Inert gas asphyxiation is a form of

asphyxiation Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can ...

which results from breathing a physiologically

inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

in the absence of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

, or a low amount of oxygen, rather than

atmospheric air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

(which is composed largely of

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

and oxygen). Examples of physiologically inert gases, which have caused accidental or deliberate death by this mechanism, are

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

and

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of 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. According to the

U.S. Chemical Safety and Hazard Investigation Board The U.S. Chemical Safety and Hazard Investigation Board (USCSB), generally referred to as the Chemical Safety Board or CSB, is an independent U.S. federal agency charged with investigating industrial chemical accidents. Headquartered in Washing ...

, 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 from accidental nitrogen asphyxiation between 1992 and 2002. Hazards with inert gases and the risks of asphyxiation are well established. 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 Euthanasia (from el, εὐθανασία 'good death': εὖ, ''eu'' 'well, good' + θάνατος, ''thanatos'' 'death') is the practice of intentionally ending life to eliminate pain and suffering. Different countries have different eut ...

, using a gas-retaining plastic hood device colloquially referred to as a

suicide bag A suicide bag, also known as an exit bag or hood, is part of a euthanasia device consisting of a large plastic bag with a drawcord used to commit suicide through inert gas asphyxiation. It is usually used in conjunction with a flow of an inert ...

. Nitrogen asphyxiation has been approved in some places as a method of

capital punishment Capital punishment, also known as the death penalty, is the state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to conclude that ...

, but has not yet been used for this purpose.

Process

When humans breathe in an asphyxiant gas, such as pure

neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypt ...

, 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. Accordingly, the human subject detects little abnormal sensation as the oxygen level falls. This leads to

asphyxia Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can ...

tion (death from lack of oxygen) without the painful and traumatic feeling of suffocation (the

hypercapnic alarm response Hypercapnia (from the Greek ''hyper'' = "above" or "too much" and ''kapnos'' = "smoke"), also known as hypercarbia and CO2 retention, is a condition of abnormally elevated carbon dioxide (CO2) levels in the blood. Carbon dioxide is a gaseous pr ...

, which in humans arises mostly from carbon dioxide levels rising), or the side effects of poisoning. In

scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for " Self-Contained Underwater Breathing Apparatus", was coined by Chr ...

rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen i ...

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. 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.

Physiology

A typical human

breath Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen. All aerobic creatures need oxygen for cellu ...

es between 12 and 20 times per minute at a rate influenced primarily by

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

concentration, and thus pH, in the

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...

. With each breath, a volume of about 0.6 litres is exchanged from an active lung volume (tidal volume + functional residual capacity) of about three litres. The normal composition of the

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

is about 78%

, 21%

, and 1%

, 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 Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an awareness of self and environment or to respond to any human or environmental stimulus. Unconsciousness may occur as the r ...

in cases of accidental asphyxia can occur within one minute. Loss of consciousness results from critical hypoxia, when arterial oxygen saturation is less than 60%. "At oxygen concentrations

n air N, or n, is the fourteenth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''en'' (pronounced ), plural ''ens''. History ...

of 4 to 6%, there is loss of consciousness in 40 seconds and death within a few minutes". At an altitude over , 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. 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 Cyanosis is the change of body tissue color to a bluish-purple hue as a result of having decreased amounts of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Body tissues that show cyanosis are usually in locations ...

and

cardiac arrest Cardiac arrest is when the heart suddenly and unexpectedly stops beating. It is a medical emergency that, without immediate medical intervention, will result in sudden cardiac death within minutes. Cardiopulmonary resuscitation (CPR) and possi ...

. About seven minutes of oxygen deprivation causes death of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

. In a 1963 study by RAF Institute of Aviation Medicine, subjects were asked to hyperventilate in a nitrogen atmosphere. Among the results: The study 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

chicken The chicken (''Gallus gallus domesticus'') is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster or cock is a term for an adu ...

s or

cane toads The cane toad (''Rhinella marina''), also known as the giant neotropical toad or marine toad, is a large, terrestrial true toad native to South and mainland Central America, but which has been introduced to various islands throughout Ocean ...

by placing the animals in a container in which the atmosphere lacks oxygen and consists of an asphyxiant gas (one or more of

), causing the animals to lose

consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...

. Argon and nitrogen are important components of a gassing process which seem to cause no

pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...

