Three of the United States’ largest suppliers of medical-grade nitrogen have prohibited its usage for capital punishment after Alabama carried out the nation’s first execution using the gas earlier this year. The companies—Airgas, Air Products, and Matheson Gas—confirmed to The Guardian on Sunday they’d taken steps to prevent their products from being associated with the death penalty. “Airgas has not, and will not, supply nitrogen or other inert gases to induce hypoxia for the purpose of human execution,” the company, which has 24 branches in Alabama, told the outlet. A spokesperson for Matheson Gas explained that producing nitrogen for execution was “not consistent with our company values.” The blockade comes after the 22-minute execution of death row inmate Kenneth Eugene Smith, 58, on Jan. 26. The convicted murderer appeared to “shake and writhe on the gurney” for at least two minutes, according to an Associated Press reporter who witnessed it. Critics have raised concerns about the previously untested method, arguing it remains unclear whether nitrogen hypoxia is a humane, painless, and dignified way to die. “People are entitled to basic human rights, even as you are trying to kill them,” Robert Dunham, director of the Death Penalty Policy Project, told The Guardian.