Inert gas asphyxiation

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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). In humans the experience is painless and involves little distress, as it is caused merely by lack of oxygen, and not due to carbon dioxide buildup in the bloodstream, which is the cause of distress in human suffocation. In other mammals, the sensations due to low-oxygen atmospheres are variable. Hazards with inert gases and the risks of asphyxiation are well-established.[1]

Hypoxic atmospheres have been used as a method of animal slaughter in animals such as chickens, where it is known as controlled atmosphere killing.

An occasional cause of accidental death in humans, inert gas asphyxiation has also 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 odor and taste. As such, the human subject detects no abnormal sensation. This leads to asphyxiation (death from lack of oxygen) without the painful and traumatic feeling of suffocation, or the side effects of poisoning.

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%.[2] "At oxygen concentrations [in air] of 4 to 6%, there is loss of consciousness in 40 seconds and death within a few minutes".[3] 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.[2] 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.[2]

Loss of consciousness may be accompanied by convulsions[2] and is followed by cyanosis and cardiac arrest. About 7 minutes of oxygen deprivation causes death of the brainstem.

Animal slaughter[edit]

CO2 stunning[edit]

It has been suggested that inert gas asphyxiation could be a more humane method of animal stunning than CO2.

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.[4][5][6]

Domestic turkeys are averse to high concentrations of CO2 (72% CO2 in air) but not low concentrations (a mixture of 30% CO2 and 60% argon in air with 3% residual oxygen).[7]

Controlled atmosphere killing[edit]

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.[8][9]

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 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.[10]

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.[11] For this reason, the use of inert gas hypoxic atmospheres for euthanasia can be species-specific.

Accidental deaths and injury[edit]

Hypoxic atmospheres on land[edit]

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,[12] 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.[13]

A laboratory assistant died in Scotland in 1999, apparently from asphyxiation, after liquid nitrogen spilled in a basement storage room.[14]

During a pool party in Mexico in 2013, 8 partygoers were rendered unconscious and one 21 year old male was put into a coma after liquid nitrogen was poured into the pool. .[15]

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.[16] 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.[17]


Main article: Suicide bag

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.[18] 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.

Capital punishment[edit]

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".[19] The idea was then proposed by Lawrence J. Gist II, an Attorney at Law, under the title, International Humanitarian Hypoxia Project.[20]

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.[21]

Nitrogen asphyxiation is not currently used by any government as an execution method.[citation needed]

See also[edit]


  1. ^
  2. ^ a b c d Fisher PW. "High altitude respiratory physiology, chapter 2 in USAF Flight Surgeon's Guide", n.d. Retrieved on 2007-02-07.
  3. ^ DiMaio V & DiMaio D. "Forensic Pathology, Second Edition. Chapter 8, Asphyxia", ISBN 0-8493-0072-X, 2001. Retrieved on 2007-01-27.
  4. ^ "When is carbon dioxide stunning used in abattoirs?". RSPCA. Retrieved June 14, 2013. 
  5. ^ Jong, E.C., Barnett, J.L. and Hemsworth, P.H., (2000). The aversiveness of carbon dioxide stunning in pigs and a comparison of the CO2 stunner crate vs. the V-restrainer. Applied Animal Behaviour Science, 67: 67-76
  6. ^ Raj, A.B.M. and Gregory, N.G., (1995). Welfare implications of the gas stunning of pigs 1. Determination of aversion to the initial inhalation of carbon dioxide or argon. Animal Welfare, 4: 273-280
  7. ^ Raj, M., (1999). Aversive reactions of turkeys to argon, carbon dioxide and a mixture of carbon dioxide and argon. Veterinary Record, 138:592-593 DOI:10.1136/vr.138.24.592
  8. ^ PETA's Animal Times, UK, Autumn 2005
  9. ^ The Animal Welfare Foundation of Canada article The Disposal of Spent Laying Hens by Jacqueline Wepruk
  10. ^ Hoen, T. and Lankhaar, J., (1999). Controlled atmosphere stunning of poultry. Poultry Science, 78(2): 287-9
  12. ^ "Hazards of Nitrogen Asphyxiation". U.S. Chemical Safety and Hazard Investigation Board. 2003-06-11. Archived from the original on 2007-02-03. Retrieved 2007-02-15. 
  13. ^ "Shuttle Tragedy". Time. 1981-03-31. Retrieved 2007-01-27. 
  14. ^ "UK: Scotland Inquiry after man dies in chemical leak". BBC. 1999-10-25. Retrieved 2007-01-27. 
  15. ^ "Liquid nitrogen at Jägermeister Mexican pool party sickens guests: one man in a coma, eight others ill". NY Daily Times. Retrieved 2014-11-16. 
  16. ^ "2 College Students Found Dead Inside Helium Balloon in Florida". Fox News. 2006-06-04. 
  17. ^ gas embolism helium death
  18. ^ Helium suicide cases
  19. ^ Creque, S.A. "Killing with kindness – capital punishment by nitrogen asphyxiation" National Review. 1995-9-11.
  20. ^
  21. ^ How to Kill a Human Being - in search of a painless death,

External links[edit]