Alabama, unless stopped by the courts, intends to strap Kenneth Eugene Smith to a gurney Thursday and use a gas mask to replace breathable air with nitrogen, depriving him of oxygen, in the nation’s first execution attempt with the method.

The Alabama attorney general’s office told federal appeals court judges last week that nitrogen hypoxia is “the most painless and humane method of execution known to man.” But what exactly Smith, 58, will feel after the warden switches on the gas is unknown, some doctors and critics say.

“What effect the condemned person will feel from the nitrogen gas itself, no one knows,” Dr. Jeffrey Keller, president of the American College of Correctional Physicians, wrote in an email. “This has never been done before. It is an experimental procedure.”

Keller, who was not involved in developing the Alabama protocol, said the plan is to “eliminate all of the oxygen from the air” that Smith is breathing by replacing it with nitrogen.