Sue Ogrocki/AP

Today, Alabama will attempt to kill 58-year-old Kenneth Eugene Smith with nitrogen gas—an untested execution method that experts warn could cause Smith unnecessary suffering while potentially putting others in the vicinity in harm’s way.

The state attempted to kill Smith, who was convicted of a murder-for-hire plot, in 2022 using lethal injection and failed. According to CBS News, this time Smith will be strapped to a gurney with a mask over his face as he’s forced to breathe in a lethal amount of nitrogen gas until he asphyxiates. The UN High Commissioner for Human Rights, Ravina Shamdasani, has called for state officials to halt the execution, warning that this method could be akin to torture and violate human rights treaties.

“Nitrogen gas has never been used in the United States to execute human beings,” Shamdasani said in a statement released a little over a week ago. “The American Veterinary Medical Association recommends giving even large animals a sedative when being euthanized in this manner, while Alabama’s protocol for execution by nitrogen asphyxiation makes no provision for sedation of human beings prior to execution.”

Many medical experts have expressed similar concerns to those made by Shamdasani, pointing out critical flaws in Alabama’s euthanasic techniques that could prolong the execution process, including Smith potentially choking on his own vomit. There’s also the looming risk of exposing others to the odorless and colorless gas, including Smith’s spiritual adviser, who had to sign a waiver and agreed to be at least 3 feet away from Smith during the execution. Despite these potential threats, Alabama’s Attorney General has called the method “the most painless and humane method of execution known to man.”

Many have also cited the state’s troubling history with botched executions, including the execution of Joe Nathan James Jr. in 2022, whose painful, multi-hour-long death may have the been longest execution by lethal injection in US history. Even Smith’s previous execution did not go according to plan because prison employees failed to set his IV line, and were unable to kill him before his death warrant expired. This setback, alongside two other botched killings, would eventually cause Alabama to pause executions until July 2023.

According to the Washington Post, Smith’s legal team has made a last-minute plea to save Smith’s life, filing a petition to the Supreme Court, calling the nitrogen gas method unconstitutional. However, the courts already rejected a separate request on Wednesday, giving no reasons or specific dissents.