CNS Percentage and OTU: Tracking Oxygen Exposure

Maximum operating depth answers “how deep”. It says nothing about how long. Oxygen exposure is tracked on two separate clocks, because acute nervous-system toxicity and cumulative lung irritation respond differently to partial pressure and time.

Two clocks, two injuries

• CNS percentage tracks central nervous system oxygen toxicity. It is acute and dominated by partial pressure. Its worst outcome is a convulsion, which underwater can be fatal regardless of how healthy the diver is.
• OTU, the oxygen tolerance unit, tracks pulmonary oxygen toxicity. It is cumulative and dominated by duration. Its outcome is lung irritation — burning on inhalation, coughing, reduced vital capacity — which is unpleasant and reversible rather than immediately dangerous.

A short exposure to a high partial pressure loads CNS heavily and OTU barely. Hours at a mildly raised partial pressure do the reverse. They are genuinely independent budgets, which is why serious planning carries both.

How the CNS clock works

Reference tables — most familiarly the NOAA oxygen exposure limits — give a maximum single-exposure duration for each oxygen partial pressure. Time spent at a given partial pressure is expressed as a percentage of that allowance, and the percentages from each part of the dive add up.

The allowance falls steeply as partial pressure rises, and that steepness is the point. This is why the difference between a 1.4 and a 1.6 ATA ceiling is not a small one, and why 1.6 is generally treated as contingency rather than a working level. The mechanics of the ceiling itself are covered in PPO₂ safety limits.

Reaching 100% does not mean a convulsion occurs. It means you have spent the budget the tables allocate, and the underlying data on human CNS toxicity is limited and variable — susceptibility differs between divers and between days, and carbon dioxide retention, exertion, cold, and immersion all appear to matter. Prudent practice plans well below the limit rather than treating it as usable capacity.

What OTU tracks

OTU accumulates as a function of oxygen partial pressure and time, with exposure counting from roughly 0.5 ATA upward. Unlike CNS, the number grows steadily rather than steeply, so it is long dives and repeated days that build it rather than brief deep moments.

Recreational single-day Nitrox diving rarely approaches pulmonary limits, and most recreational computers display CNS rather than OTU. OTU becomes a real constraint on extended exposures, on consecutive days of oxygen-rich diving, and in technical and decompression diving, where daily and multi-day allowances are planned explicitly.

Accumulating across a diving day

Neither clock resets on surfacing. CNS loading decays — a common planning approximation halves it roughly every 90 minutes on the surface — so a surface interval reduces accumulated exposure without clearing it. OTU decays more slowly still, which is what makes multi-day trips the relevant case.

This runs in parallel with inert gas accumulation, on a completely separate schedule. A day of diving can be comfortable on nitrogen and tight on oxygen, or the reverse, and a rich mix improves one while worsening the other. That trade is the whole reason both get tracked; see surface intervals for the inert gas half.

Planning against the limits

In practice: choose the leanest mix that meets the dive rather than the richest the depth permits, since the rich mix costs oxygen budget for decompression benefit you may not need. Set your computer to the analyzed mix so its tracking is real. Keep workload low, because carbon dioxide retention appears to raise CNS susceptibility. Add conservatism across consecutive diving days, and treat any oxygen-toxicity symptom as a reason to end the exposure rather than something to push through.

A commonly taught planning target is to keep a single dive below80% CNS and the diving day below 100%, leaving margin for delays and individual variability. Your agency, computer, or technical-diving procedure may specify a lower limit.

The mix decision is where most of the leverage sits — the best-mix guide covers why the richest theoretical mix is often not the right operational one. Use the MOD calculator to verify the measured mix's depth ceiling before tracking its exposure.