Inert gas asphyxiation

**Inert gas asphyxiation** is a form of asphyxiation which results from breathing a physiologically inert gas in the absence of oxygen, or a low amount of oxygen, rather than atmospheric air (which is largely composed of nitrogen and oxygen).

According to the U.S. Chemical Safety and Hazard Investigation Board, in humans, "breathing an oxygen deficient atmosphere can have serious and immediate effects, including unconsciousness after only one or two breaths. The exposed person has no warning and cannot sense that the oxygen level is too low." In the US, at least 80 people died due to accidental nitrogen asphyxiation between 1992 and 2002. Hazards with inert gases and the risks of asphyxiation are well established.

The term "physiologically inert" is used to indicate a gas which has no toxic or anesthetic properties and does not act upon the heart or hemoglobin. Instead, the gas acts as a simple diluent to reduce oxygen concentration in inspired gas and blood to dangerously low levels, thereby eventually depriving all cells in the body of oxygen.

* Examples of physiologically inert gases, which have caused accidental or deliberate death by this mechanism, are argon, helium, nitrogen and methane.

An occasional cause of accidental death in humans, inert gas asphyxia with gases including helium, nitrogen, methane and argon has been used as a suicide method. Inert gas asphyxia has been advocated by proponents of euthanasia, using a gas-retaining plastic hood device colloquially referred to as a suicide bag.

Nitrogen asphyxiation has been suggested by a number of lawmakers and other advocates as a more humane way to carry out capital punishment. In April 2015, the Oklahoma Governor Mary Fallin signed a bill authorizing nitrogen asphyxiation as an alternative execution method in cases where the state's preferred method of lethal injection was not available as an option. In March 2018, Oklahoma Attorney General Mike Hunter and Corrections Director Joe M. Allbaugh announced a switch to nitrogen gas as the primary method of execution.

## Process

When humans breathe in an asphyxiant gas, such as pure nitrogen, helium, neon, argon, sulfur hexafluoride, methane, or any other physiologically inert gas(es), they exhale carbon dioxide without re-supplying oxygen. Physiologically inert gases (those that have no toxic effect, but merely dilute oxygen) are generally free of odor and taste. As such, the human subject detects little abnormal sensation as the oxygen level falls. This leads to asphyxiation (death from lack of oxygen) without the painful and traumatic feeling of suffocation (the hypercapnic alarm response, which in humans arises mostly from carbon dioxide levels rising), or the side effects of poisoning. In scuba diving rebreather accidents, there is often little sensation, however, a slow decrease in oxygen breathing gas content has effects which are quite variable. By contrast, suddenly breathing pure inert gas causes oxygen levels in the blood to fall precipitously, and may lead to unconsciousness in only a few breaths, with no symptoms at all.

## Physiology

A typical human breathes between 12 and 20 times per minute at a rate primarily influenced by carbon dioxide concentration, and thus pH, in the blood. With each breath, a volume of about 0.6 litres is exchanged from an active lung volume (tidal volume + functional residual capacity) of about 3 litres. Normal Earth atmosphere is about 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, and other gases. After just two or three breaths of nitrogen, the oxygen concentration in the lungs would be low enough for some oxygen already in the bloodstream to exchange back to the lungs and be eliminated by exhalation.

Unconsciousness in cases of accidental asphyxia can occur within 1 minute. Loss of consciousness results from critical hypoxia, when arterial oxygen saturation is less than 60%. "At oxygen concentrations [in air] of 4 to 6%, there is loss of consciousness in 40 seconds and death within a few minutes". At an altitude over 43,000 ft (13,000 m), where the ambient oxygen concentration is equivalent to 3.6% at sea level, an average individual can perform flying duties efficiently for only 9 to 12 seconds without oxygen supplementation. The US Air Force trains air crews to recognize their individual subjective signs of approaching hypoxia. Some individuals experience headache, dizziness, fatigue, nausea and euphoria, and some become unconscious without warning.

Loss of consciousness may be accompanied by convulsions and is followed by cyanosis and cardiac arrest. About 7 minutes of oxygen deprivation causes death of the brainstem.

The Royal Air Force Institute of Aviation Medicine did a study in which subjects were asked to hyperventilate in a nitrogen atmosphere. Among the results: "When the duration of over-ventilation with nitrogen was greater than 8-10 sec the subject reported a transient dimming of vision. In the experiments in which nitrogen breathing was carried out for 15-16 sec the subject experienced some general clouding of consciousness and impairment of vision. Vision was frequently lost in these experiments for a short period. In the few experiments in which nitrogen was breathed for 17-20 sec unconsciousness supervened and was accompanied on most occasions by a generalized convulsion. The duration of the interval between the start of over-ventilation with nitrogen and the onset of symptoms was 12-14 sec." Unfortunately they did not report how much discomfort the subjects felt.

## Animal slaughter

Some animal species are better equipped than humans to detect hypoxia, and these species are more uncomfortable in low-oxygen environments that result from inert gas exposure.

## Euthanasia of animals

Diving animals such as mink and burrowing animals, such as rodents and rats, are sensitive to low-oxygen atmospheres and (unlike humans) will avoid them, making purely hypoxic techniques possibly inhumane for them. For this reason, the use of inert gas (hypoxic) atmospheres (without CO2) for euthanasia, is also species-specific.

### Relation to controlled atmosphere killing

*Controlled atmosphere killing* (*CAK*) or *controlled atmosphere stunning* (*CAS*) is a method for slaughtering animals such as chickens or cane toads by placing the animals in a container in which the atmosphere lacks oxygen and consists of an asphyxiant gas (one or more of argon, nitrogen or carbon dioxide), causing the animals to lose consciousness. Argon and nitrogen are important components of a gassing process which seem to cause no pain, and for this reason many consider some types of controlled atmosphere killing more humane than other methods of killing.

However, "stunning" is often done using carbon dioxide. If carbon dioxide is used, controlled atmosphere killing is not the same as inert gas asphyxia, because carbon dioxide at high concentrations (above 5%) is not biologically inert, but rather is toxic and also produces initial distress in a number of animal species. The addition of toxic carbon dioxide to hypoxic atmospheres used in slaughter without animal distress is a complex and highly species-specific matter, which also depends on concentration of carbon dioxide.

## Accidental deaths and injury

Accidental nitrogen asphyxiation is a possible hazard where large quantities of nitrogen are used. It causes several deaths per year in the United States, which is asserted to be more than from any other industrial gas.

* In one accident in 1981, shortly before the launch of the first Space Shuttle mission, five technicians lost consciousness and two of them died after they entered the Orbiter aft compartment. Nitrogen had been used to flush oxygen from the compartment as a precaution against fire. They were not wearing air packs because of a last-minute change in safety procedures.

* During a pool party in Mexico in 2013, eight party-goers were rendered unconscious and one 21-year-old male was put into a coma after liquid nitrogen was poured into the pool.

* In 2015, a technician at a health spa was asphyxiated while conducting unsupervised cryotherapy using nitrogen.

* Occasional deaths are reported from recreational inhalation of helium, but these are very rare from direct inhalation from small balloons. The inhalation from larger helium balloons has been reportedly fatal. A fatal fall from a tree occurred after the inhalation of helium from a toy balloon, which caused the person to become either unconscious or light headed.