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Chemical Safety Information - Glossary
Asphyxiant
An Asphyxiant is a material capable of reducing the level of oxygen in the body to dangerous levels. Most commonly, asphyxiants work by merely displaying air in an enclosed environment. This reduces the concentration of oxygen below the normal level of around 19% which can lead to breathing difficulties, unconciousness or even death.
The danger is (in theory) posed by any gas which is potentially present at high levels in the environment. Special hazards are posed by liquid nitrogen, since the evaporation of a fairly small volume of liquid can create a large volume of nitrogen gas, displacing enough oxygen from a small room to pose a potentially fatal hazard.
When liquid nitrogen evaporates, because the nitrogen produced is very cold it readily forms a layer of cool, oxygen-depleted gas at floor level. If a person becomes light-headed as a result of lack of oxygen and collapses, they may fall to floor level and will then be breathing an atmosphere very poor in oxygen. Death can quickly result. Consequently, it is essential that liquid nitrogen be used only in well ventilated areas. If there is the possibility of the generation of large volumes of inert gas very quickly (for example, through a liquid nitrogen Dewar going "soft" or a superconducting liquid-helium-cooled magnet becoming resistive and blowing off the helium) at least two people should be present during routine operations. Non-trivial quantities of liquid nitrogen should never be transported in a lift (elevator) along with people.
Some asphyxiants act directly upon the oxygen-carrying ability of the blood. Carbon monoxide, for example, binds strongly to the haemoglobin molecule, which is responsible for the transport of oxygen around the body. This reduces the amount of haemoglobin which is free to transport oxygen, possibly to dangerous levels. Carbon monoxide is produced in potentially large amounts when material is burnt with a limited supply of oxygen. It is generated at often substantial levels when cigarettes are smoked.
