Death Penalty Policy Project Director Robert Dunham said the method is slow, with veterinarians saying it could take up to seven minutes to die by breathing in nitrogen.

Additionally, he argues, Smith's execution is problematic because a judge overrode an Alabama's jury recommendation to sentence Smith to life in prison.

"Kenneth Smith is being executed, despite the fact that 11 of the 12 jurors in his case voted for life. So this is a person who never should have been sentenced to death in the first place," Dunham said. "But Alabama had a law that permitted the trial judge to override the recommendation of the jury. It no longer does that; it is impermissible and no state in the United States permits a judge to override a jury vote for life. But Alabama, after abandoning that process, has refused to overturn the death sentences of the individuals who were sentenced to death in, in essentially violation of the community's judgment."

Alabama botched Smith's previously scheduled execution by lethal injection in November 2022 when multiple attempts to insert an intravenous line failed.

Smith, 58, has sued the Alabama Department of Corrections, arguing that the proposed method comes with dangerous risks, including that the mask's seal with his face might be broken allowing in oxygen, botching the execution. Such a scenario could induce a stroke or leave Smith in a permanent vegetative state, he argued. A federal judge ruled against him, saying that he is unlikely to show the new method amounts to cruel or unusual punishment. Robert Grass, a lawyer representing Smith, said he planned to appeal.

Smith was convicted of beating and stabbing the wife of an Alabama pastor in her own home in 1989 for $1,000. Within days of the murder, the pastor confessed that he was having an affair and had ordered the murder and then he committed suicide. The two other men convicted of the murder along with Smith have died: one while serving a life sentence for the crime and the other by lethal injection.

U.S. states have found it increasingly difficult to obtain barbiturates used in lethal-injection execution protocols, in part because of a European ban preventing pharmaceutical companies from selling drugs to be used in executions. As a result, some states have sought to revive older methods such as firing squads, while Alabama, Mississippi and Oklahoma have introduced new gas-based protocols.

According to Amnesty International, the United States executed 18 people in 2022.