hovering over the plant," said Karawan.

Karawan said there was a 20-minute delay in notifying outside agencies of the leak because a computer predicted the gas cloud would stay within the plant's boundaries.

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 wasn't broadcast until 10:09, said Kanawha Valley emergency services coordinator Bill White.

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, and 13 remained hospitalized Monday.

Doctors predicted quick recovery, but Stanley Miller, 30, one of six Carbide workers injured, was in serious condition Monday with eye injuries. The rest were in satisfactory condition.

Many of the injured residents said their homes were engulfed by fumes before they heard any warning.

White blamed Carbide.

"They didn't man their communications radio" which was set up this spring as part of a revamped emergene pesticide ingredient but that the source of the steam has not been determined.

Steam would have set off a chemical reaction capable of blowing out the three gaskets that failed and allowed the substance to escape, Karawan said.

An internal Union Carbide memo dated Nov. 28, 1983, rated aldicarb oxime as being rated in the same class of toxins such as MIC or phosgene, a poison gas used in World War I, said Jerry Dotson, an aide to Waxman. Waxman is chairman of the Energy and Commerce Committee's health subcommittee, which investigated the safety of U.S. chemical plants following Bhopal accident.

All three were rated in the memo as "Class 4" substances on a 1-6 scale, with 6 being the most toxic.

The memo said all are cancer-causing and exposure can result in mutations, genetic damage, "fertility impairment in humans, irreversible central nervous system disturbances (and) cumulating organ toxicity that is irreversible."