Two kinds of oxygen toxicity
Central nervous system (CNS) oxygen toxicity affects the brain. It can happen quickly when oxygen partial pressure is high, and its most dangerous sign is a convulsion. A convulsion underwater is very likely to cause the diver to lose their regulator and drown. This is the type that matters most to recreational divers.
Pulmonary oxygen toxicity affects the lungs. It builds up over long exposures to raised oxygen, such as long decompression dives or hours of chamber treatment, and causes chest irritation, coughing and reduced lung function. Technical divers track it with units called OTUs.
Partial pressure
What matters is not the percentage of oxygen in a gas but its partial pressure (PO2): the oxygen fraction multiplied by the absolute pressure at depth. At the surface, air has a PO2 of 0.21 bar. At 30 m (4 bar absolute) it is 0.84 bar. The deeper you go and the richer the mix, the higher the PO2.
Training agencies use two limits:
- 1.4 bar as the maximum for the working part of a dive.
- 1.6 bar as an absolute maximum, used for contingency planning and, in technical diving, for resting decompression stops.
Maximum operating depth
Each gas mix has a maximum operating depth (MOD): the depth at which its PO2 reaches the limit. You find it by dividing the PO2 limit by the oxygen fraction, which gives the absolute pressure, then converting that to depth.
| Gas | MOD at 1.4 bar | MOD at 1.6 bar |
|---|---|---|
| Air (21%) | About 57 m (186 ft) | About 66 m (217 ft) |
| Nitrox 32 | About 34 m (111 ft) | About 40 m (132 ft) |
| Nitrox 36 | About 29 m (95 ft) | About 34 m (113 ft) |
| Nitrox 40 | About 25 m (82 ft) | About 30 m (99 ft) |
For air, nitrogen narcosis and the usual 40 m recreational limit come long before oxygen toxicity. For enriched air nitrox, the oxygen limit is the one that counts, which is why every nitrox tank is labelled with its MOD.
Warning signs
CNS oxygen toxicity can strike without warning. When warnings do come, they can include twitching (often of the lips or face), visual changes such as tunnel vision, ringing in the ears, nausea, irritability, dizziness and unusual anxiety. Divers learn the memory aid "ConVENTID": convulsions, vision, ears, nausea, twitching, irritability, dizziness. If any appear, ascend to reduce the PO2 at once, signal your buddy and end the dive.
Exposure time, cold, hard work and raised carbon dioxide from heavy breathing are thought to increase the risk.
Prevention
- Analyse every nitrox tank yourself and label it with the oxygen percentage and MOD.
- Set your dive computer to the right mix and a PO2 limit of 1.4 bar.
- Plan the dive's deepest point well inside the MOD.
- Keep exertion low and breathe slowly and deeply.
- For deep technical dives, use trimix with less oxygen and track oxygen exposure across the whole dive.
Frequently asked questions
Can you get oxygen toxicity diving on air?
Only at depths far beyond recreational limits. Air reaches a PO2 of 1.4 bar at about 57 m (186 ft).
Why is oxygen toxic?
At high partial pressures, oxygen produces reactive molecules faster than the body can neutralise them, which affects the brain and lungs.
What is the CNS clock?
A percentage many dive computers show to track oxygen exposure against published limits over a dive or a day. 100 percent means you've reached the limit.