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The Supreme Court on Wednesday refused to stop Alabama from executing an inmate by nitrogen hypoxia, a controversial and untested method that has prompted legal pushback and a rebuke from the United Nations. Kenneth Eugene Smith is scheduled to be the first person in the United States to be put to death with nitrogen gas.

The justices rejected arguments by Smith's lawyers that it would be unconstitutional for the state to attempt a second execution after a failed lethal injection in 2022.

Smith also has asked the 11th U.S. Circuit Court of Appeals to block the execution. That court has not yet issued its ruling. His execution is currently scheduled for Thursday, Jan. 25.

The use of nitrogen gas will be a capital punishment first, even though it has not only been denounced by some medical professionals but also by veterinarians who oppose its use on animals. In 2020, the American Veterinary Medical Association advised against the use of nitrogen gas as a way to euthanize most mammals, calling it "distressing." One of the few uses of nitrogen gas in animal euthanasia is with chickens. 

Experts have repeatedly warned that nitrogen toxicity may cause the condemned person to suffer unnecessarily, while at the same time threatening the health of others in the room.  

What is nitrogen hypoxia?

Nitrogen hypoxia is a process where pure nitrogen gas —or nitrogen gas at concentrations high enough to be lethal— is inhaled to the point of causing asphyxiation. It is a new alternative to more common forms of capital punishment, like lethal injection and electrocution, which are the two methods used most prevalently across the U.S., according to the Death Penalty Information Center. 

The nonprofit organization noted that "lethal gas" is authorized broadly as an execution method in seven states, including Alabama, Mississippi and Oklahoma, although only those three have specified the use of nitrogen for this purpose, and only Alabama has released a formal execution protocol for nitrogen hypoxia. The protocol was issued last August, and the version made available to the public is heavily redacted.

Breathing nitrogen through a mask could in theory cause a person to lose consciousness before oxygen deprivation leads to death, and Alabama officials have insisted that this is most likely how the scheduled execution will happen. 

But Smith's legal team, while seeking an injunction to halt the execution plans that was ultimately rejected, accused the state of using him as a "test subject" for a lethal experiment. The United Nations' human rights office called on Alabama to stop the execution, noting there is "no scientific evidence to prove" that execution by nitrogen inhalation will not cause "grave suffering." 

What do we know about inhaling nitrogen gas?

During the execution, officials said Smith will be strapped to a gurney and forced to breathe nitrogen through a gas mask until his body is depleted entirely of oxygen and shuts down. Although it has never been used before inside the death chamber, the consequences of too much nitrogen inhalation — usually accidentally in industrial settings — are well-documented.

Nitrogen exists organically in the atmosphere, and actually composes around 75% of the air that humans and animals breathe every day. But the colorless and odorless gas is only safe to inhale when it is mixed with an appropriate concentration of oxygen; otherwise, breathing it in is toxic. 

The U.S. Chemical Safety Board issued a series of bulletins in the early 2000s that reviewed cases of nitrogen asphyxiation over the previous decade. In one bulletin that references information from the Compressed Gas Association, the board notes that when enough nitrogen is introduced to deplete oxygen in the air to less than 10%, effects on the human body can be lethal, potentially causing "inability to move, loss of consciousness, convulsions" and death.   

Alyssa Spady contributed to this report.

In:
• Supreme Court of the United States
• Capital Punishment

Stephen Smith

Stephen Smith is a senior editor for CBSNews.com.