An Alabama prisoner will soon become the first in the US to be executed by nitrogen gas. Veterinarians won't even use the method on most animals.

• An Alabama prisoner is about to undergo an experimental execution using nitrogen gas on Tuesday.
• The inmate's attorney told a federal appeals court on Friday it constitutes cruel and unusual punishment.
• Even veterinarians refrain from using nitrogen gas in euthanasias for most animals.

An Alabama prisoner is scheduled to be executed by nitrogen gas on Tuesday — an experimental form of capital punishment that has prompted a global outcry from human rights organizations.

Kenneth Eugene Smith survived a lethal injection attempt in September 2022, when officials from Alabama's Department of Corrections tried and failed for four hours to find a vein to insert an IV line. A federal judge then ruled that an execution by nitrogen hypoxia can go forward.

The UN High Commissioner for Human Rights has criticized the planned execution, saying it violates international human rights treaties, including the prohibition on "torture or other cruel, inhuman, or degrading treatment or punishment."

Critics have also pointed to the American Veterinary Medical Association's 2020 euthanasia guidelines, which say nitrogen hypoxia creates an "anoxic environment that is distressing for some species." The AVMA now says the gas can be used under certain conditions for chickens, turkeys, and pigs, but is deemed "unacceptable" for other mammals.

On Friday, a federal appeals court heard arguments from Smith's attorney in a last-minute effort to block the execution, saying a second execution attempt would constitute cruel and unusual punishment. Alabama's solicitor general argued that the state has "adopted the most painless and humane method of execution known to man," according to the Associated Press.

The nitrogen hypoxia execution method, which remains untested, involves a mask being fitted to the prisoner's face, which pipes in nitrogen gas. But Smith's lawyer has argued there is a risk that the mask's seal could leak in oxygen, causing a prolonged and terrifying execution.

Federal appeals judges on Friday also asked what would happen if Smith vomited into the mask — to which Alabama's solicitor general responded that the state would not stop the nitrogen gas once it began flowing.

"If he vomits during the execution with the mask on, you're telling me that the state will not stop the execution, they will permit him to choke on his vomit?" one judge asked, according to the Associated Press. The solicitor general replied that the risk of vomiting was low and that Smith would be unconscious almost immediately.

Smith was convicted of the brutal murder-for-hire scheme against Elizabeth Sennett in 1988. Smith and another man each accepted $1,000 from Sennett's husband, a preacher who had taken out an insurance policy on his wife and died by suicide before he could be criminally charged, according to AL.com. Authorities said Sennett had been stabbed and beaten with a fireplace instrument in a scene that was staged to look like a home invasion.

Smith confessed to the crime, and the other man was executed by lethal injection in 2010, according to the Associated Press.

In a recent interview with The Guardian, Smith told the news outlet he has been "sick to his stomach" about his upcoming second journey to the death chamber.

"I am not ready for that. Not in no kind of way. I'm just not ready, brother," he said.