Oxygen deficient atmospheres (those which have an oxygen content equal to or less than 19.5% at sea level, or are above 14,000 feet). If you are working in a normal atmosphere you can assume an oxygen level of 21%. However, if you are working in an area with poor ventilation, or in a confined space, you will need to measure the oxygen level.

The normal atmosphere at altitudes above 14,000 feet is considered oxygen deficient. The standard requires workers above this altitude to have an oxygen enriched atmosphere.