Oxygen Toxicity

Harmful effects of breathing oxygen at high partial pressure; the acute CNS form can cause a sudden underwater convulsion, so divers cap PO2 at 1.4 bar.

Oxygen toxicity occurs when the partial pressure of oxygen (PO2) in the breathing gas gets high enough to damage the body. Divers worry mainly about the central nervous system (CNS) form, which can strike with little warning at PO2 above roughly 1.4 to 1.6 bar and culminate in a full convulsion. A seizure on the surface is survivable, a seizure underwater usually means a lost regulator and drowning, which is why oxygen limits are among the hardest rules in diving.

This is the entire reason nitrox has depth limits. Recreational agencies cap working PO2 at 1.4 bar, so EAN32 has a maximum operating depth of 33.7 m (110 ft), with 1.6 bar reserved as a contingency or for resting decompression stops on pure oxygen at 6 m. Warning signs are remembered as VENTID: visual disturbance, ears ringing, nausea, twitching, irritability, dizziness, but a convulsion can arrive first. Exertion, cold, and CO2 retention raise the risk.

A slower pulmonary form, tracked as oxygen tolerance units, irritates the lungs after long cumulative exposures and matters mostly for technical, rebreather, and hyperbaric chamber exposure. For everyday nitrox diving the discipline is simple: analyze your gas, mark the tank, set your dive computer to the real mix, and respect the MOD you calculated. Repetitive high-PO2 days add up too, which is why nitrox divers on liveaboards keep an eye on the CNS percentage their computer tracks.