, 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

. 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 some 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 rats and

mink Mink are dark-colored, semiaquatic, carnivorous mammals of the genera '' Neogale'' and '' Mustela'' and part of the family Mustelidae, which also includes weasels, otters, and ferrets. There are two extant species referred to as "mink": ...

s 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

is also species-specific.

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. 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 lightheaded. In 2015, a technician at a health spa was asphyxiated while conducting unsupervised

cryotherapy Cryotherapy, sometimes known as cold therapy, is the local or general use of low temperatures in medical therapy. Cryotherapy may be used to treat a variety of tissue lesions. The most prominent use of the term refers to the surgical treatment, s ...

using nitrogen. In 2021, six people died of asphyxiation and 11 more were hospitalized following a liquid nitrogen leak at a poultry plant in Gainesville, Georgia.

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... asnot 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

, 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 Philip Haig Nitschke (; born 8 August 1947) is an Australian humanist, author, former physician, and founder and director of the pro-euthanasia group EXIT (Australia), Exit International. He campaigned successfully to have a legal euthanasia la ...

, 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 ndtotally legal" manner. Nitschke said that nitrogen is "undetectable even by autopsy, which was important to some people". Nitschke produced a

3D printed 3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer co ...

pod, " Sarco", that fills with nitrogen at the push of a button, claiming to cause its user to become unconscious within a minute and then die of oxygen deprivation.

Capital punishment

Execution by nitrogen asphyxiation was discussed briefly in print as a theoretical method of

in a 1995 ''

National Review ''National Review'' is an American conservative editorial magazine, focusing on news and commentary pieces on political, social, and cultural affairs. The magazine was founded by the author William F. Buckley Jr. in 1955. Its editor-in-chief ...

'' article. 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 MP

Michael Portillo Michael Denzil Xavier Portillo (; born 26 May 1953) is a British journalist, broadcaster and former politician. His broadcast series include railway documentaries such as '' Great British Railway Journeys'' and '' Great Continental Railway Jour ...

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 Mary Fallin (; née Copeland; born December 9, 1954) is an American politician who served as the 27th governor of Oklahoma from 2011 to 2019. A member of the Republican Party, she was elected in 2010 and reelected in 2014. She was the first and s ...

Oklahoma Oklahoma (; Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a state in the South Central region of the United States, bordered by Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...

signed a bill allowing nitrogen asphyxiation as an alternative execution method. Three years later, in March 2018, Oklahoma announced that, due to the difficulty in procuring lethal injection drugs, nitrogen gas will be used to carry out executions. After making "good progress" in designing a nitrogen execution protocol, but not actually carrying out any executions, Oklahoma announced in February 2020 it had found a new reliable source of lethal injection drugs, but would continue working on nitrogen execution as a contingency method. In March 2018, Alabama became the third state (after Oklahoma and Mississippi), to authorize the use of nitrogen asphyxiation as a method of execution. In the case ''

Bucklew v. Precythe ''Bucklew v. Precythe'', 587 U.S. ___ (2019), was a United States Supreme Court case regarding the standards for challenging methods of capital punishment under the Eighth Amendment to the United States Constitution. In a 5–4 decision, the Cour ...

'', decided 1 April 2019, the U.S. Supreme Court ruled that a

Missouri Missouri is a state in the Midwestern region of the United States. Ranking 21st in land area, it is bordered by eight states (tied for the most with Tennessee): Iowa to the north, Illinois, Kentucky and Tennessee to the east, Arkansas t ...

death row Death row, also known as condemned row, is a place in a prison that houses inmates awaiting execution after being convicted of a capital crime and sentenced to death. The term is also used figuratively to describe the state of awaiting execution ...

inmate could not avoid death by lethal injection and choose inert gas asphyxiation using nitrogen, since it has never been used in any execution in the world.

