As it becomes increasingly difficult to obtain the drugs needed for lethal injections, several states have sought to resurrect bygone ways of killing death-row inmates.

In Utah, which outlawed death by firing squad in 2004, lawmakers voted last week to reinstate that execution method should the state run out of drugs for lethal injection. Tennessee chose the electric chair last year as its own backup method. Last week, the Alabama House passed a bill that would do the same. Similar measures in Virginia, Missouri, and Wyoming failed last year.

But in Oklahoma, a bill is advancing that would introduce an entirely new and untested method of execution: death by nitrogen inhalation.

“It’s probably the best thing we’ve come up with since the start of executing people by government,” the bill’s sponsor, Rep. Mike Christian (R) told the Oklahoman.

How would such an execution work?

Nitrogen gas itself is odorless and nontoxic, and makes up 78 percent of the atmosphere. It only becomes lethal when someone breathes it in at high concentrations, and only then because that person is therefore not getting enough oxygen.

The proposed law is vague on the exact procedure, but Christian has said that it would be cheap and simple. Some kind of bag or breathing mask would be placed around the inmate’s head. Nitrogen gas would be pumped in, displacing any oxygen. The inmate would start to feel lightheaded, possibly euphoric, which are symptoms of oxygen deprivation. Painless death would soon follow.

Or that’s what’s supposed to happen, though nobody really knows for certain. (Generally speaking, medical professionals refuse to conduct research into killing methods.)

The idea dates back decades—as early as 1995, when National Review suggested that smothering people with nitrogen might be more humane than killing them with caustic chemicals in a gas chamber. That article pointed to several instances in which people accidentally died of nitrogen asphyxiation.

In 2003, the U.S. Chemical Safety Board issued a bulletin warning of inadvertent death by nitrogen. Between 1992 and 2002, 80 such deaths were recorded. In most cases, the victims had no idea they were walking into area with a high concentration of nitrogen and they succumbed quickly.

For instance, two people at a Delaware refinery died in 2005 trying to get a roll of duct tape out of a chamber filled with nitrogen. Nobody saw the first worker go inside. The foreman who descended to rescue him also disappeared inside the chamber. The two were found passed out at the bottom, and soon pronounced dead.

These kinds of accidents occur because it’s hard for humans to detect when there is not enough oxygen in the air.

Breathing serves two purposes: It replenishes our blood with oxygen, and it removes the carbon dioxide that we produce. Human bodies are much more sensitive to the buildup of carbon dioxide, which triggers a searing, desperate urge to breathe. That is the feeling of suffocation. We are much less likely to detect a lack of oxygen, and will happily breathe air that is deficient in oxygen for far longer than is healthy.

The U.S. Air Force Flight Surgeon’s Manual specifically warns aviators against hypoxia for this reason. When people breathe air that is extremely deficient in oxygen, research suggests that they rarely notice in time. A couple of breaths, and they are soon unconscious.

In January, pilot Ross Detwiler described for Aviation Week the feeling of hypoxia from his stints in a chamber that simulates the rarefied air at high altitudes. He wrote:

These symptoms include, but are not limited to, a feeling of happiness or well being (à la Mrs. D), a feeling of over confidence, a feeling of tenseness, perspiring, changes in breathing rate and heart rate, belligerence and, eventually, unconsciousness and death.

The feelings associated with hypoxia can be too subtle even for trained pilots to recognize. “Such is the insidious nature of oxygen deprivation,” the Federal Aviation Administration cautioned in a brochure. “It sneaks up on the unwary and steals the first line of sensory protection—the sense that something is wrong—dreadfully wrong.”

Is death by nitrogen really more humane?

Because it is viewed as a quick and probably painless way to die, death by hypoxia has become popular among people seeking to commit suicide. The procedure typically involves what is called an exit bag—a plastic tent wrapped around the head that is filled with an inert gas like nitrogen or helium.  (The method is similar to what has been discussed in Oklahoma.)

