On today's dive boats, you're hard-pressed not to see one or two of the telltale green and yellow tanks that signal somebody aboard is scuba diving with Enriched Air Nitrox—EANx or commonly just called "enriched air" or "nitrox." It’s been used in recreational diving for decades, and every major training agency has a training program for scuba diving with enriched air.

Diving with nitrox increases your no stop dive time for longer dives and/or shorter surface intervals, all else being equal and gas supply allowing. Nitrox is an effective tool when used properly—but potentially dangerous if not. It has shallower depth limits and other concerns you don't have with air within recreational diving limits that you learn about in the PADI Enriched Air Diver course.

As PADI’s content development executive and a PADI Enriched Air instructor trainer, I’ve learned a thing or two about nitrox—so let’s dive in.

What is Nitrox? How is it Different from Air?

In recreational diving, enriched air nitrox is any nitrogen/oxygen gas mixture with additional oxygen added. The air you breathe while scuba diving is about 21% oxygen and 71% nitrogen. By contrast, nitrox will have a higher oxygen proportion—in recreational diving, usually 32 or 36%. This is why you will often hear the mixes referred to as "Nitrox 32" or "Nitrox 36." You may also hear the terms nitrox-enriched air or EANx, where EA stands for enriched air and Nx stands for nitrox, though it's common to drop the "x." So, if you encounter a mix at your dive shop labeled EANx36 or EAN36, it is a gas mix with 36% oxygen, 64% nitrogen.

How is it Different Diving with Nitrox vs. Air?

The purpose of EANx is not the additional oxygen, but having less nitrogen. By reducing the proportion of nitrogen you breathe, you can extend your bottom time without increasing your theoretical risk of decompression sickness (DCS). As you learned in the PADI Open Water Diver course, the pressure at depth causes nitrogen from the gas you breathe to dissolve in your bloodstream and tissues. The deeper (i.e. higher pressure) and longer you dive, the more nitrogen dissolves into your tissues. Within limits, this nitrogen has no effect and slowly leaves your body after you surface.

But, if you surface with too much dissolved nitrogen in your tissues, bubbles can form and cause DCS. Staying within the no stop limits caps the theoretical amount of dissolved nitrogen so that you can ascend directly to the surface if necessary (but otherwise make your safety stop), with low DCS risk (it is never 0 risk). If you exceed this limit (like tec and commercial divers), however, you have to make mandatory decompression stops in stages to release nitrogen as you come up to keep your risk of DCS low.

Imagine you’re doing a deep dive to 100 feet. Based on the Recreational Dive Planner (RDP), you hit your no decompression limit (NDL) at 20 minutes on a typical air dive. But, diving nitrox changes this. Because you're breathing less nitrogen, less dissolves into your body—all else being equal—allowing a longer no-decompression limit. Using EANx32, for example, your no stop limit is 30 minutes at 100 feet.

For tables diving, you either use special tables for the blend you're using, or air tables with an Equivalent Air Depth (EAD) that adjusts your limits. If, for example, you are at 70 feet diving EANx36, your EAD is 50 feet because you're absorbing nitrogen as if you were 20 feet shallower with air. Using the RDP, your single dive no stop time doubles from 40 minutes to 80 minutes. Using EANx with a computer (which is most common today), the EANx advantage plus a multilevel profile means most dives—even repetitive dives—are limited by how much gas you have, not by no stop time.

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How to Earn a Nitrox Diving Certification

Becoming a PADI Enriched Air Diver is not difficult, but it's important. You complete an eLearning course during which you learn about EANx theory, equipment considerations and managing the risks. This is followed by in-person instruction with a PADI pro, which takes a day or two. This gives you hands-on experience analyzing EANx (so you know for sure which blend you're using), setting your dive computer for that blend and filling out an enriched air log.

The Pros and Cons of Nitrox Diving

Like everything in life, there are advantages and disadvantages to using enriched air.

Benefits of Enriched Air Nitrox Diving

In recreational diving, as you saw above, the benefit of nitrox diving is that you have longer bottom times without the mandatory decompression stops (technical divers use it for other reasons).

The best use of nitrox is in the 50- to 100-foot range. No stop times for dives shallower than 50 feet are so long that even making repetitive dives with air you'll usually reach your planned ascent pressure before you run out of no stop dive time. But, using EANx36 at 60 feet, for example, means you’ll have a no stop limit of 80 minutes or more (depending on your computer) for your first dive, and that doesn't count what you gain as you go up if you follow an ascending profile. Similarly, while repetitive dives must still account for dissolved nitrogen, they're usually longer because you didn't absorb as much nitrogen on the previous dive. In most instances using EANx, you are limited by your gas supply, not your no decompression limit.

This applies to what you were taught, by the way, regarding diving well within limits. With enriched air, you can dive longer and still be conservative—a dive "to" the limit with air may be planned as well within limits using EANx. Again, you can't guarantee zero risk of DCS, but staying well within the limits reduces practical DCS risk for most divers.

Interested in diving nitrox? Start your adventure here with PADI's Enriched Air Diver course.

Disadvantages of Nitrox Diving

EANx diving's benefit does not come without a cost. Oxygen, while useful and essential to life, can become hazardous in high concentrations (high partial pressures). Breathing oxygen at an increased partial pressure for prolonged periods (i.e. while diving) can lead to oxygen toxicity. Signs include convulsions or seizures, visual distortions, ringing ears, nausea, twitching, irritability and dizziness. Oxygen toxicity hits are very rare in diving, but if they happen underwater, they happen without warning and can lead to drowning if a diver loses the regulator while convulsing.

With enough pressure, any percentage of oxygen can cause toxicity, so as a PADI Enriched Air Diver, you learn how to determine the maximum depth for the EANx blend you're using. This issue doesn't apply to air in recreational diving because have to go well beyond established depth limits for oxygen toxicity to be likely, where as with nitrox the depth limits are within recreational diving depths.

