The prisoner is Kenneth Smith, who was found guilty for the 1988 murder-for-hire of Elizabeth Sennett.
And the use of nitrogen gas - a method untested in judicial executions - has provoked an outcry from death-penalty abolitionists who say it runs the significant risk of crossing over into a form of torture.
"How long it's going to take him to die is just not clear. And I think that the state, however, has all the time in the world. So there have been executions in the United States that have actually gone on for hours."
Dr. Joel Zivot is an anesthesiologist and associate professor at Emory University's School of Medicine, and he told Reuters there's a chance Smith's death will not be swift.
"So, when he's breathing nitrogen gas, assuming that there's no leak, assuming that it all is conducted properly, there's a pretty good chance that he'll have a seizure. That we know this, you know, from, again, some animal and human studies. So if he has a seizure, then he'll stop breathing. And, he may again aspirate the contents of his stomach."
Alabama first tried to execute Smith in 2022 by lethal injection, but aborted the attempt after several hours of failed efforts to insert an intravenous needle into his body.
Alabama Solicitor General Edmund LaCour told federal judges last week that the state has since developed "the most painless and humane method of execution known to man."
Smith's lawyers disagree, and seek a court-ordered halt to the second execution attempt, scheduled for 6 p.m. on Thursday.
