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,[1] rather than atmospheric air (which is largely composed of nitrogen and oxygen). Examples of physiologically inert gases, which have caused accidental or deliberate death by this mechanism, are argon, helium, nitrogen and methane. 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.[2]

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.[3] Hazards with inert gases and the risks of asphyxiation are well established.[4]

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.[5][6] 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.[7]


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.[8] 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.[3]

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; however, the experience is still less aversive than CO2 exposure.[9]


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's 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%.[10] "At oxygen concentrations [in air] of 4 to 6%, there is loss of consciousness in 40 seconds and death within a few minutes".[11] At an altitude over 43,000 ft (13,000 m), where the ambient oxygen concentration is equivalent to a concentration of 3.6% at sea level, an average individual can perform flying duties efficiently for only 9 to 12 seconds without oxygen supplementation.[10] 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.[10]

Loss of consciousness may be accompanied by convulsions[10] and is followed by cyanosis and cardiac arrest. About 7 minutes of oxygen deprivation causes death of the brainstem.

The RAF Institute of Aviation Medicine did a study[12] 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

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.[13][14] However, "stunning" is often done using carbon dioxide.[15] 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.[16] 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.[17][18][19]

Euthanasia of animals

Diving animals such as rats, minks and burrowing animals 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.[9][20]

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,[21] 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.[22]

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.[23][24]

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.[25] 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.[26]

In 2015, a technician at a health spa was asphyxiated while conducting unsupervised cryotherapy using nitrogen.[27][28]


Use of inert gas for suicide was first proposed by a Canadian, Dr Bruce Dunn.[29] 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."[30] Dunn collaborated with other researchers, notably the Canadian campaigner, John Hofsess, who in 1997 formed the group "NuTech" with Derek Humphry and Philip Nitschke.[31] Two years later, NuTech had streamlined Dunn's work by using readily-available party balloon cylinders of helium.[32]

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.[33] 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.[34]

After attempts were made by authorities to control helium sales in Australia, a new method was introduced that instead uses nitrogen.[35] 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.[36] 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.[37] Nitschke said, "[nitrogen] was undetectable even by autopsy, which was important to some people".[38]

Capital punishment

Execution by nitrogen asphyxiation was discussed briefly in print as a theoretical method of capital punishment in a 1995 National Review article, "Killing with kindness – capital punishment by nitrogen asphyxiation".[39] The idea was then proposed by Lawrence J. Gist II, an attorney at law, under the title, International Humanitarian Hypoxia Project.[40]

In a televised documentary in 2007, the British political commentator and former MP 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.[41]

In April 2015, Governor Mary Fallin of Oklahoma signed a bill allowing nitrogen asphyxiation as an alternative execution method.[5] Three years later, in March 2018, Oklahoma announced that, due to the difficulty in procuring lethal injection drugs, nitrogen gas would be its primary method of carrying out executions.[42] In March 2018, Alabama became the third state (along with Oklahoma and Mississippi), to authorize the use of nitrogen asphyxiation as a method of execution.[43]

The case Bucklew v. Precythe, decided 1 April 2019, ruled that a Missouri death row inmate could not avoid death by lethal injection and choose inert gas asphyxiation using nitrogen, since that was not prescribed in the state and had yet to be used as an execution method.

Oklahoma Attorney General Mike Hunter expects to submit a nitrogen gas execution protocol to court for review before 2020, setting the stage for executions to resume in 2020.[44]

