(NewsNation)  — The state of Alabama is poised to carry out a first-of-its-kind execution using nitrogen gas Thursday, sparking debate over whether the method is humane.

Death row inmate Kenneth E. Smith is scheduled for execution by nitrogen hypoxia. The method involves being fitted with a respirator-style face mask and inhaling pure nitrogen.

Smith is on death row for committing a capital murder in 1988. He’s one of two men convicted in the murder-for-hire killing of Elizabeth “Liz” Sennett, the wife of a deeply indebted preacher who wanted to collect insurance, the Associated Press reported.

The United Nations Human Rights Office called on Alabama to halt the execution, calling the method “inhumane” and “torture,” but a federal judge ruled last week that Alabama can move forward with it.

Why execution by nitrogen gas?

After his failed execution in 2022, Smith filed a lawsuit asking the state to execute him by nitrogen hypoxia instead of the most common option, lethal injection, according to the American Bar Association (ABA).

Smith, however, still feels anxiety heading into next week, he told NPR in an exclusive interview.

“I’m still carrying the trauma from the last time,” he told NPR. “I’m being treated for PTSD, and I struggle daily. So when I got this date, my level of anxiety this time was not even close to what I faced last time. Everybody is telling me that I’m going to suffer. Well, I’m absolutely terrified.”

The drugs used for lethal injections also became harder for prisons to get their hands on once commercial manufacturers started refusing to allow their products to be used in executions, according to the ABA.

What is it and how does it work?

Nitrogen is safe to breathe but only if mixed with an appropriate amount of oxygen, according to the U.S. Chemical Safety and Hazard Investigation Board.

Nitrogen hypoxia occurs when the subject inhales pure nitrogen, depriving their body of enough oxygen to keep functioning.

One 2015 study cited literature reporting that breathing pure nitrogen would cause someone to lose consciousness within 17-20 seconds. Their heart should stop beating within 3-4 minutes, according to the report.

Has it been tried before?

Alabama is one of three states, along with Oklahoma and Mississippi, that allow nitrogen hypoxia executions, but none has used it until now.

The 2015 report, commissioned for Oklahoma, cited a decades-old study conducted on human volunteers who hyperventilated on pure nitrogen gas for varying lengths of time.

The process has also been cited as a preferred method by some right-to-die groups, according to Oklahoma’s report.

Smith’s attorneys, however, say the protocol comes with unknowns. They also argued in court filings that the American Veterinary Medical Association has said nitrogen hypoxia is an acceptable method of euthanasia for pigs but not other mammals because it could create an “anoxic environment that is distressing for some species,” The AP reported.

Is it painless and humane?

Opinions are mixed. The U.N. has called the method “torture,” while the 2015 Oklahoma study determined it was “humane” and said the method “can assure a quick and painless death.”

Robin Maher, executive director of the Death Penalty Information Center, joined “NewsNation Now” on Tuesday, where she raised concerns about the efficacy and ethics of what she called a “completely untested” process.

“There’s no way to test its efficacy, no way to make sure the prisoner is not suffering gratuitous pain and suffering, and there’s no way of knowing it’ll even work correctly,” Maher said.