Kenneth Smith, 58, was convicted for his role in a murder in 1988 and sentenced to death.

In 2022, Smith survived an attempted execution by lethal injection when his executioners repeatedly failed to insert an intravenous line for the drugs used to flow into his body.

They had tried veins in his hands and arms, and finally, as has been reported by The New York Times, one near his heart.

Lethal injections were introduced in 1982 in the US, where they are the most common method of execution. Lethal injections contain one or more drugs, including anesthetics, barbiturates and paralytic agents (muscles relaxants).

Some states use fentanyl, a pain killer that is about 50 times stronger than heroin and increasingly considered to be responsible for many accidental drug overdoses.  

But now, the US State of Alabama, where Smith has been held on death row for more than two decades, successfully attempted his execution a second time using a new method known as nitrogen hypoxia.

What is nitrogen hypoxia?

Hypoxia is a form of asphyxiation. The US National Institutes of Health describes it as "a state in which oxygen is not available in sufficient amounts." And as we all know, we need oxygen to breathe and live.

Smith will be forced to inhale pure nitrogen via a respirator mask, which his executioners will have strapped to his face. That will be connected to a canister of pressurized nitrogen.

Lots of nitrogen is in the air around us, and it's safe when it's correctly balanced with the oxygen in the atmosphere. But when nitrogen concentrations get too high and oxygen levels too low, it is potentially lethal.

As Smith inhales the gas, it will increase the concentration of nitrogen in his body to lethal levels. The nitrogen will essentially squeeze any oxygen out of his respiratory system, depriving tissues and organs of oxygen, causing low blood flow and, ultimately, death.

Nitrogen in the air around us

We breathe nitrogen every day — the air around us, the atmosphere, is made up of about 78% nitrogen and only 21% oxygen. The rest consists of water vapor, argon, neon, helium, hydrogen and xenon. Those are known as "permanent gases".

There are also a range of "variable gases" in the atmosphere. They include methane, ozone and carbon dioxide — with concentrations that can vary from day to day and region to region.

At a concentration of 78% in the atmosphere, nitrogen is safe to breathe. But it grows dangerous and potentially fatal once levels of nitrogen reach 80% or more.

Some health and safety guidelines, including from the US Occupational Safety and Health Administration, advise people working in closed environments to ensure that oxygen levels are at an absolute minimum of 19.5%. But even at that level, nitrogen in the atmosphere can cause "unnoticeable physiological effects" — that's according to a report by the US Chemical Safety and Hazard Investigation Board. 

The problem is that you cannot detect dangerous levels of nitrogen with your sense of smell. Nitrogen gas is odorless, colorless, tasteless, and inert. It does not react with other chemical substances — and is nonflammable and nonexplosive, too. So, you need special gas detectors to notice it.

But you can look out for common symptoms of nitrogen poisoning or oxygen deficiency, especially if you work in an industry that uses nitrogen — which many do, for instance, to protect against corrosive oxidation.

What happens if you inhale high levels of nitrogen, and not enough oxygen?

At 16% oxygen, a person may experience an increased pulse and breathing rate and find it difficult to think and concentrate or coordinate their movements.

At 14%, they may experience an abnormal feeling of fatigue or effects on their emotional state and judgment.

At 12.5%, a person will experience a severe inability to breathe, causing permanent heart damage, nausea and vomiting.

And at below 10%, they will be unable to move or experience convulsions, a loss of consciousness, and death.

Nitrogen use in animal euthanasia and human assisted suicide

In 2020, the American Veterinary Medical Association reported on the use of nitrogen, among other gases, in the euthanasia of farm animals. It said that nitrogen hypoxia can be distressing for animals but suggested it could be used "after [the animals] are rendered unconscious."

There is also a movement for the use of nitrogen as a form of assisted suicide. whereby a person inhales the gas in a closed, nitrogen-filled chamber or pod.

Edited by: Derrick Williams