Inert gas asphyxiation
Inert gas asphyxiation is a form of asphyxiation which results from respiration of inert gas in the absence of oxygen rather than atmospheric air (a mixture of oxygen and the inert nitrogen). The painful experience of suffocation is not caused by lack of oxygen, but because carbon dioxide builds up in the bloodstream, instead of being exhaled as under normal circumstances. With inert gas asphyxiation, carbon dioxide is exhaled normally, and no such pain experience occurs.
An occasional cause of accidental death in humans, inert gas asphyxiation has been used as a suicide method, and has been advocated by proponents of euthanasia (using helium or nitrogen in a device called a suicide bag). Nitrogen asphyxiation has been suggested as a more humane way to carry out capital punishment, but so far this use of inert gas has not been attempted by any country, state or territory.
When humans breathe in pure nitrogen, helium, argon, sulfur hexafluoride, methane, or any other chemically inert gas(es), they exhale carbon dioxide without resupplying oxygen. Inert gases are generally free of odour and taste. As such, the human subject detects no abnormal sensation. This leads to asphyxiation (death from lack of oxygen, not from poison) without the painful and traumatic feeling of suffocation, or from side effects of poisonous interactions.
Some species of animals are equipped to detect hypoxia better than humans are, and these species are more uncomfortable in low-oxygen environments that result from inert gas exposure.
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". As this procedure provides an atmosphere completely devoid of oxygen, the sequence of effects should be expected to occur even more quickly. 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 recognise their individual subjective signs of approaching hypoxia. Some individuals experience headache, dizziness, fatigue, nausea, or euphoria, but some become unconscious without warning. Equivalent training is unlikely for a condemned individual, making unconsciousness without warning probable, although as much as a 30-second warning is possible.
As with many gas killing systems, CO2 partly acts by displacing oxygen so the brain cannot function and brain death ensues. However, CO2 also has a direct anaesthetic effect which results in loss of consciousness more quickly than with some other low oxygen gas mixtures such as argon and nitrogen mixtures. At high concentrations CO2 gas is a narcotic/anesthetic and a poison, therefore it is not a physiologically inert gas.
In many countries, CO2 stunning is mainly used on pigs. A number of pigs enter a chamber which is then sealed and filled with 80% to 90% CO2 in air. It has been shown that pigs lose consciousness within 13 to 30 seconds. Following this, immediate slaughter is essential so the animal does not start to recover. Systems have been developed which stun groups of pigs (this improves welfare and can improve meat quality); groups of pigs are moved from a small pen into an elevator (gondola) which lowers them into a pit containing the CO2.
There have been animal welfare concerns about CO2 stunning because it irritates the respiratory tract. However, research has produced conflicting results with some showing pigs tolerate CO2 stunning and others showing they do not.
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 it more humane than other methods of killing.
Controlled atmosphere stunning has been used both within processing plants and on-farm for euthanizing poultry. Anoxia in broiler chickens generated through argon or a mixture of argon and carbon dioxide or hypercapnic hypoxia seems can be used under commercial conditions. Meat tenderness and drip losses improve. The blood spots, especially those on the thighs and breasts caused by electrical stunning and hanging, disappear altogether.
Portable units are available for the on-farm slaughter of livestock. One portable unit type that costs US$2,500 can be pushed down the aisles of a barn, with 200–250 chickens per load being placed inside a CO2 enriched chamber. Chickens are unconscious within 30 seconds of being in the chamber.
CO2 induction is a common way to euthanize laboratory rodents, such as mice, rats or guinea pigs. In 2013, the American Veterinary Medical Association issued new guidelines for carbon dioxide induction, stating that a flow rate of 10% to 30% volume/min is optimal for the humane euthanization of laboratory mice.
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. For this reason, the use of inert gas hypoxic atmospheres for euthanasia can be species-specific.
Hypoxic atmospheres on land
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. For example 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.
Occasional deaths are reported from recreational inhalation of helium, but these are very rare from direct inhalation from small balloons. Entering large helium balloons has been fatal. Accidental falls have been fatal after a person inhaled from a balloon, and lost consciousness. Lung rupture and fatal gas embolism has occurred from inhaling from a pressure tank, and although this is reported as helium inhalation death, it is a different process from inert gas asphyxiation.
In rebreather diving users employ a system in which carbon dioxide is removed by a scrubber, and oxygen is added by a separate addition system. If the oxygen addition system fails for any reason, a diver may black out without prior symptoms, and drown. 
A method of suicide based on self-administration of helium in a bag called an "exit bag" or suicide bag, has been described by some medical euthanasia advocacy groups. Originally such bags were used with helium, but after attempts were made by authorities to control helium sales in Australia, a method was advocated that used nitrogen.
After a number of accidents in which humans suffocated in nitrogen without any warning, the suggestion was made in 1995 that hypoxic atmospheres be used for the humane killing of humans.
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".
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.
Nitrogen asphyxiation is not currently used by any government as an execution method.
- Gas chamber
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- Capital punishment debate
- Positional asphyxia
- Suicide bag
- Charcoal-burning suicide
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