Oxygen keeps us alive, so it feels strange to call it a hazard, yet under high pressure it can become one. When you breathe oxygen at a partial pressure far above normal, as can happen deep on enriched-air mixes, it can irritate the lungs or, more seriously, trigger a convulsion. The good news is that recreational divers who follow depth limits and gas rules almost never encounter it. Understanding oxygen toxicity is mostly about respecting a few simple boundaries.
Partial pressure is the key idea
At the surface, oxygen makes up about 21% of the air and sits at a comfortable partial pressure. As you descend, pressure multiplies, so the oxygen you breathe presses on your body far more strongly than it does on land.
Dive computers and tables track this as the partial pressure of oxygen. Keeping it within a safe ceiling is what prevents toxicity, and it is a number your training teaches you to respect.
The two forms it can take
Central nervous system toxicity is the sudden, serious form. It can appear on very deep dives or on nitrox pushed too deep, and its signs are worth memorizing:
- Twitching, especially of the face or lips
- Tunnel vision or ringing in the ears
- Nausea, dizziness or irritability
- In the worst case, a convulsion underwater
Pulmonary toxicity is the slower form, from very long exposures, and mainly matters to technical and commercial divers, not to recreational ones.
Nitrox: a benefit with a limit
Enriched air, or nitrox, adds oxygen and reduces nitrogen, which lets you stay longer at moderate depths. That is a real advantage, but the extra oxygen means a shallower maximum depth than on regular air.
This is why nitrox divers analyze every tank and set a maximum operating depth. Follow that number and nitrox is safe and comfortable. Ignore it and you approach oxygen toxicity, which is exactly what the limit exists to prevent.
Why recreational divers rarely see it
Standard recreational depth limits keep the partial pressure of oxygen well below the danger zone on ordinary air. You would have to go far deeper than recreational training allows to risk it.
Proper gas analysis, sensible depth planning and honest limits mean the vast majority of divers will never experience a single symptom. It is a hazard you manage on paper long before you ever get near it.
Learn gas planning with us
At Nautilus our PADI instructors teach depth discipline from your very first dive, and our deeper sites like Bushnell are always planned with conservative limits. If nitrox interests you, we can explain how to analyze a tank and set a safe maximum depth.
Come learn the numbers in warm, forgiving water with small groups. Browse our courses and dives and keep reading more guides to build real confidence in how diving works.
Important
This is general educational information, not medical advice. Always complete the medical questionnaire honestly, dive within your training and, if in doubt, consult a diving physician or contact DAN (Divers Alert Network). In an emergency in Colombia, the nearest hyperbaric chamber is in Coveñas.