EANx Cylinders

To help with these concerns, EAN cylinders are clearly marked so they won’t be confused with standard air. Tanks filled with nitrox are marked with a yellow and green tank band at the top, just below the crown. The oxygen content (mix percentage) is written on a label or tag, along with the maximum depth to which you can use that mix and the diver's name. The diver must personally analyze the gas. You also sign a log indicating your percent EANx and maximum depth the blend can be used.

What Nitrox Won't Do

Enriched air is sometimes talked about like it’s a cure-all for every diving downside. But that’s not the case, so let’s take a moment to bust a few myths.

Myth 1 - Using nitrox means you're less likely to get narked.

This sounds reasonable on the surface. If nitrogen causes narcosis and you're breathing less of it, you should have less narcosis. The trouble is, the depth range where you start worrying about narcosis (100 to 130 feet) is also where you have to stop using nitrox because of the risk of oxygen toxicity. And even if that wasn’t the case, oxygen is thought to be about as narcotic as nitrogen (not always but usually — gets complex), so you're one step up and one step back anyway. The preventive value of nitrox is "very small, so marginal that I think it should be discounted,” writes Dr. Peter Bennett in the chapter on inert gas narcosis in The Physiology and Medicine of Diving. And note, it's gas narcosis, not nitrogen narcosis.

Myth 2 - Nitrox reduces your gas consumption.

This also sounds reasonable. If there's more oxygen in each breath, presumably, you don't have to take another one so soon, and your gas consumption rate is lower. Unfortunately, it doesn't work that way because most of the oxygen you breathe is exhaled unused anyway. What drives the next breath is the need to exhale carbon dioxide, and that's not affected by what's in your tank.

Myth 3 - Nitrox allows you to dive with less fatigue.

The theory here is that the work of off gassing nitrogen is a major cause of diving fatigue, so less of it should leave you less tired. Many nitrox divers swear it's true, but Bennett cites a blinded study that proved otherwise. Using unmarked tanks, one group of divers was given nitrox, another was given air, and both were asked later how they felt. "There was no difference," says Bennett. "It's a placebo effect."

Myth 4 - You can dive deeper with enriched air.

Some divers think those green-and-yellow cylinders look serious and "techie," and associate that with going deep. As you just learned, though, EANx introduces a shallower depth floor and the penalty for going below it—potentially dying—is serious. However, perhaps contributing to the myth is that tec divers use EANx, but not for the deep part of their dives. They use it for decompressing in the shallows while ascending from a deep dive. It's not a deep diving gas, but it is a decompression gas for tec divers.

Getting the Right Nitrox Mix

The pre-mixes offered at most dive shops—usually 32 or 36 percent—make life simple. Just pick up the tank, analyze the mix , confirm the maximum depth and enter it into your computer.

But you can use other blends. Sometimes there's a "best fit" for a particular dive (something you learn about as a PADI Enriched Air Diver) and your dive shop can often blend a tank with whatever percentage you want, as long as it's below the recreational limit of 40 percent.

Custom mixes like this really can give you the best bang for the buck. For example, using EANx32 at 70 feet has an equivalent air depth of 56 feet (a table NDL of 55 minutes)—but if you’re using EANx40 (highest allowable oxygen without advanced cert), you have an EAD of 45 feet (table limit 100 minutes). If the entire dive will be at about 70 feet (a "square" dive instead of a multilevel dive) this will give you more dive time, especially if you make a repetitive dive to the same depth. In places where a given mix is particularly useful due to popular dive site depth, dive centers commonly stock that EANx blend.

Nitrox and Recreational Scuba Equipment Safety

Standard scuba equipment can be used for oxygen concentrations up to 40 percent, according to long standing and accepted dive community practices. Since this is also the maximum recreational blend limit for nitrox diving, there has never been gear-related incident recreational diving equipment used within recreational nitrox limits. (Note: Some manufacturers use a more conservative rule. Always follow a manufacturer's recommendations for their gear.)

There is one exception to this—a scuba tank must be specially marked for safety as you learned, and often it must rated for pure oxygen. This is because while oxygen does not burn, it does enhance the flammability of everything that it contacts. Under high concentrations (percent in this case, not partial pressure), most substances ignite more easily, and burn hotter and more rapidly.

One common method of making nitrox (especially custom mixes) requires putting pure oxygen in the tank first, followed by specially filtered oxygen compatible air second. Because the tank contains pure oxygen during blending, even though the final blend will be less than 40 pecent, the cylinder must meet oxygen service standards. This means the materials (O-rings, valve seats, etc.) and any lubricants must be rated for use with pure oxygen. The tank must also be specially cleaned, which is done annually.

Earn a Nitrox Diving Certification

PADI’s Enriched Air Diver course includes eLearning that can be completed in a few short hours. In person learning, is done in a day or two, gives you the chance to analyze a tank, set your dive computer for nitrox, and fill out an enriched air log with a PADI pro there to guide you.

Nitrox Formulas and Definitions

• Normoxic nitrox: Regular air, or cleaned air with 21 percent oxygen
• EAN, EANx, enriched air, enriched air nitrox: Any mix with more than 21 percent oxygen
• EAD: Equivalent Air Depth. The relationship between nitrogen absorbed at a particular depth breathing nitrox and the depth at which an equivalent absorption rate would occur breathing air.
• PPO2 (or PO2): Oxygen partial pressure, pressure in atmospheres x O2 percentage = PPO2
• MOD or maximum depth: Max depth = *(1.4 ÷ mix%) - 1) X 33
• Best Mix: 1.4 ÷ ((Depth + 33) ÷ 33) = best mix