Sentence:  "Enriched air" or "<WSD>nitrox</WSD>" is a mixture of air and oxygen; the oxygen component ranges to about 50% but is used normally at 32 to 40%. [...] Safe enriched air <WSD>nitrox</WSD> diving practices established for the scientific diving community might be of some value and could help avoid accidents due to ignorance of its hazards and use of improper procedures. 
Meaning of the word marked with <WSD> tags:  A mixture of nitrogen and oxygen, the nitrogen content being lower than what is normally present in air, which is used in place of air as a breathing gas.