Asphyxiant

Simple asphyxiants are inert, essentially non-toxic gases such as nitrogen, argon and helium. They do no direct harm, but in an enclosed space they push oxygen out of the air. Carbon dioxide is often grouped with them, but it is not just an oxygen displacer: at elevated concentrations it has its own toxic effects, which is why it has an occupational exposure limit and a NIOSH IDLH value based on effects such as intoxication and unconsciousness. The danger is that they are often odorless and give no warning, and workers can lose consciousness within moments of entering an oxygen-deficient space.

Chemical asphyxiants interfere with oxygen transport or use. Carbon monoxide binds to hemoglobin in the blood and blocks oxygen carriage. Hydrogen cyanide and hydrogen sulfide interfere with cell use of oxygen. Chemical asphyxiants can be dangerous at very low concentrations, whatever the oxygen level of the air.

Asphyxiation is a major cause of deaths in confined spaces, and rescuers are often killed because they enter without protection. Safe work depends on testing the atmosphere before entry, continuous ventilation and a rescue plan. Compressed and liquefied gases, including cryogenic liquids, can release large volumes of gas as they evaporate.

Simple asphyxiant is not a UN GHS hazard class, but OSHA HazCom defines it and requires label elements for it: the signal word Warning and the statement "May displace oxygen and cause rapid suffocation", with no pictogram. Air-purifying respirators do not supply oxygen and cannot protect against an oxygen-deficient atmosphere.