Oxygen (O) : A gaseous element present in the atmosphere in about 21 percent concentration; it is essential to combustion, is one of the four parts of the fire tetrahedron, and is the essential gas in respiration
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}