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Users of freezers may be exposed to potential harm when nitrogen escapes from a cryogenic storage vessel and reduces the surrounding oxygen concentration to an unacceptably low level. 

Individuals using or maintaining the freezing equipment need to be aware that although nitrogen itself is non-toxic, it can reduce the oxygen concentration of atmospheric air locally to levels that may become hazardous to health and potentially fatal.

The Occupational Safety and Health Administration (OSHA) specifies that a hazardous atmosphere may include one where the oxygen concentration is below 19.5% (or above 23.5%).  Atmospheric air contains about 21% oxygen, and reductions in this concentration are increasingly harmful, diminishing mental alertness such that a person may not recognize symptoms nor realize they are in danger.

There may be no visual indication of an oxygen depleted atmosphere, but sometimes a white-water vapor cloud may be present due to the cooling effect of the nitrogen. A vapor cloud usually indicates some oxygen depletion, but do not assume the absence of a cloud means there is a normal atmosphere.

To guard against oxygen deficiencies, ICs are responsible for ensuring oxygen monitoring devices are installed and maintained in any indoor location where compressed gases or cryogenic liquids are stored and/or dispensed in a manner that could create the potential for the displacement of oxygen. 

For more information, visit the Division of Occupational Health and Safety, NIH Protocol for Use and Maintenance of Oxygen Monitoring Devices at https://www.ors.od.nih.gov/sr/dohs/safety/Pages/oxygen.aspx.