Alabama plans to give death row inmates a new option - death by oxygen deprivation -- but it's unclear exactly how the never-used method would be carried out.

Gov. Kay Ivey last week signed into law a bill to authorize executions by nitrogen hypoxia. Nitrogen is an inert gas that makes up about 78 percent of the air people breathe. With nitrogen hypoxia, death would occur from breathing only nitrogen, without the life-sustaining oxygen that makes up about 20 percent of the air.

Two other states -- Oklahoma and Mississippi -- have authorized executions by nitrogen hypoxia. Two weeks ago, Oklahoma Attorney General Mike Hunter announced it would become that state's main method when it has a protocol finalized. That is expected to take several months and the state hasn't released any details. The difficulty of obtaining the drugs for lethal injection led to the decision, Hunter said.

The Alabama Department of Corrections has no specifics yet about how it will implement the new law, including the possible cost and whether it would require a new death chamber.

"The Department of Corrections will follow the law of the state and adhere to the protocols and method of an execution as ordered by the Alabama Supreme Court," DOC spokesman Bob Horton said.

Sen. Trip Pittman, R-Montrose, the sponsor of the bill, said he got the idea of nitrogen hypoxia executions from Oklahoma. Pittman said he believes it will be a humane method. He said it's important to maintain the death penalty for the worst offenders.

"I think the death penalty is a deterrence and I think it's the appropriate punishment for people that commit certain heinous crimes," Pittman said.

Last year, Pittman sponsored a bill to authorize executions by firing squad as an alternative to lethal injection. That came after a court ruling postponed the execution of Tommy Arthur for the seventh time in 15 years. Arthur had challenged the state's lethal injection method and suggested the firing squad as an alternative. Arthur was finally executed by lethal injection in May 2017 for a 1982 murder.

Pittman said the Department of Corrections opposed his firing squad bill. He said the DOC was initially neutral on the nitrogen hypoxia bill and eventually supported it after problems with one of the state's most recent executions and because of concerns about obtaining lethal injection drugs.

Lethal injection remains Alabama's main method of execution. Inmates can also still opt for the electric chair, though none have done so since Alabama adopted lethal injection. Nitrogen hypoxia becomes a third option.

The new law says that inmates can choose nitrogen hypoxia over lethal injection. It says that if lethal injection is ruled unconstitutional or becomes otherwise unavailable, nitrogen hypoxia becomes the state's primary method. The new law takes effect June 1.

Federal courts have upheld the legality of lethal injections through repeated challenges by inmates. Alabama and other states have at times experienced difficulties in obtaining the drugs.

Alabama's last execution, of Michael Wayne Eggers on March 15, was carried out without incident. But the previous one, of Doyle Lee Hamm on Feb. 22, was called off after medical personnel could not find a vein to inject the lethal drugs.

Robert Dunham, executive director of the Death Penalty Information Center, said there's no basis for the assertion that nitrogen hypoxia is a humane or relatively painless form of execution.

"Nitrogen hypoxia may work, it may not work," Dunham said. "We don't know. It's experimental. And it's unethical to do experiments so that's the dilemma that states are in.

Dunham said it's not clear if the executions would be carried out in a vacuum chamber or with just a death mask. The Death Penalty Information Center is a non-profit organization that provides the media and the public analysis and facts about capital punishment.

Dunham points to studies that found problems with the use of nitrogen hypoxia in the euthanasia of animals. The American Veterinary Medical Association Guidelines for the Euthanasia of Animals in 2013 says that nitrogen hypoxia is an unacceptable method for most mammals unless an anesthesia is also used and that an anesthesia can delay death.

"So, it's either not going to be fast or it's not going to be appropriate," Dunham said.

The World Society for the Protection of Animals recommends against using nitrogen hypoxia for euthanasia for multiple reasons, including that it causes distress before unconsciousness.

Pittman said there may be ways to mitigate those concerns, including with a sedative or anesthesia before the loss of oxygen. He also questions whether those concerns are overstated.

"People that are worried about a little discomfort when a sentence of death is being carried out, I wish they had shown the same compassion to their victims when they ruthlessly, in most cases, murdered them," Pittman said.

A report prepared at the request of an Oklahoma lawmaker by criminal justice faculty members at Oklahoma's East Central University concluded that nitrogen hypoxia would be a humane method of execution.

The report cited, among other things, a 1961 study that found that people lost consciousness after breathing pure nitrogen for about 20 seconds and experienced no physical discomfort. The report said the method would be simple and noted that nitrogen is used in industry and is readily available.

Dunham said he doesn't think Alabama, or any other state, wants torturous executions. But he said the failure of the Hamm execution and the lack of a clear explanation by the Department of Corrections showed a lack of transparency that could undermine public trust.

This story was updated at 10:20 p.m. with additional comments from Robert Dunham.