ASSOCIATED PRESS WRITER

BOB EGELKO

SAN FRANCISCO (AP)—Prisoners executed by cyanide gas experience ‘‘suffocation and air hunger and intense physical stress’‘ before they die, a witness testified Monday in a lawsuit seeking to shut down California’s gas chamber.

‘‘It’s like being held under water,’‘ said Dr. Kent Olson, medical director of San Francisco’s regional Poison Control Center.

Even involuntary gasps and tremors, often reported by witnesses in the final stages of gas chamber executions, must be presumed to reflect conscious pain ‘‘at some level,’‘ he said.

The American Civil Liberties Union is arguing before U.S. District Judge Marilyn Hall Patel that death in San Quentin’s gas chamber is cruel and unusual punishment. The ACLU filed the lawsuit last year in response to California’s first scheduled execution in 25 years.

The state contends that the only provable pain caused by the gas chamber is the mental torment associated with an impending execution.

Gas, once a common method of execution, is now used by only five states, including California, where 196 prisoners have died in the San Quentin gas chamber since 1938. The other four states are Arizona, Maryland, Mississippi and North Carolina.

No court has ever declared the gas chamber to be unconstitutional but the case has the potential to shut down gas chambers nationwide. That is unlikely, given the conservative majority on the U.S. Supreme Court, but the lawsuit could accelerate challenges to executions by lethal gas.

Even if the ACLU prevails, it will not halt executions in the state, which would be conducted by lethal injection, an option under a law that took effect this year.

Olson, a toxicologist who specializes in diagnosing and treating poisoning, said he believes any response to the stimulus of being poisoned shows some degree of conscious pain. ‘‘If they are not brain-dead, they are experiencing it at some level,’‘ he said.

Cyanide is ‘‘‘very rapid-acting, very rapidly absorbed … but there’s still a time when it’s building up in the cells’‘ and depriving them of oxygen, Olson said.

During that period, he said, animals in experiments have squealed and tried to escape from their cages, in apparent pain and terror.

‘‘What they’re experiencing … is cruel,’‘ he said. ‘‘It’s not an appropriate way of euthanizing an animal.’‘

He cited an American Veterinary Medical Association committee report earlier this year that rejected cyanide as a method of execution for dogs, while accepting other gases if the animal is heavily sedated.

‘‘It is outside the standards of human decency to kill an animal or a human with a method that is unnecessarily painful and associated with a great deal of terror,’‘ Olson said.

California instituted the gas chamber in 1937, claiming it was a more humane alternative to hanging. Nearly 200 people have died in the San Quentin chamber.