Asphyxiant gas
An asphyxiant gas is a non-toxic or minimally toxic gas which dilutes or displaces the oxygen containing atmosphere, leading to death by asphyxiation if breathed long enough. Toxic gases in large enough concentrations to cause asphyxia lead to death by other mechanisms such as interaction with the respiratory system by competing with oxygen (such as carbon monoxide) or causing direct damage (such as phosgene). Because asphyxiant gases are relatively inert, their presence in large quantities may not be noticed until the effects of elevated blood carbon dioxide are recognized by the body. Asphyxiation is not an intrinsic gas property, but arises from its ability to cause death by asphyxiation without causing other symptoms. Notable examples of asphyxiant gases are nitrogen, argon, and helium. The earth's atmosphere is made of 79% asphyxiant gases (mainly nitrogen), and 21% oxygen. This is an example of how all safe, breathable atmospheres are made up of a high enough concentration of oxygen together with at least one asphyxiant gas.

# Examples of asphyxia by asphyxiant gas
This means that asphyxiant gases are normally hazardous only in special circumstances, due to the exclusion of the naturally abundant oxygen in Earth's atmosphere:
- Environmental gas displacement
breathing in a confined space, combined with accidental gas leaks such as mines, submarines, refrigerators, or other confined spaces
fire extinguisher systems that flood spaces with inert gases - such as computer data centers and sealed vaults
due to a large-scale release of gas, even larger areas can be affected given enough asphyxiant gas, such as during the Lake Nyos disaster in which volcanically-released carbon dioxide was responsible for the death of 1,800 people.
- breathing in a confined space, combined with accidental gas leaks such as mines, submarines, refrigerators, or other confined spaces
- fire extinguisher systems that flood spaces with inert gases - such as computer data centers and sealed vaults
- due to a large-scale release of gas, even larger areas can be affected given enough asphyxiant gas, such as during the Lake Nyos disaster in which volcanically-released carbon dioxide was responsible for the death of 1,800 people.
- Direct administration of asphyxiant gas
exclusive administration of a gas, such as from inhaling the contents of a balloon filled with helium
inadvertent administration of asphyxiant gas in respirators
asphyxiant gases are also used in suicide and erotic asphyxiation.
- exclusive administration of a gas, such as from inhaling the contents of a balloon filled with helium
- inadvertent administration of asphyxiant gas in respirators
- asphyxiant gases are also used in suicide and erotic asphyxiation.
- Contained asphyxiant gas environment
climbing inside an inflatable balloon filled with helium
- climbing inside an inflatable balloon filled with helium

# Handling of asphyxiant gases

## US
The handling of compressed asphyxiant gases and the determination of appropriate environment for their use is regulated in the United States by the Occupational Safety and Health Administration (OSHA) and National Institute for Occupational Safety and Health (NIOSH). OSHA requirements for the use of asphyxiant gases mandate the provision of respiratory protective devices under the Respiratory Protection Standard . This includes "respirator selection, an evaluation of the worker's ability to perform the work while wearing a respirator, the regular training of personnel, respirator fit testing, periodic workplace monitoring, and regular respirator maintenance, inspection, and cleaning." Containers should be labeled according to OSHA's Hazard Communication Standard . These regulations were developed in accordance with the official recommendations of Compressed Gas Association (CGA) pamphlet P-1. The specific guidelines for prevention of asphyxiation due to displacement of oxygen by asphyxiant gases is covered under CGA's pamphlet SB-2, Oxygen-Deficient Atmospheres. Specific guidelines for use of gases other than air in back-up respirators is covered in pamphlet SB-28, Safety of Instrument Air Systems Backed Up by Gases Other Than Air.

### Odorized asphyxiant gas
Because of the potential danger of inadvertent asphyxiant gas leaks in the workplace, there have been proposals to odorize some commonly used atmospheric gases such as nitrogen and argon so that their presence can be detected by smell. However, the Compressed Gas Associated has published a position paper arguing against this practice. Their arguments include concern that odorized gas will lead to more relaxed handling of the gases, that the ability to smell is variable among workers, and that there are impracticalities involved in assigning a different smell to each gas.

# UK
In the United Kingdom gasses are covered by COSHH.
The asphyxiant, and potentially explosive, natural gas is given an odour artificially.

# Historical aspects
The dangers of excess concentrations of non-toxic gases has been recognized for centuries within the mining industry. The concepts of black damp (or "stythe") and afterdamp reflect an understanding that certain gaseous mixtures could lead to death with prolonged exposure. Early mining deaths due to mining fires and explosions were often a result of encroaching asphyxiant gases as the fires consumed available oxygen. Early self-contained respirators were designed by mining engineers such as Henry Fleuss in order to help in rescue efforts after fires and floods. While canaries were typically used to detect carbon monoxide, tools such as the Davy Safety Lamp and the Geordie lamp were useful for detecting methane and carbon dioxide, two asphyxiant gases. When methane was present, the lamp would burn higher; when carbon dioxide was present, the lamp would gutter or extinguish. Modern methods to detect asphyxiant gases in mines led to the Federal Mine Safety and Health Act of 1977 in the United States which established ventilation standards in which mines should be "...ventilated by a current of air containing not less than 19.5 volume per centum of oxygen, not more than 0.5 volume per centum of carbon dioxide..."