Inert gas asphyxiation

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 but euphoria—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.

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.

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. Crude simulation of oxygen transport through the lungs and bloodstream suggests that the partial pressure of oxygen in arterial blood would be about 50% of saturation 1 minute after switching gases and would reach zero within 3 minutes.

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, 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.

Relation to controlled atmosphere killing

Controlled atmosphere killing (CAK) or controlled atmosphere stunning (CAS) is a method for slaughtering animals such as chickens 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. 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.

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.

Accidental deaths and injury

Accidental nitrogen asphyxiation is a possible hazard where large quantities of nitrogen are used.

Accidental nitrogen asphyxiation causes about eight deaths per year in the United States, which is asserted to be more than from any other industrial gas. This figure comes from the fact that some 80 people were recorded killed in such accidents from 1992 to 2002.

In a notable accident in 1981, shortly before the launch of the first Space Shuttle mission, two technicians lost consciousness and one of them died after they entered the Orbiter aft compartment which was pressurized with pure nitrogen as a precaution against fire.

A laboratory assistant died in Scotland in 1999, apparently from asphyxiation, after liquid nitrogen spilled in a basement storage room.

During a pool party in Mexico in 2013, 8 party-goers were rendered unconscious and one 21-year-old male was put into a coma after liquid nitrogen was poured into the pool.

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 accidental fall from a tree occurred after the inhalation of helium from a toy balloon, causing the person to become either unconscious or light headed.

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

Suicide

Use of inert gas for suicide was first proposed by a Canadian, Dr Bruce Dunn. Dunn commented that, "...the acquisition of a compressed gas cylinder, an appropriate pressure reducing regulator, and suitable administration equipment... [was] "not inaccessible to a determined individual, but relatively difficult for a member of the public to acquire casually or quickly." Dunn collaborated with other researchers, notably the Canadian campaigner, John Hofsess, who in 1997 formed the group "NuTech" with Derek Humphry and Philip Nitschke. Two years later, NuTech had streamlined Dunn's work by using readily-available party balloon cylinders of helium.

The method of suicide based on self-administration of helium in a bag, a colloquial name being the "exit bag" or suicide bag, has been referenced by some medical euthanasia advocacy groups. Originally such bags were used with helium, and 30 deaths were reported with use of them from 2001 to 2005, and another 79 from 2005 to 2009. This suggested to one set of reviewers that the popularity of the technique was increasing, as also did the increase in helium suicides in Sweden during the latter half of the same decade.

After attempts were made by authorities to control helium sales in Australia, a new method was introduced that instead uses nitrogen. Nitrogen became the main gas promoted by euthanasia advocates, such as Philip Nitschke, who founded a company called Max Dog Brewing in order to import canisters of nitrogen into Australia. Nitschke stated that the gas cylinders can be used for both brewing and, if required, to end life at a later stage in a "peaceful, reliable [and] totally legal" manner. Nitschke said, "[nitrogen] was undetectable even by autopsy, which was important to some people".

Capital punishment

After a number of accidents in which humans suffocated in nitrogen without any warning, the suggestion was made in 1995 that hypoxic atmospheres might be used for a more humane form of execution.

Execution by nitrogen asphyxiation was discussed briefly in print as a theoretical method of capital punishment in a National Review article, "Killing with kindness – capital punishment by nitrogen asphyxiation". The idea was then proposed by Lawrence J. Gist II, an attorney at law, under the title, International Humanitarian Hypoxia Project.

In a televised documentary in 2007, the British political commentator (and former Member of Parliament) Michael Portillo examined execution techniques in use around the world and found them unsatisfactory; his conclusion was that nitrogen asphyxiation would be the best method.

In April 2015, Governor Mary Fallin of Oklahoma signed a bill allowing nitrogen asphyxiation as an alternative execution method.