A leading authority on the effects of the poison gas that has killed hundreds of people in India this week said yesterday that Americans could face a similar disaster should a mishap occur at a similar chemical factory in this country.

The chemical, called methyl isocyanate, has been used for more than 50 years with no major accidents in the United States, he pointed out.

On the other hand, he emphasized, many plants across the nation use the substance or one of its toxic chemical relatives. Trucks and trains loaded with these substances crisscross the country with some frequency.

"The concerns being expressed about similar hazards in this country are by no means overreactions," said Yves Alarie, who has been studying the health effects of such chemicals for more than 10 years. Alarie is a toxicologist at the University of Pittsburgh School of Public Health.

Methyl isocyanate is mainly used to make a common family of insecticides known as the carbamates. The insecticides are much less toxic to humans than their parent compound.

Closely related to methyl isocyanate are about a dozen other forms of isocyanate, all highly toxic, Alarie said. These are turned into a wide variety of synthetic materials used as industrial coatings and resins and agricultural fertilizers and pesticides. Probably the best known product is polyurethane, which is made into products ranging from foam rubber to varnish.

Alarie said billions of pounds of isocyanates are produced annually in the United States by several manufacturers, including Union Carbide Corp., which owns the factory in Bhopal, India, and American Cyanamid Co. Union Carbide's plant at Institute, W.Va., is believed to be the only one in the United States that makes the methyl form.

"Isocyanates have been made in this country on a big scale for over 50 years and there haven't been any major accidents, but it is a danger. It's a real danger," Alarie said. "The American chemical industry has generally been a safe one, but you are never going to eliminate all accidents."

Alarie said there have been numerous small incidents in which two or three workers were killed. "If there were to be a big accident where this chemical would escape, it could be a real disaster," he said.

Alarie said he was especially worried about the shipment of isocyanates by truck or rail from manufacturers to users. Tank cars of methyl isocyanate, for example, travel by rail from the Union Carbide plant in West Virginia to a pesticide factory operated by E.I. du Pont de Nemours & Co. in La Porte, Tex. Several other pesticide makers also make purchases from the same plant and have the substance shipped overland.

A spokesman for American Cyanamid said his company buys methyl isocyanate for shipment to a pesticide plant in Resende, Brazil. He said it is shipped in lots of 30,000 to 60,000 pounds overland from West Virginia to freighters that sail to Rio de Janeiro.

Alarie said the effects of accidental spills on human beings can be judged from what happens to laboratory animals exposed to isocyanates.

Animals that inhale the gas in heavy concentrations experience an immediate swelling of the tissues that line the air passages to the lungs. The swelling can be enough to close passages, leading to the equivalent of a massive asthma attack.

Victims surviving this effect suffer massive lung damage that causes fluids to be secreted and to accumulate in the lungs. In a day or so, Alarie said, the victim can, in effect, drown in the fluids.

Animals exposed to lower doses of isocyanates may escape these effects but still suffer much tissue destruction within the lungs, he said, leaving them particularly vulnerable to ordinary respiratory infections that can prove fatal.

Despite its severe effects, Alarie said, methyl isocyanate is not a persistent substance. On contact with water, it is converted into methyl amine, a much less toxic substance that is completely excreted in a few days, and harmless carbon dioxide.

Alarie said methyl isocyanate vapors in the air are soon converted when they react with water vapor.