Inert gas asphyxiation
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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, rather than atmospheric air (which is oxygen and nitrogen). 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.
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 persons died due to 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, using a gas-retaining plastic hood device colloquially referred to as a suicide bag. At least 109 persons committed suicide using helium inhalation, the most popular inert gas for this use, from 2001 to 2009, in Australia.
Nitrogen asphyxiation has been suggested by a number of lawmakers and other advocates, as a possibly more humane way to carry out capital punishment.
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 (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 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 recognize their individual subjective signs of approaching hypoxia. Some individuals experience headache, dizziness, fatigue, nausea, euphoria and some become unconscious without warning.
Relation of inert gas asphyxia 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 animals 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
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. This figure comes from the fact that some 80 persons 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.
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. An accidental fall from a tree caused from loss of consciousness and/or light-headedness as a direct result from the inhalation have been fatal indirectly after a person inhaling from a toy balloon.
A method of suicide based on self-administration of helium in a bag, a slang 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. Two persons have been reported in the forensic literature to have committed suicide using nitrogen inhalation.
One person was reported to have committed suicide by inhaling argon without oxygen.
Hypoxic methane has been used for suicide.
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". 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 early 2015, the State of Oklahoma administratively approved the use of "nitrogen hypoxia" as a secondary capital punishment method. 
- “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.
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