For the first time of its kind in the world, authorities in the American state of Alabama used nitrogen gas to carry out an execution of a prisoner, sparking great controversy worldwide.

Although about half of the American states still apply death penalty laws with various methods, some states still allow execution by hanging, firing squad, or the electric chair, nitrogen gas execution has become one of the most controversial methods in the world.

According to what the British "BBC" network reported, the prisoner Kenneth Eugene Smith, accused in the 1988 murder case of a preacher's wife, was initially scheduled to be executed by lethal drugs in November 2022, but after their failure, the state used nitrogen gas to execute him, using a mask focused on the prisoner's face with pure nitrogen gas.

- How and when is nitrogen gas deadly to humans? And how did the American state use it in the execution?

Nitrogen gas is an inert, colorless, odorless gas that does not support human life, and it is not toxic by itself; nitrogen makes up more than three-quarters of the Earth's atmosphere.

Although every person needs oxygen to stay alive, air consists of 78% nitrogen, and only 21% oxygen, with small amounts of other gases.

However, inhaling nitrogen gas in its pure concentrated form leads to suffocation of oxygen reaching the brain, a process called hypoxia due to nitrogen.

Oxygen deprivation occurs in the body when inhaling a high concentration of nitrogen gas; this leads to replacement of oxygen in the body and causes death.

The American "CNN" network quoted Dr. Jonathan Groner, a professor of surgery at Ohio State University College of Medicine, as saying that execution using nitrogen gas primarily causes respiratory failure.

One study prepared for legislators in Oklahoma, who are studying allowing executions with nitrogen gas, referenced research concluding that without oxygen, inhaling only 12 breaths of pure nitrogen will cause sudden loss of consciousness, according to the "BBC" network.

Doctors and activists warned of the catastrophic risks of nitrogen gas, which may not fulfill its function in killing, but rather leave symptoms ranging from violent convulsions to living in a conscious state without any physical response, and perhaps leakage of gas from the mask causing others in the room to be killed.

United Nations experts criticized the execution of prisoner Smith, affirming that it may constitute torture, as they said in a press statement: "We are concerned that oxygen deprivation by nitrogen may lead to a painful and humiliating death."