In the event of future chemical leaks, employes of Union Carbide Corp. will alert the community before determining whether the leak is likely to escape the plant, company Chairman Warren M. Anderson said today.

Union Carbide came under criticism this week for waiting 20 minutes to warn residents about a chemical leak that sickened 142 people last Sunday in nearby Institute. Company officials said they inferred from a computer tracking system that the toxic cloud would not drift beyond the plant.

"We're going to change our way," Anderson said at a news conference here. "The game that we have to play now is that I think you pull the cord first and apologize later that it was not necessary, rather than not pulling the cord and apologizing later that you didn't.

Anderson acknowledged that Sunday's leak, eight months after a leak at Union Carbide's plant in Bhopal, India, killed 2,000 and injured thousands more, has damaged the company's credibility here. But he said it would eventually prove to be "a blip on the curve."

He declined to answer questions about the chemicals involved in the leak or how it occurred, saying the company's investigation would determine those facts by next week.

Plant officials initially identified the escaped gas as primarily aldicarb oxime, a pesticide ingredient, with lesser amounts of dichloromethane, a solvent also known as methylene chloride; carbon monoxide; carbon dioxide and sulfur compounds.

Environmental Protection Agency and state officials said today that Union Carbide's initial reports to them showed that the mixture was about two-thirds methylene chloride, a suspected carcinogen in humans, and one-third aldicarb oxime.

The EPA began a review of methylene chloride in May under the Toxic Substances Control Act after it found that the chemical might pose a "significant cancer risk" to humans, said Janet Luffy of EPA's regional office in Philadelphia.

Methylene chloride is widely used in paint thinners and strippers, foam products and photograph coatings. It is also used to decaffeinate coffee, in which it leaves virtually no residue, according to toxicologists.

EPA and state environmental officials said their preliminary examinations of Sunday's leak show that Union Carbide employes had pumped some of the mixture into an empty reactor vessel when a meter malfunctioned, causing a primary tank to overflow.

Carl Beard, director of the State Air Pollution Control Commission, said workers later pumped the overflow out of the reactor tank and assumed, based on test data, that it was empty. But 500 gallons remained in the tank for about a week until steam entered through a leaky valve, heated the mixture and forced it through three ruptured gaskets into the air, he said.

It is unclear how much of the mixture left the tank, but the released vapor likely contained more methylene chloride than aldicarb oxime, according to George Rusch, manager of general toxicology for Allied Chemical Corp., which makes aldicarb oxime for Union Carbide.

Rusch said aldicarb oxime has an extremely high boiling point and would have been slow to vaporize.

"If it did manage to vaporize, some of it, it would rapidly condense once it hit the cool air," he said, adding that methylene chloride has a relatively low boiling point and vaporizes readily.

Toxicologists said methylene chloride is lighter than air and would have been more likely to float high out of harm's way while aldicarb oxime, which is heavier, would have hugged the ground. Residents complained chiefly Sunday of eye irritation, nausea, dizziness and headaches.

The immediate effects of inhaling methylene chloride, according to toxicologists, include drowsiness, dizziness, nausea and tingling fingers. EPA spokesman Al Heier said tests in which rats received oral doses of both chemicals showed that methylene chloride, which can also cause eye irritation, is slightly more toxic.

State health department officials have said they will keep a register of residents who sought hospital treatment and monitor their health.

Anderson said today that the computer had been programmed to track only the plant's three most toxic chemicals. He did did not say whether other chemicals would be added to the tracking system. He also did not elaborate on the kind or magnitude of leaks that would warrant early warnings in the future, but said, "We'd rather be accused of crying wolf."

Anderson added that he believed that Institute would be able to resume a comfortable relationship with plant.

"At one stage of the game, there was a lot of pride in this valley," he said. "This industry has had a good track record in this valley. I feel optimistic . . . . There's a reservoir of good will between this community and Union Carbide, and I think we can build on that."