A 2009 article in the Journal of Medical Ethics described video footage of four suicides resulting from people wearing face masks hooked up to helium tanks. There was some twitching. “Purposeless movements of the extremities were disconcerting for [the] attendants,” the researchers write. But they go on to conclude that the method seems humane. “The dying process of oxygen deprivation with helium is potentially quick and appears painless,” they write.

At Rep. Christian’s behest, professors at East Central University recently produced a report on death by nitrogen. They recommend it as “a humane method to carry out a death sentence.” Among the procedure’s merits, they cite the ease of administration and widespread availability of nitrogen canisters. (None of the three professors, though, appear to have backgrounds in medicine or biology. The Oklahoman also pointed out that one of the professors helped Christian campaign for state representative.)

The Supreme Court will soon review the constitutionality of Oklahoma’s lethal injection protocol. Oklahoma botched the execution of Clayton Lockett last year by administering an untested mixture of drugs. In January, the court ordered the state to halt executions by lethal injection until Oklahoma’s new cocktail of chemicals could be reviewed.

Oklahoma already has two backup options for killing inmates. If lethal injection is ruled unconstitutional, the law provides for execution by electrocution. And if electrocution is also ruled unconstitutional, Oklahoma allows death by firing squad.

Christian’s bill, HB 1879, would put death by nitrogen second in line, after lethal injection and before electrocution. Like the new laws in Tennessee and Utah, HB 1879 would also make it easier to use the backup options. Lethal injection would not have to be ruled unconstitutional in order for executioners to cue up the nitrogen death bag. As long as one method is “unavailable” for some reason (say, a drug shortage), officials would have the freedom to move down the list of killing protocols.

Christian’s idea may soon spread. Taking Oklahoma’s lead, corrections officials in Louisiana filed a report in February recommending that the state freely allow the use of death by nitrogen as an alternative to lethal injection.

“The research reviewed suggests that this method would be the most humane method and would not result in discomfort or cruel and unusual punishment to the subject,” the committee of lawyers and penitentiary officers wrote.

What does the animal research show?

Though research into killing humans with inert gases is clearly unethical, the question has been raised for other species.

It may be worth noting, for instance, that the World Society for the Protection of Animals (now World Animal Protection) has recommended against euthanizing animals using nitrogen or other inert gases—in part because some animals have different biology; in part because the process can take longer than lethal injection, especially in newborns; and in part because there are readily available alternatives.

The preferred method is death by injection of pentobarbital, which is one of the drugs that penitentiaries have had a hard time obtaining for human executions. (Texas is on its last dose, which it plans to use next month, after the state’s top criminal appeals court halted an execution scheduled for Wednesday.)

In the 1970s, Canadian researchers in conjunction with the Calgary Humane Society conducted a chilling experiment in which they killed 20,500 animals in nitrogen chambers over the course of four years. The cats and dogs sometimes howled or twitched as they died, though it’s impossible to know if these were reflexive movements or indications of suffering.

To get at that question, the researchers put a cat into the chamber until it was knocked out, then took the cat out before it died. After repeating this procedure a couple times, they noted that “in no case did the animal display fear of the chamber,” which is perhaps evidence that the nitrogen chamber was not unpleasant for the cat.

The researchers write that the benefits of death by nitrogen are that it costs less—”about 10 cents per kilogram”—and that it is less anguishing for a pound worker than holding down a cat or dog and injecting him with a lethal shot.

But in recent years, veterinary groups have generally agreed that death by hypoxia is inhumane for most animals. A recommendation to the European Union in 1997 took a much more concerned view of the twitching observed in cats and dogs who die in a nitrogen chamber:

[Nitrogen gas hypoxia] causes unconsciousness in dogs and cats in 1-2 min with hyperpnoea [deep breaths] for about 10s before collapse. After collapse there are vocalizations, opisthonos [abnormal back spasms], convulsions and gasping. Kittens and puppies are resistant to anoxia; they fall unconscious but fail to die. This is not an acceptable method.

The European Union’s rules for euthanizing animals used for research prohibit death by nitrogen or argon gas except for birds, rodents, and pigs. Similarly, the American Veterinary Medical Association’s latest euthanasia guidelines say that death by nitrogen or argon is acceptable for turkeys, chicken, and pigs, but not for other mammals.