CNS% and OTU

Posted 24 June 2026 · Field Note #16

Two oxygen clocks: CNS and OTU

Oxygen is the one gas that helps you both stay down comfortably and decompress faster — richer oxygen mixes speed off-gassing because they leave less room for inert gas. But oxygen is also a drug, and like any drug the dose matters. Breathe it at a high enough partial pressure (ppO₂) for long enough and it turns toxic. Divers track that dose with two separate "clocks," because oxygen can hurt you in two different ways.

The first clock is CNS% — central nervous system toxicity. This is the acute risk, and the frightening one: at high ppO₂, oxygen can provoke a convulsion with little warning. Underwater, a seizure usually means losing your regulator and drowning. CNS load builds faster the higher your ppO₂, which is why agencies cap working ppO₂ (commonly 1.4 for the active part of a dive, with 1.6 a typical ceiling for deco) and why divers keep CNS under 100% on a single dive. CNS partly recovers on the surface between dives, but it still accumulates over a repetitive day, so watch it across dives too. It is a percentage of an estimated limit, not a precise countdown, so treat it conservatively.

The second clock is OTU — oxygen tolerance units, a measure of pulmonary toxicity. This is the chronic risk: prolonged oxygen exposure irritates the lungs, causing chest soreness, coughing and reduced lung capacity over long or repetitive days. A single dive rarely troubles it. Multi-day decompression projects, long oxygen-rich deco hangs, or repetitive closed-circuit diving can stack it up meaningfully, and it clears slowly — over days, not hours.

The root mechanism is the same: oxygen at pressure generates reactive molecules faster than the body neutralises them, stressing nerve tissue acutely and lung tissue cumulatively.

The practical takeaway: for ordinary recreational nitrox dives, neither clock is likely to be your limiting factor — depth and gas usually bite first. But once you push into serious decompression, long deco days, or rebreather projects, track both. Watch CNS% within and across dives, and keep an eye on OTU when the oxygen exposure piles up day after day.

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⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.