See also


  1. European Industrial Gases Association (2009), Hazards of Inert Gases and Oxygen Depletion, IGC Doc 44/09/E
  2. “Argon, hydrogen, helium, and nitrogen are inert gases that cause asphyxiation if present in high enough concentration to dilute O2 in the inspired air to dangerous levels. [...] Asphyxia, which is synonymous with respiratory failure, can be defined as insufficient oxygen at the cellular level. [...] Simple asphyxiants are gases that are physiologically inert. They do not suppress cardiac output or alter the function of the hemoglobin. Rather, they cause asphyxiation only when present in high enough concentration to lower the concentration of O2 in the inspired air to levels at which the SaO2 and PaO2 fall, resulting in inadequate O2 delivery to tissues.” Quoted from Simple Asphyxiants, Mark Wilkenfield, M.D. Chapter 34, pp. 556–7. in: Environmental and Occupational Medicine Editors William N. Rom, Steven B. Markawitz. 4th Edition, Publisher: Lippincott Williams & Wilkins, 2007. ISBN 0781762995, 9780781762991.
  3. Chemical Safety Board Bulletin This is a government summary of accidental 80 nitrogen inhalation deaths, mostly in occupational circumstances. For a presentation form with additional material, see
  4. "HAZARDS OF INERT GASES AND OXYGEN DEPLETION". Singapore: Asia Industrial Gases Association.
  5. "Oklahoma Gov. Mary Fallin signs bill allowing nitrogen asphyxiation as alternative execution method".
  6. Associated Press (18 April 2015). "Oklahoma governor signs 'foolproof' nitrogen gas execution method". The Guardian.
  7. Mark Berman (March 18, 2018). "Oklahoma says it will begin using nitrogen for all executions in an unprecedented move". Washington Post.
  8. Rebreather physiology review.
  10. Paul W. Fisher. "2 - High Altitude Respiratory Physiology". USAF Flight Surgeon’s Guide. USAF School of Aerospace Medicine. Archived from the original on 16 March 2007. Retrieved 5 March 2015. With [prolonged or acute hypoxia] there may be convulsions and eventual failure of the respiratory center.CS1 maint: BOT: original-url status unknown (link)
  11. "Forensic Pathology 2e, Chapter 8: Asphyxia".
  13. PETA's Animal Times, UK, Autumn 2005
  14. The Animal Welfare Foundation of Canada article The Disposal of Spent Laying Hens by Jacqueline Wepruk Archived 2012-12-04 at the Wayback Machine
  15. Andy Coghlan (Feb 3, 2018). "A more humane way of slaughtering chickens might get EU approval". New Scientist.
  16. Archie Campbell (Mar 10, 2018). "Humane execution and the fear of the tumbril". New Scientist.
  17. "When is carbon dioxide stunning used in abattoirs?". RSPCA. Archived from the original on April 9, 2014. Retrieved June 14, 2013.
  18. Jong E.C.; Barnett J.L.; 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. doi:10.1016/S0168-1591(99)00103-3.
  19. Raj, A.B.M. and Gregory, N.G., (1995).
  20. "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.
  21. "Hazards of Nitrogen Asphyxiation" (PDF). U.S. Chemical Safety and Hazard Investigation Board. 2003-06-11. Archived from the original (PDF) on 2007-02-03. Retrieved 2007-02-15.
  22. "Shuttle Tragedy". Time. 30 March 1981.
  23. "Liquid nitrogen at Jägermeister Mexican pool party sickens guests: one man in a coma, eight others ill". NY Daily Times. Retrieved 2014-11-16.
  24. Bracher, Paul (18 June 2013). "Awful Idea: Liquid Nitrogen at a Pool Party".
  25. "2 College Students Found Dead Inside Helium Balloon in Florida". Fox News. 2006-06-04.
  26. Lung ruptures and fatal gas embolisms have occurred from inhalation from a pressure tank and although this was reported as a helium inhalation death, it differs greatly from the process of inert gas asphyxiation. See gas embolism helium death
  27. Saeidi, Mahsa (2015-11-11). "Coroner rules cryochamber death due to lack of oxygen". KTNV. Archived from the original on 2016-10-18. Retrieved 2016-10-17.
  28. Mcgee, Kimberley; Turkewitz, Julie (2015-10-26). "Death of Woman in Tank at a Nevada Cryotherapy Center Raises Questions About Safety". The New York Times. ISSN 0362-4331. Retrieved 2016-01-26.
  29. Dunn B, Nitrogen and other Inert Gases. In: Smith K, Docker C, Hofsess J, Dunn B, "Beyond Final Exit." Published by the Right to Die Society of Canada 1995, pp.65-71. The book was distributed internationally by Canadian, American, and British organizations. See also Exit (Right-to-Die Organisation)
  30. "Dunn B, Beyond Final Exit, p.70. Dunn's work and other chapters from Beyond Final Exit was also publicised on the Canadian website (now offline) called DeathNET. Côte R, In Search of Gentle Death, Corinthian Books 2012, page 314.
  31. See: Euthanasia device
  32. Humphry, Derek (2014). "Obtaining drugs via the internet and using helium hood kits".
  33. Grassberger M, Krauskopf A (2007). "Suicidal asphyxiation with helium: report of three cases". Wien. Klin. Wochenschr. 119: 323–5. doi:10.1007/s00508-007-0785-4. PMID 17571238.
  34. Austin A, Winskog C, van den Heuvel C, Byard RW (May 2011). "Recent trends in suicides utilizing helium". J. Forensic Sci. 56: 649–51. doi:10.1111/j.1556-4029.2011.01723.x. PMID 21361949.
  35. Harding BE, Wolf BC (Sep 2008). "Case report of suicide by inhalation of nitrogen gas". Am J Forensic Med Pathol. 29: 235–7. doi:10.1097/PAF.0b013e318183240c. PMID 18725778.
  36. "Death in a Can: Australia's Euthanasia Loophole". VICE Australia. VICE Media LLC. 2014-07-04. Retrieved 2015-04-11.
  37. Sexton, Mike (18 December 2012). "Euthanasia campaigner under scrutiny". ABC. Retrieved 6 May 2013.
  38. Orr, Aleisha (3 May 2013). "Euthanasia group to show West Aussies how to die 'well'". WAToday. Retrieved 8 May 2013.
  39. Creque, S.A. "Killing with kindness – capital punishment by nitrogen asphyxiation" National Review. 1995-9-11.
  40. International Humanitarian Hypoxia Project.
  41. "How to Kill a Human Being – in search of a painless death". VideoSift.
  42. "Oklahoma Turns to Nitrogen Gas for Executions".
  43. "Alabama 3rd state to allow execution by nitrogen gas".
  44. Oklahoma poised to build own execution device.
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