Alabama authorities were due to execute death row inmate Alan Miller on Thursday, but on Monday a federal judge blocked the execution from being carried out.

The reason was that Miller had opted to be executed by nitrogen hypoxia, a currently untried execution method that the state authorized in 2018.

Its authorization means inmates can choose to be executed via that method if they wish, which is what Miller claims he did by filling in a form and leaving it for a prison worker to collect, according to the AP news agency.

Prison officials said they have no record of this. As of this week, the state was not fully prepared to carry out an execution using that method.

Miller said while testifying that he chose the nitrogen hypoxia method because it reminded him of the nitrous oxide gas used by dentists and that seemed better than a lethal injection as he is scared of needles.

U.S. District Judge R. Austin Huffaker, Jr., issued an injunction to block the execution from going ahead by any method other than nitrogen hypoxia. He noted that Alabama may soon be able to carry out the method, AP reported.

Miller was sentenced to death after being convicted of a workplace shooting on August 5, 1999, that killed three people in Shelby County, Alabama. Miller was mentally ill, according to a psychiatrist hired by the defense, CBS reported. He has been on death row since 2000, according to the Shelby County Reporter.

Professor Rob Chilcott, head of toxicology at the University of Hertfordshire in the U.K., told Newsweek about how nitrogen can affect the body and why the as-yet untested execution method could kill.

“Hypoxia is the medical term which describes an insufficient amount of oxygen in the body,” he said. “Normally, the air we breathe contains about 21 percent oxygen. Adding excess nitrogen to air will dilute the concentration of oxygen and, depending on the ‘dose’ of nitrogen, will cause varying degrees of hypoxia.

“If sufficient nitrogen is added to lower the oxygen concentration to less than about 5 percent, the person breathing the air will rapidly become unconscious. In the absence of any intervention, death will occur shortly after.”

Nitrogen can be highly dangerous outside the context of a controlled execution, too. Chilcott said there had been around 60 deaths in the U.K. from nitrogen exposure over the past five years.

“Normally, the human body monitors carbon dioxide levels in the blood, not oxygen,” he said. “For example, when we exercise we produce excess carbon dioxide. This stimulates the respiratory center of the brain, resulting in an increased respiratory rate—we breathe harder and faster to remove the excess carbon dioxide from our bodies.

“However, artificially adding excess nitrogen to air will reduce blood oxygen levels but not increase the amount of carbon dioxide in the blood. Thus, inhaling a nitrogen rich atmosphere will not produce a physiological signal to alert the body to a lack of oxygen. This is why inert gases such as nitrogen and helium are highly dangerous.”