Nitrox is a mixture of nitrogen and oxygen. In recreational scuba diving, nitrox is usually a mixture of 21- 40% oxygen and 60-79% nitrogen. A particular mixture of nitrox is referred to by the percentage of oxygen in the mixture. Therefore, two common nitrox mixtures, Nitrox 32 (NOAA Nitrox I) and Nitrox 36 (NOAA Nitrox II), contain 32% and 36% oxygen respectively. Air, which contains only trace amounts of gases other than nitrogen and oxygen, can be considered Nitrox 21 as it contains approximately 21% oxygen. The terms "enriched air" and "nitrox" are sometimes used interchangeably, but they are not the same. "Nitrox" refers to any mixture of nitrogen and oxygen, while "enriched air" refers to mixtures of oxygen and nitrogen that contain more than 21% oxygen. Special training and certification are required to dive with nitrox.