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. Since nitrogen is the primary gas responsible for decompression sickness (DCS) and nitrogen narcosis, reducing your nitrogen exposure allows for longer no-decompression limits (NDLs) and shorter surface intervals.
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 deepest you can safely go before the partial pressure of oxygen (PPO2) exceeds safe limits. The MOD formula is:
Where is the fraction of oxygen (e.g., 0.32 for EAN32) and PPO2 is your chosen oxygen limit in ATA.
For example, at the standard recreational PPO2 limit of 1.4 ATA:
- EAN32: MOD = (1.4 / 0.32 − 1) × 10 = 33.8m
- EAN36: MOD = (1.4 / 0.36 − 1) × 10 = 28.9m
- Air (21%): MOD = (1.4 / 0.21 − 1) × 10 = 56.7m
Try it: MOD Calculator
Benefits of Diving with Nitrox
- Longer no-decompression limits. By absorbing less nitrogen at depth, you can stay longer before reaching your NDL. The advantage is significant — at 18m, EAN32 can give you roughly 45–50% more bottom time compared to air.
- Shorter surface intervals. Less nitrogen absorption means faster off-gassing during surface intervals, letting you get back in the water sooner on multi-dive days.
- Reduced fatigue. Many divers report feeling less tired after Nitrox dives compared to air dives at similar depths and durations. While scientific evidence is limited, this is a widely reported anecdotal benefit.
- Added safety margin. Even if you plan your dive using air tables, breathing Nitrox provides an extra buffer against decompression sickness due to reduced nitrogen loading.
No-Decompression Limits: Air vs Nitrox
Approximate NDLs in minutes. Based on DSAT recreational tables for illustration. Always use your dive computer for actual planning.
| Depth | Air (21%) | EAN32 | EAN36 |
|---|---|---|---|
| 12m | 130 min | 200 min(+54%) | 200 min |
| 15m | 80 min | 125 min(+56%) | 163 min |
| 18m | 56 min | 82 min(+46%) | 108 min |
| 21m | 40 min | 57 min(+43%) | 72 min |
| 24m | 30 min | 40 min(+33%) | 50 min |
| 27m | 20 min | 30 min(+50%) | 37 min |
| 30m | 16 min | 22 min(+38%) | 28 min |
| 33m | 13 min | 17 min(+31%) | Exceeds MOD |
Risks and Considerations
Oxygen Toxicity
The primary risk of Nitrox is oxygen toxicity. At depth, the partial pressure of oxygen increases. If PPO2 exceeds safe limits (typically 1.4 ATA for recreational diving), Central Nervous System (CNS) oxygen toxicity can cause convulsions — which underwater can be fatal. Read more in our PPO2 Safety Limits guide.
- Shallower maximum depth. Higher oxygen percentages mean shallower MODs. While air can theoretically be breathed to56.7m (at 1.4 ATA PPO2), EAN32 is limited to about 33m and EAN36 to about 29m. Exceeding MOD is extremely dangerous.
- 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. Tanks used for mixes above 40% oxygen require oxygen-clean equipment. Standard recreational Nitrox mixes (up to 40%) can typically use standard scuba equipment, though practices vary by region and operator.
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 MOD Calculator to practice calculating Maximum Operating Depths for different gas mixes and PPO2 limits.