Liquid hydrogen and helium are sufficiently cold to liquefy air. Nitrogen will evaporate more rapidly from the liquefied air than oxygen. This will leave behind a liquid air mixture that has a higher concentration of oxygen than normal air. When this evaporates, an atmosphere with a higher concentration of oxygen is created.
Liquid oxygen is also a frequently used cryogenic material and is transported by road and stored at premises. A release of liquid oxygen will inevitably result in elevated oxygen levels as the oxygen evaporates into the atmosphere.
Other situations where an oxygen-enriched atmosphere might occur because of certain chemical reactions include:
- Decomposition of hydrogen peroxide or another oxidising source
- Chemical oxygen generators
- Electrolysis of water
By far the most likely situation for an oxygen enrichment to be encountered is through the transport/storage of cryogenic oxygen.
Cryogenic materials can create a fire hazard through three possible routes:
- The cryogenic material is a liquefied flammable gas (e.g. methane as liquefied natural gas (LNG))
- The cryogenic material is liquefied oxygen; as this evaporates, the atmospheric oxygen concentration will be raised, increasing the potential for combustible materials to burn
- Certain cryogenic materials (liquid hydrogen and helium) are sufficiently cold to liquefy air and when this evaporates nitrogen will be released first; when oxygen subsequently evaporates this will also lead to an increase in the atmospheric concentration of oxygen
Knowledge and understanding
Understand all associated hazard knowledge
- Control measureRecognise consequences on processes using cryogenic materials
- Control measureSubstance identification