"The 'fire tetrahedron' is one of the elementary topics we learn in fire protection: Heat, fuel, oxygen and an uninhibited chemical chain reaction in appropriate combinations are needed to sustain combustion. We also learn that the atmosphere we breathe includes about 20.5 percent oxygen by volume. Oxygen-enriched atmospheres, on the other hand, are those environments that the United States Occupational Safety and Health Administration describes as containing more than 23.5 percent oxygen by volume. Health care facilities, industrial operations and research laboratories are just a few of the places where liquid or gaseous oxygen may be used in large quantities, and environments may be oxygen-rich."
Coffee Break Training - Fire Protection Series No. FP-2014-13
United States Fire Administration: http://www.usfa.fema.gov/