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Oxygen-enriched atmosphere

Hazard Knowledge

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