What is oxygen enrichment?

The concentration of oxygen in confined spaces is a concern from two standpoints. Too little oxygen can cause asphyxiation. Excessively high or "enriched" levels of oxygen above normal concentration in fresh air can dramatically promote or accelerate combustion and other chemical processes.

Elevated concentrations of O₂ can cause ordinary combustible materials to become flammable or explosive (see Confined Space Safety 101 for a primer on confined space risks).

The definition of oxygen enrichment varies between different standards and regulatory documents. OSHA 1910.146, and 1926 Subpart AA reference 23.5% as the concentration above which the atmosphere is hazardous because of oxygen enrichment. However, OSHA 1915 Subpart B, “Standard for shipyard employment” specifies 22%. The latter value is consistent with non-mandatory recommendations from groups such as the National Fire Protection Association (NFPA). The most conservative standards specify 22% as the concentration above which the atmosphere is deemed to be hazardous due to oxygen enrichment.