It was the first time anywhere in the world that a human being has been executed by nitrogen hypoxia.

The state attorney called it "perhaps the most humane method of execution ever devised" - but the prisoner's legal team said he was the test subject for a lethal experiment.

Alabama has one of the highest per capita execution rates in the United States, with 165 people currently on death row.

But like other states that have the death penalty, it has struggled to source drugs used for lethal injections. In 2010, pharmaceutical companies around the world started restricting sales to prison agencies.

Alabama, Oklahoma and Mississippi permitted nitrogen gas as an alternative in 2022, and it was used for the first time on Kenneth Eugene Smith.

He was convicted in 1988 of killing a pastor's wife and has already survived a botched execution, when prison authorities couldn't find a suitable vein for a lethal injection.

How did the nitrogen execution work?

A heavily redacted protocol released by the courts sets out how the nitrogen was be used.

Smith was secured to a gurney in the death chamber. He was forced to breathe a high concentration of nitrogen gas from a pressurised cannister through a tightly fitting mask strapped to his face.

Under the protocol, the gas was administered for 15 minutes or for five minutes after an ECG monitor showed his heart had stopped, whichever was longer.

But there are huge gaps in the information released, including details on how the oxygen-monitoring equipment is calibrated, how the nitrogen hypoxia system is operated, and how the system is shut down.

Nitrogen makes up 78% of the air we breathe. But increasing the concentration reduces the amount of oxygen available and that is ultimately fatal - people have died in workplace accidents due nitrogen asphyxiation.

Alabama state authorities say breathing a high concentration of nitrogen would cause a person to lose consciousness in seconds and die within minutes.

But even a small amount of air leaking into the mask could have slowed his death. Smith's lawyers argued in court that he may choke on his own vomit or be left in a vegetative state.

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UN's calls to halt the execution

The United Nations had called on Alabama to stop the execution, warning that there is no scientific evidence to prove that execution by nitrogen inhalation will not cause "grave suffering".

The UN High Commissioner for Human Rights added that it "could amount to torture or other cruel, inhuman or degrading treatment or punishment under international human rights law".

There have been experiments on animals.

According to the American Veterinary Medical Association, rats show signs of panic and distress when forced to inhale the gas, including open-mouth breathing and seizure-like behaviour.

And veterinary scientists in the US and Europe advise that nitrogen should not be used to euthanise large mammals on welfare grounds unless they are first sedated.

There have been many means of execution in the past - including firing squads, electric chairs and lethal injections.

But the executioner has to pull a trigger, flick a switch, or activate a plunger. They perform the final act that kills.

With nitrogen hypoxia, it was Smith who was killing himself with every involuntary breath.