Straight from ScubaDiving.com:
In recreational diving terms, enriched air nitrox (EANx) refers to any nitrogen/oxygen gas mixture with more than the 21 percent oxygen found in normal air. For example, 32 percent and 36 percent oxygen are common; you’ll hear them called nitrox 32 (or 36) or EAN32 (or 36). To clearly identify them from air cylinders, tanks filled with EAN are marked with a yellow and green tank band at the top, just below the crown. The oxygen mix percentage is written on a label or tag, which is important because the percentage affects your depth limits and no stop dive time.
How Does EANx Nitrox Work?
Enriched air has more oxygen than air does, which reduces the percentage of nitrogen. As a result, you inhale less nitrogen throughout the dive than if you’re using regular air.
We all learned in our Open Water Diver course that, when you dive, the water pressure causes nitrogen from the air you’re breathing to dissolve in your bloodstream and tissues. The higher the pressure (deeper the dive) and the more time under pressure (bottom time), the more nitrogen dissolves in. If you surface with too much dissolved nitrogen in your tissues, bubbles can form causing decompression sickness (DCS). Staying within the no stop limits limits the theoretical amount of dissolved nitrogen so that you can ascend directly to the surface (but make your safety stop). If you exceed this limit (as do tec and commercial divers), you have to make mandatory decompression stops in stages as you come up to keep the risk of DCS low. Based on the Recreational Dive Planner, for example, a diver on air at 100 feet reaches his or her no-decompression after 20 minutes. At 60 feet, the diver’s maximum time would be 55 minutes. But, nitrox changes these numbers.
Because you’re breathing less nitrogen, less dissolves into your body, all else being equal, allowing a longer no-decompression limit. This can be calculated using an equivalent air depth (EAD), which is a shallower depth at which nitrogen would be going into solution at the same rate if you were using air. If, for example, at 105 feet with EANx32 has an EAD of 90 feet, so you’re absorbing nitrogen as if you were 15 feet shallower. Therefore, the diver’s ordinary no-decompression limit of 15 minutes extends to 25 minutes — that’s a 66 percent increase in bottom time.