Disclaimer: Do not attempt to dive with enriched air unless you have completed the appropriate speciality course. Doing so without complete training can be very dangerous.
Enriched air can mean any gas blend other than the standard 21% oxygen / 79% nitrogen that comprises what we call "air." For our purposes in this article, enriched air will refer to a blend of oxygen and nitrogen in which the oxygen content has been raised, more commonly called "Nitrox" (or EANx, Enriched Air Nitrox). We won't be concerned with Tri-mixes and the like---usually the domain of tech diving.
Nitrox blends increase the oxygen content of a cylinder, which simultaneously lowers the nitrogen content. This is usually accomplished by partially filling a cylinder with air (21% oxygen / 79% nitrogen), then "topping it off" with pure, 100% oxygen. Some basic math tells us the final oxygen content.
For example, imagine we fill a cylinder to 2500 PSI with air, then fill to 3000 PSI with pure oxygen. What is the final oxygen content? 500 PSI (3000 PSI - 2500 PSI) is 100% oxygen, while 21% of 2500 PSI is also pure oxygen. 2500 PSI * 0.21 = 525 PSI. Add the two together to get 500 PSI + 525 PSI = 1025 PSI oxygen. Divide by the total contents of the cylinder to get the percentage of pure oxygen, 1025 / 3000 = 0.3416 = 34.16% oxygen. That means the remaining 65.84% must be nitrogen (ignoring trace amounts of other gases in the air). We call our blend is EANx35, Enriched Air Nitrox 35 (you round up with enriched air blends).
Why enriched air?
What advantages does Nitrox provide divers? Nitrox reduces exposure to nitrogen compared to diving with air. Nitrogen is known to instigate Decompression Sickness (DCS), so less exposure is always good.
Divers take advantage of this benefit in two ways. Bottom times provided by dive tables and computers are controlled by estimates of nitrogen exposure. Less exposure means more bottom time. So the maximum bottom time on a Nitrox dive to a given depth is always higher than the maximum bottom time of an air dive to the same depth. My dive table says that an air dive to 60 ft (18 m) has a maximum bottom time of 47 minutes. The same dive on EANx32 has a maximum bottom time of 77 minutes, half an hour longer!
Not everyone wishes to dive for an hour and a half, however. The other advantage, then, is that with two identical dive profiles, one with air and one with Nitrox, the Nitrox dive will have a higher buffer of nitrogen exposure, significantly decreasing any risk of DCS. This isn't a big deal for any one dive, but for many consecutive multi-day dives, where DCS causes are less understood, this buffer becomes important. The benefit of Nitrox also decreases required surface intervals, allowing a diver to easily fit four dives in a day.
What's the catch?
Diving with Nitrox introduces a few more variables to track. For one, you must always check your cylinder for what percentage Nitrox it contains. The blender uses the above calculations, but any errors can affect your diving in a bad way, so for safety it is not only recommended, but required that the diver personally check their enriched air before every dive. Shops will have the appropriate equipment for doing so. I would refuse to dive with an enriched air mix I have not personally checked.
Fortunately, most dive computers come equipped for enriched air, making it straight-forward to dive with Nitrox. However, Nitrox requires modified dive tables, due to different exposures. Shops sell tables for common mixes such as EANx32 and EANx36, and your Nitrox specialty course may even come with them. The most versatile method is using something called Equivalent Air Depth (EAD).
Equivalent Air Depth calculations tell you, given a specific Nitrox blend, at what depth you would have to dive with air to get the same nitrogen exposure. With this depth, you can use normal dive tables to plan dives. Since Nitrox contains less nitrogen, this depth is always less than the actual dive depth. We can compute EAD as
EAD = (1 - O%) * (D + 10) / 0.79 - 10,
where O% is the percentage of oxygen, and D is the depth of the dive, in meters. Suppose we are planning a dive to 60 ft (18 m) with EANx34 and don't have an appropriate table. We compute our EAD as (1 - 0.34) * (18 + 10) / 0.79 - 10 = 13.39 m, or 45 ft. We then use our ordinary dive tables, entering 45 ft (or 13.39 m) as the dive depth. This gives me a 63 minute bottom time.
Diving with air within recreational limits, oxygen exposure never reaches critical limits---our body is always able to metabolize the available oxygen. However, with Nitrox oxygen content is increased to amounts that could be dangerous.
For instance, it is known that exposure to oxygen partial pressure beyond 1.6 can be fatal. Oxygen partial pressure is the portion of pressure due to oxygen at a given depth. Air at 1 atm has an oxygen partial pressure of 0.21 * 1 = 0.21. At 2 atm it's 0.21 * 2 = 0.42. With this knowledge, we can calculate a maximum allowable depth on any gas blend. For air, this maximum is 1.6 / 0.21 = 7.619 atm, which corresponds to a depth of about 66 m (215 ft). In practice, it is recommended (and wise) to use 1.4 as the maximum oxygen partial pressure, to build in a safety buffer. For air then, we wouldn't want to exceed a depth of 1.4 / 0.21 = 6.6 atm, or 56 m (185 ft). I'm guessing that won't be a problem for most of you. :)
We can repeat these calculations for our Nitrox blends. Given EANx36, the maximum depth is 1.4 / 0.36 = 3.8 atm, or 28 m (93 ft), which is above the recreational limits! Keep this in mind for high oxygen content blends. What is the maximum allowable depth for EANx40 (the highest oxygen content permitted with most certifications)?^1^
This is a maximum partial pressure in a single dive, but over multiple dives, your body accumulates excess oxygen that it can't burn. Given enough time this oxygen can become toxic. Therefore, you must also track accumulation of oxygen over a running 24 hour period. You will learn how to do this and be given the appropriate tables in your Enriched Air Diver specialty course.
If these advantages sound appealing to you, I highly recommend completing your Enriched Air specialty. Most regular divers will do it at some point. Don't let the calculations and variables deter you; they become easier over time. Again, let me reiterate that you should not dive with Nitrox unless you have completed such a course. It can be done in an evening, with no dives required (at least with PADI).
1. 82.5 ft (25 m)