The planned execution of a death row inmate by the state of Alabama on Thursday evening will be carried out by a procedure that has never been used for capital punishment in the United States.

The inmate, Kenneth Smith, who was convicted in a 1988 stabbing murder, will be put to death by inhaling nitrogen gas, a method known as nitrogen hypoxia.

Supporters of the method say it is fast and painless. But earlier this month, the United Nations Human Rights Office urged Alabama to stop the execution, saying it could amount to torture and be in violation of human rights treaties that the United States has agreed to.

Alabama would be the first state to use nitrogen hypoxia, but other states are interested in employing the method.

What is nitrogen hypoxia?

Hypoxia is a medical term for a state of insufficient oxygen in the body. Nitrogen, a colorless, odorless gas, makes up about 78 percent of the air inhaled by humans. But under the method of nitrogen hypoxia, the person breathes in only nitrogen, leading within minutes to unconsciousness and then death from lack of oxygen.

How will the nitrogen be administered?

According to the protocol released by Alabama prison officials, members of the “execution team” will strap Mr. Smith to a gurney in the state’s execution chamber in Atmore. A mask will be placed on his head and nitrogen will be released into it, depriving him of oxygen. Many experts compare this process to putting a plastic bag over someone’s head, although in that situation, the person would be inhaling carbon dioxide rather than nitrogen.

Why is this method being used now?

The standard method of execution since the 1980s has been lethal injection of drugs that stop the heart. But states have had problems with lethal injections for years.

Some states have had trouble obtaining sufficient quantities of drugs for lethal injections.

Even when they have the proper dosage, many executions have been botched because the team that administers the injection has been unable to locate appropriate veins.

This is what happened to Mr. Smith in Alabama. He was to be executed by lethal injection in November 2022, but a team of people repeatedly failed to properly insert an intravenous line.

This problem often occurs because medical ethics rules prohibit doctors and other health care professionals from assisting in an execution. So the injections are usually administered by inexperienced prison workers, said Dr. Joel Zivot, an associate professor of anesthesiology at Emory School of Medicine and an expert on physician participation in lethal injection.

Lethal injection also involves drugs that, if given incorrectly, can result in significant pain and suffering.

Supporters of using nitrogen hypoxia believe it is an easier and more humane alternative because it does not require an injection and is swift and painless.

Has nitrogen hypoxia been used in other situations?

Several years ago, Dr. Philip Nitschke, an Australian doctor and founder of Exit International, which advocates medically assisted suicide, developed a pod in which a patient could flip a switch and release the flow of nitrogen. He recently told The New York Times that he had witnessed about 50 deaths due to nitrogen hypoxia.

Is there research on the impact of nitrogen on humans?

Very little, which is why some people feel it should not be used in state executions. Most of the reports in medical journals are about nitrogen exposure in leaks and industrial accidents that killed workers, and in suicide attempts.

In a 1963 experiment to study the effect of brief hypoxia on three healthy volunteers, “most of them had seizures within 15 to 20 seconds of breathing pure nitrogen,” Dr. Zivot said.

What are the drawbacks to this method?

Doctors say that the prisoner could vomit in his mask, not only causing him to choke but also loosening the seal, which would allow oxygen to rush in, diluting the nitrogen.

Mr. Smith’s lawyers have argued that this is a likely scenario for Mr. Smith, who, they said, has been vomiting continuously in recent days, which they associate with the PTSD he has suffered after the 2022 botched execution.

Alabama corrections officials said that Mr. Smith would not be given food after 10 a.m. Thursday morning to reduce the likelihood of vomiting when he is on the gurney.

Mr. Smith’s head and body will be tightly bound to the gurney, to prevent thrashing and a displacement of the mask. It is not known whether he will be given a sedative before the execution, which would further reduce the likelihood of thrashing. But Dr. Zivot noted that sedating a patient is a medical procedure and would typically require a physician’s involvement.

Veterinarians have generally stopped using nitrogen to euthanize animals, who showed severe signs of distress. Critics and supporters of the method strongly disagree over whether a human would feel distress with nitrogen.

“Nobody really knows what’s going to happen,” said Dr. Jeffrey Keller, president of the American College of Correctional Physicians. “So will he choke? Will he vomit? Will the mask fit or will the nitrogen leak out? Will that nitrogen harm anybody else who is standing nearby? Nobody knows any of this. It’s an experiment.”