What is Nitrox? A Complete Guide for Divers

Nitrox, also known as Enriched Air Nitrox (EAN or EANx), is a breathing gas mixture that contains a higher percentage of oxygen than standard air. While normal air is approximately 21% oxygen, 78% nitrogen, and about 1% argon and other trace gases, Nitrox typically contains between 22% and 40% oxygen with a corresponding reduction in nitrogen.

How Does Nitrox Work?

The key principle behind Nitrox is simple: by increasing the oxygen percentage in your breathing gas, you decrease the nitrogen percentage. Lower nitrogen exposure can lengthen no-decompression limits and improve repetitive-dive options. It does not make the mix meaningfully less narcotic at a given depth; the narcosis guide owns that separate question. For the wider context, compare Nitrox with air, oxygen, Trimix, Heliox, and Argon in the diving gases guide.

The two most common Nitrox mixes are:

• EAN32 — 32% oxygen, 68% nitrogen. The most popular recreational Nitrox mix, offering a good balance of extended bottom time and usable depth range.
• EAN36 — 36% oxygen, 64% nitrogen. Provides even longer no-decompression limits but with a shallower maximum operating depth.

Maximum Operating Depth

Every Nitrox mix has a Maximum Operating Depth (MOD): the oxygen-based ceiling for that mix and selected PPO₂ limit. More oxygen means a shallower MOD, so the same change that reduces nitrogen loading also narrows the usable depth range.

The dedicated MOD guide owns the formula, worked arithmetic, reference table, rounding convention, and calculator. Use the full MOD calculator for a measured mix rather than copying a memorised depth.

Benefits of Diving with Nitrox

• Longer no-decompression limits. By absorbing less nitrogen at depth, you can stay longer before reaching your NDL. The exact gain belongs to the table or computer model you are using. See the NDL guide for published depth examples and algorithm differences.
• Potentially shorter surface intervals. When the dive profile leaves you with less modeled nitrogen loading, a planning table or computer may permit a shorter interval before a repetitive dive. Nitrox does not make your tissues off-gas faster.
• Potential decompression margin. Nitrox can be used to bank margin or to extend time. The EAD guide explains that planning choice and its limits.

Many divers also say they feel less tired after Nitrox dives. Controlled studies have not confirmed it: a small double-blind chamber trial comparing EAN36 with air at 18 m59 ft found no measurable difference in post-dive fatigue, and a Divers Alert Network review found no reliable evidence for the effect. Choose a mix for its decompression and depth consequences, not for an expected reduction in tiredness.

Analyzing and Labeling a Mix

1. Calibrate the analyzer. In clean air it should read 20.9%; follow the instrument's instructions and local procedure.
2. Sample a steady, low flow. Crack the valve or use a flow restrictor, let the reading stabilize, and avoid blasting gas directly across the sensor.
3. Verify the result. Re-check an unstable or unexpected reading. Many operators accept a fill within ±1 percentage point of the ordered mix, but plan from the measured value, not the order.
4. Label and configure. Record the measured O₂ percentage, its MOD at the chosen PPO₂, the date, and your initials; then set the computer to that measured mix.

Risks and Considerations

• Shallower oxygen-based maximum depth. Higher oxygen percentages mean shallower MODs. Using the seawater approximation of 10 m33 ft per atmosphere and rounding down, the PPO₂ calculation for air produces 56m187ft (at 1.4 ATA PPO₂), EAN32 is limited to about 33m111ft and EAN36 to about 28m95ft. The air figure is not a recommended diving depth: training limits, narcosis, gas density, gas supply, and decompression obligations impose other constraints. Exceeding the selected MOD increases oxygen-toxicity risk.
• Gas analysis is mandatory. You must analyze your tank before every dive to verify the actual oxygen percentage and calculate the correct MOD. Assuming the mix without verification can be deadly.
• Equipment considerations. Oxygen-service requirements depend on how the gas is made and which components it touches, not only on the final percentage. A cylinder filled by partial-pressure blending receives pure oxygen before the air top-up, so it must be oxygen-clean even when it finishes as EAN32. Divers Alert Network advises that components exposed to more than about 25% oxygen in normal use be oxygen-compatible and oxygen-clean; some regulations and shops use 40% instead. Follow the equipment manufacturer and fill station, and see the Nitrox blending guide for why the fill method matters.

Getting Started with Nitrox

To dive with Nitrox, you need to complete an Enriched Air Nitrox specialty certification from a recognized training agency. The course covers gas analysis, MOD calculations, oxygen exposure tracking, and Nitrox dive planning. Once certified, you can rent or fill Nitrox tanks at most dive centers worldwide.

Use our Nitrox calculator to practice calculating Maximum Operating Depths for different gas mixes and PPO₂ limits, or to find the best mix for a planned depth.