Notes

{{reflist, refs= {{cite book, author1=Paul W. Fisher , title=USAF Flight Surgeon's Guide , publisher=USAF School of Aerospace Medicine , url=http://wwwsam.brooks.af.mil/af/files/fsguide/HTML/Chapter_02.html , access-date=5 March 2015 , archive-date=16 March 2007 , chapter=2 - High Altitude Respiratory Physiology , quote=With rolonged or acute hypoxiathere may be convulsions and eventual failure of the respiratory center. , url-status=bot: unknown , archive-url=https://web.archive.org/web/20070316011544/http://wwwsam.brooks.af.mil/af/files/fsguide/HTML/Chapter_02.html {{cite web, url=http://www.charlydmiller.com/LIB/forensicpathasphyxia.html, title=Forensic Pathology 2e, Chapter 8: Asphyxia, work=charlydmiller.com PETA's Animal Times, UK, Autumn 2005 {{Cite web , last=Wepruk , first=Jacqueline , title=The Disposal of Spent Laying Hens , url=https://awfc.ca/english/works/pub/disposehens.htm , url-status=dead , archive-url=https://web.archive.org/web/20121204073853/https://awfc.ca/english/works/pub/disposehens.htm , archive-date=2012-12-04 , access-date=2012-12-06 , publisher=The Animal Welfare Foundation of Canada {{Cite web , url=http://www.landfood.ubc.ca/animalwelfare/publications/pdfs/theses/Joanna-thesis.pdf , title=ALTERNATIVES TO CARBON DIOXIDE EUTHANASIA FOR LABORATORY RATS , access-date=2012-02-07 , archive-url=https://web.archive.org/web/20130320052350/http://www.landfood.ubc.ca/animalwelfare/publications/pdfs/theses/Joanna-thesis.pdf , archive-date=2013-03-20 , url-status=dead {{cite news , url=http://www.csb.gov/safety_publications/docs/SB-Nitrogen-6-11-03.pdf , title=Hazards of Nitrogen Asphyxiation , publisher=U.S. Chemical Safety and Hazard Investigation Board , date=2003-06-11 , access-date=2007-02-15 , archive-url=https://web.archive.org/web/20070203150950/http://www.csb.gov/safety_publications/docs/SB-Nitrogen-6-11-03.pdf , archive-date=2007-02-03 , url-status=dead {{cite news , url=http://www.time.com/time/magazine/article/0,9171,922499,00.html , archive-url=https://web.archive.org/web/20080206123546/http://www.time.com/time/magazine/article/0,9171,922499,00.html , url-status=dead , archive-date=February 6, 2008 , title=Shuttle Tragedy , publisher=Time , date=30 March 1981, url-access=subscription Creque, S.A. "Killing with kindness – capital punishment by nitrogen asphyxiation" National Review. 1995-9-11. {{cite web, url=http://www.videosift.com/video/How-to-Kill-a-Human-Being-in-search-of-a-painless-death, title=How to Kill a Human Being – in search of a painless death, work=VideoSift "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 Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can ...

, which is synonymous with

respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gre ...

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. Chemical Safety Board Bulletin
{{Webarchive, url=https://web.archive.org/web/20160304190717/http://www.csb.gov/assets/1/19/SB-Nitrogen-6-11-031.pdf , date=2016-03-04 This is a government summary of accidental 80 nitrogen inhalation deaths, mostly in occupational circumstances. For a presentation form with additional material, se

{{Webarchive, url=https://web.archive.org/web/20160304191159/http://www.csb.gov/assets/1/19/Nitrogen_Asphyxiation_Bulletin_Training_Presentation.pdf , date=2016-03-04 {{cite journal , pmid = 21361949 , doi=10.1111/j.1556-4029.2011.01723.x , volume=56 , title=Recent trends in suicides utilizing helium , date=May 2011 , vauthors=Austin A, Winskog C, van den Heuvel C, Byard RW , journal=J. Forensic Sci. , issue=3 , pages=649–51, s2cid=30521706 {{cite journal, url=https://web.archive.org/web/20141204230210/http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/6041/SPUMS_V27N1_13.pdf?sequence=1 , title=Rebreather physiology review , first=RW (Bill) , last=Hamilton , journal=SPUMS Journal , volume=27 , issue=1 , date=March 1997

External links

NIOSH respirator fact sheet

U.S. Chemical Safety and Hazard Investigation Board

Restraint asphyxia

Humane Society of the United States comparison of controlled-atmosphere killing (CAK) to electrical water-bath stunning
Controlled Atmosphere Killing
People for the Ethical Treatment of Animals

by Michael P. Copeland, J.D., Thom Parr, M.S., and Christine Pappas, J.D., Ph.D.
''Execution by Nitrogen Hypoxia: Search for Scientific Consensus''
by Kevin M. Morrow, J.D. Animal welfare Causes of death Execution methods Meat industry Nitrogen Poultry farming Slaughter methods Toxicology Asphyxia