Oxygen toxicity and nitrox: ppO2 limits and MOD
In short
Oxygen becomes toxic to the brain at high partial pressure and can cause a convulsion underwater, sometimes without warning. Recreational nitrox divers keep ppO2 at or below 1.4 bar (ata), with 1.6 as a contingency maximum. At 1.4, the maximum operating depth (MOD) of EAN32 is 33 m (111 ft), and of EAN36, 28 m (95 ft).
The wall drops away below you, and the big fish always seem a little deeper. On EAN32, your floor is 33 m (111 ft). Below it, you pass the oxygen limit nitrox divers keep to, and you risk a convulsion that can come with no warning.
What is nitrox, and why do divers use it?
Nitrox, or enriched air, is a breathing gas with more oxygen than the 21% in air. The most common blends are EAN32 (32% oxygen) and EAN36 (36%). Less nitrogen means your body absorbs less of it, so you get longer no-stop times, as long as your computer is set for the mix. Nitrox doesn’t let you go deeper (a common myth), and it doesn’t remove the risk of decompression sickness.
What is oxygen toxicity?
Oxygen toxicity is damage caused by breathing oxygen at a high partial pressure. It takes two forms:
- Central nervous system (CNS) toxicity, the one that matters on nitrox dives. It can cause a convulsion. Underwater, a convulsion can mean losing your regulator and drowning.
- Pulmonary (lung) toxicity, from long exposures. DAN describes it as rarely permanent, and it’s rare on recreational no-stop dives.
The risk grows with partial pressure and time. NOAA adds hard work, carbon dioxide buildup and cold or heat stress to the list of factors.
What ppO2 limits do nitrox divers use?
Recreational nitrox divers keep the partial pressure of oxygen (ppO2) at or below 1.4 bar (ata) during the dive and treat 1.6 as an absolute maximum, for contingencies only. DAN calls 1.4 or less the “green light” zone for open-circuit scuba, 1.4 to 1.6 the “yellow light” zone, and above 1.6 a level recreational divers should not exceed. Some operators set lower limits.
The formula comes from Dalton’s law: ppO2 = oxygen fraction × absolute pressure. EAN32 at 30 m (99 ft), where the pressure is 4 bar (4 ata), gives 0.32 × 4 = 1.28.
How do you calculate the maximum operating depth (MOD)?
The MOD is the depth where your ppO2 reaches your limit. Divide the limit by the oxygen fraction, subtract 1, and multiply by 10 m (or 33 ft). Always round down.
Worked example
MOD of EAN32 at a ppO2 of 1.4
| Step | Metric | Imperial |
|---|---|---|
| 1. Pressure where ppO2 reaches 1.4 | 1.4 ÷ 0.32 = 4.375 bar | 1.4 ÷ 0.32 = 4.375 ata |
| 2. Remove the atmosphere | 4.375 − 1 = 3.375 | 4.375 − 1 = 3.375 |
| 3. Turn it into depth | 3.375 × 10 = 33.75 m | 3.375 × 33 = 111.4 ft |
| 4. Round down | 33 m | 111 ft |
The same steps give the other common values:
| Mix | MOD at ppO2 1.4 | Contingency depth at 1.6 |
|---|---|---|
| EAN32 | 33 m (111 ft) | 40 m (132 ft) |
| EAN36 | 28 m (95 ft) | 34 m (113 ft) |
Rounding down keeps you on the safe side. Use fractions (0.32), not percentages (32), and the oxygen percentage you analyzed yourself. All the formulas are in the dive theory formulas guide.
Try a question from the app
maximum operating depth
What is the maximum operating depth of EAN32 at a ppO2 of 1.4 bar?
Show the answer
Answer C. 1.4 ÷ 0.32 = 4.375 bar; minus 1 = 3.375; × 10 = 33.75 m, rounded down to 33 m. 40 m is its contingency depth at 1.6 bar, and 28 m is the MOD of EAN36.
What are the warning signs of oxygen toxicity?
NOAA teaches the signs of CNS oxygen toxicity with the memory aid VENTID-C:
- Vision: tunnel vision, blurred vision.
- Ears: ringing or roaring.
- Nausea, sometimes vomiting.
- Twitching and tingling, often of the lips or face.
- Irritability, confusion, anxiety.
- Dizziness, clumsiness, unusual fatigue.
- Convulsions.
Both NOAA and DAN warn that a convulsion can come with no warning sign at all. If any sign appears, signal your buddy, ascend shallower together to lower your ppO2, and end the dive with a controlled ascent. A diver who convulses underwater needs a rescue: call emergency services once you’re out, and follow your training.
Try a question from the app
oxygen toxicity signs
On a nitrox dive near your MOD, your lips start twitching and your ears ring. What do you do?
Show the answer
Answer C. These can be warning signs of oxygen toxicity, which can lead to a convulsion. Reduce the ppO2 at once by ascending, with your buddy close, and end the dive. Your buddy’s gas is probably the same mix.
How do you dive nitrox safely?
- Get trained first. Nitrox is a specialty course, and one of the elective dives in Advanced Open Water.
- Analyze your own cylinder before every dive, even if the shop already did: DAN advises a check that doesn’t depend on the filling station. Label it with the mix, the MOD, the date and your name.
- Set your computer to the analyzed oxygen percentage and your ppO2 limit. A wrong setting is risky both ways: too low, its MOD alarm comes far too deep; too high, it gives you no-stop time your body doesn’t have.
- Watch your oxygen exposure, which your computer tracks, often as a percentage of a limit, especially over several dives a day.
- Don’t count on nitrox against narcosis. Oxygen is probably narcotic too: see nitrogen narcosis.
Which dive training levels cover nitrox and oxygen toxicity?
Advanced Open Water introduces nitrox: ppO2 limits, MOD, analysis and computer settings. Divemaster adds Dalton’s law, the best mix for a planned depth and oxygen exposure over time. The Advanced Open Water practice questions include the ppO2 limits.
Frequently asked questions
What is the MOD of EAN32?
At a ppO2 of 1.4, the MOD of EAN32 is 33 m (111 ft): 1.4 ÷ 0.32 = 4.375, minus 1, times 10 m or 33 ft, rounded down. Its contingency depth at 1.6 is 40 m (132 ft), which is a limit for emergencies, never a target.
What is the MOD of EAN36?
At a ppO2 of 1.4, the MOD of EAN36 is 28 m (95 ft): 1.4 ÷ 0.36 = 3.89, minus 1, times 10 m or 33 ft gives 28.9 m or 95.3 ft, rounded down. The richer the mix, the shallower its maximum depth.
Can you get oxygen toxicity on air?
Not within recreational limits. On air, ppO2 reaches 1.4 only around 56 m (186 ft), far deeper than the 40 m (130 ft) limit of recreational diving; NOAA gives 186 ft. With nitrox, you reach 1.4 much shallower, which is why every nitrox dive starts with the MOD.
What is the best nitrox mix for a 30 m dive?
Divide 1.4 by the absolute pressure at your planned maximum depth and round down. At 30 m (99 ft), the pressure is 4 bar (4 ata): 1.4 ÷ 4 = 0.35, so EAN35 at most. A richer mix means less nitrogen but a shallower limit. Plan with the mix you actually analyzed.
