What nitrox is
Nitrox is any mix of nitrogen and oxygen. In everyday diving the word means enriched air: a mix with more than the 21 percent oxygen found in air. Recreational nitrox usually contains between 22 and 40 percent oxygen. The two most common blends are EAN32 (32 percent oxygen) and EAN36 (36 percent oxygen). "EAN" stands for enriched air nitrox, and the number is the oxygen percentage.
The main benefit
Decompression limits depend on how much nitrogen your body absorbs. Nitrox contains less nitrogen than air, so you absorb less on each dive and can stay longer before reaching your no-decompression limits. The gain is largest in the range from about 18 to 30 m (60 to 100 ft), where on air the limits are short.
Divers sometimes describe this using equivalent air depth (EAD): the depth on air that would give the same nitrogen loading. For example, EAN32 at 30 m has roughly the nitrogen loading of air at about 24 m. Many divers also use nitrox with their computer set to air, which keeps air limits but adds an extra safety margin.
The main limit: oxygen
More oxygen brings a shallower maximum depth. Oxygen becomes toxic at high partial pressure, as explained in oxygen toxicity, so every nitrox mix has a maximum operating depth (MOD).
| Mix | MOD at PO2 1.4 bar | MOD at PO2 1.6 bar |
|---|---|---|
| EAN32 | About 34 m (111 ft) | About 40 m (132 ft) |
| EAN36 | About 29 m (95 ft) | About 34 m (113 ft) |
| EAN40 | About 25 m (82 ft) | About 30 m (99 ft) |
Exceeding the MOD raises the risk of a convulsion underwater, which can be fatal. The MOD for the planned mix is written on the tank and set in your dive computer.
Myths about nitrox
- "Nitrox is for deep diving." The opposite: its higher oxygen content makes it unsuitable for deep dives.
- "Nitrox stops narcosis." Oxygen is thought to be narcotic too, so nitrox is generally not considered a cure for nitrogen narcosis.
- "Nitrox makes you less tired." Many divers say they feel better after nitrox dives, but studies have not shown a clear effect.
Using nitrox safely
- Take an enriched air course. Dive centres won't fill nitrox for divers without certification.
- Analyse your own tank with an oxygen analyser before every dive, and sign the fill log.
- Label the tank with the oxygen percentage, the MOD and your name.
- Set your dive computer to the analysed mix.
- Plan the dive well inside the MOD and keep watch on your depth.
Equipment used with nitrox up to 40 percent oxygen generally doesn't need special cleaning in most regions, but rules and manufacturers' guidance vary. Tanks filled by some blending methods must be cleaned for oxygen service. Ask the dive centre.
Frequently asked questions
Is nitrox safer than air?
Nitrox reduces nitrogen loading, which can add a margin against decompression sickness, but it brings an oxygen depth limit. It is only safer when used correctly.
What does EAN32 mean?
Enriched air nitrox with 32 percent oxygen and 68 percent nitrogen.
Do I need a special regulator for nitrox?
For mixes up to 40 percent, most modern regulators are approved by their makers. Check the manufacturer's guidance for your model.