Concept
An asphyxiant gas, also known as a simple asphyxiant, is a nontoxic or minimally toxic gas which reduces or displaces the normal oxygen concentration in breathing air. Breathing of oxygen-depleted air can lead to death by asphyxiation (suffocation). Because asphyxiant gases are relatively inert and odorless, their presence in high concentration may not be noticed, except in the case of carbon dioxide (hypercapnia). Toxic gases, by contrast, cause death by other mechanisms, such as competing with oxygen on the cellular level (e.g. carbon monoxide) or directly damaging the respiratory system (e.g. phosgene). Far smaller quantities of these are deadly. Notable examples of asphyxiant gases are methane, nitrogen, argon, helium, butane and propane. Along with trace gases such as carbon dioxide and ozone, these compose 79% of Earth's atmosphere. Asphyxia Asphyxiant gases in the breathing air are normally not hazardous. Only where elevated concentrations of asphyxiant gases displace the normal oxygen concentration does a hazard exist. Examples are: Environmental gas displacement Confined spaces, 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 Large-scale natural release of gas, such as during the Lake Nyos disaster in which volcanically-released carbon dioxide killed 1,800 people. Release of helium boiled off by the energy released in a magnet quench such as the Large Hadron Collider or a magnetic resonance imaging machine. Climbing inside an inflatable balloon filled with helium Direct administration of gas Inadvertent administration of asphyxiant gas in respirators Use in suicide and erotic asphyxiation The risk of breathing asphyxiant gases is frequently underestimated leading to fatalities, typically from breathing helium in domestic circumstances and nitrogen in industrial environments. The term asphyxiation is often mistakenly associated with the strong desire to breathe that occurs if breathing is prevented.