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, nitrogen asphyxiation has been suggested as a more humane way to end a human life in a controlled setting such as in capital punishment.
When humans breathe in pure nitrogen, they exhale carbon dioxide without resupplying oxygen. Nitrogen is a colorless odorless tasteless gas that is about 78% of Earth's atmosphere. As the main component in air, nitrogen poses no significant risk upon discharge. As such, the subject would detect no abnormal sensation. This leads to asphyxiation without the painful and traumatic feeling of suffocation.
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 liters is exchanged from an active lung volume (tidal volume + functional residual capacity) of about 3 liters. 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, 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.
Loss of consciousness may be accompanied by convulsions and is followed by cyanosis and cardiac arrest. About 7 minutes of oxygen deprivation causes death of the cerebral cortex and presumably of the medulla oblongata, which controls breathing and heart action.
Animal slaughter 
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.
Portable units are available. Controlled atmosphere stunning has been used both within processing plants and on-farm for euthanizing poultry. 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.
Studies show that 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 is very species-specific.
Accidental deaths 
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.
Capital punishment 
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.
See also 
- Fisher PW. "High altitude respiratory physiology, chapter 2 in USAF Flight Surgeon's Guide", n.d. Retrieved on 2007-02-07.
- DiMaio V & DiMaio D. "Forensic Pathology, Second Edition. Chapter 8, Asphyxia", ISBN 0-8493-0072-X, 2001. Retrieved on 2007-01-27.
- PETA's Animal Times, UK, Autumn 2005
- The Animal Welfare Foundation of Canada article The Disposal of Spent Laying Hens by Jacqueline Wepruk
- ALTERNATIVES TO CARBON DIOXIDE EUTHANASIA FOR LABORATORY RATS
- "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.
- "Shuttle Tragedy". Time. 1981-03-31. Retrieved 2007-01-27.
- "UK: Scotland Inquiry after man dies in chemical leak". BBC. 1999-10-25. Retrieved 2007-01-27.
- Creque, S.A. "Killing with kindness – capital punishment by nitrogen asphyxiation" National Review. 1995-9-11.
- How to Kill a Human Being - in search of a painless death, Videosift.com
- Seitz KN. "The Transition of Methods of Execution in North Carolina: A Descriptive Social History of Two Time Periods, 1935 and 1983", PhD dissertation (Sociology) Virginia Polytechnical Institute and State University, 2001. Retrieved on 2007-01-23.
- "Methods of Execution". Retrieved 2007-01-25.
- "Some Examples of Post-Furman Botched Executions". Retrieved 2007-01-25.
- NIOSH respirator fact sheet
- U.S. Chemical Safety and Hazard Investigation Board
- Death Penalty Information Center
- Restraint asphyxia
- The Welfare of Birds at Slaughter: 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