Oxygen monitors should be considered in areas where inert or other asphyxiating gases at present, in confined spaces or where appreciable amounts of carbon dioxide are in use. However, for carbon dioxide detection, CO2 toxicity normally overrides the O2 depletion hazard.  For more information please see CARBSAFE - carbon dioxide detection. 

To determine if you need oxygen depletion monitoring, a risk assessment of the area and activities should be carried out. If the risk assessment identifies that in the event of a worst-case scenario oxygen levels would deplete to below 19.5% O2, you require an oxygen monitor. However, a required risk assessment often does not cover specific areas where a leak may occur (such as directly next to LN2 storage vessel) and even in situations in which oxygen depletion does not reach life-threatening levels, the effect of oxygen starvation on the health of working personnel is still a major health and safety concern.

Oxygen enrichment poses a severe fire hazard. An oxygen concentration of 23% or above is regarded as dangerously enriched, meaning materials catch fire more easily and burn more intensely. Therefore, oxygen monitors are recommended.