---
title: "Oxygen exposure"
description: "How the CNS clock and OTU accumulate on a plan, the four thresholds the planner warns at, and what the two numbers do not measure."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Oxygen exposure

import { PlanSummaryDemo } from "@/components/react/PlanSummaryDemo";
import { trimixDeco } from "@/demo/plans";

Oxygen exposure is tracked as two independent numbers because oxygen hurts you
in two unrelated ways. **CNS %** tracks the risk of a central-nervous-system
oxygen toxicity event, a seizure underwater. **OTU** tracks cumulative pulmonary
irritation, which builds over hours and days and does not seize anybody.

## The two clocks

Both are driven by the partial pressure of oxygen you are breathing at every
moment of the dive, not the label on the cylinder. On a rebreather that is the
setpoint the loop is holding.

The CNS clock is a percentage of a single-exposure limit, accumulated minute by
minute against the NOAA table, so 100 % means you have used the whole of the
exposure that table allows at the pressures you have been breathing. Below
**0.50 bar** the clock does not run at all. Between 1.50 and 1.60 bar the
allowance falls by more than half, which is why 1.60 bar is treated as a
decompression-only limit rather than a working one.

Oxygen tolerance units accumulate for any ppO₂ above 0.50 bar, at roughly one
unit per minute at 1.0 bar and about twice that at 1.6 bar. OTU is less
sensitive to pressure than the CNS clock, because pulmonary irritation is about
dose over time rather than peak exposure, so a long shallow nitrox dive can
carry more OTU than a short deep one and less CNS.

## Where the planner warns

The plan carries warnings at four thresholds, and they do not change with the
algorithm; every model reports the same figures against the same limits.

| Condition | Severity | Message |
|---|---|---|
| CNS above 80 % | Warning | `CNS 84% exceeds 80% advisory` |
| CNS above 100 % | Error | `CNS 112% exceeds 100% limit` |
| OTU above 200 | Warning | `OTU 240 exceeds 200 advisory` |
| OTU above 300 | Error | `OTU 340 exceeds 300 single-dive limit` |

The 300 OTU figure is the recommended single-dive limit, and the 200 advisory
sits below it so that a plan flags before it reaches the limit.

In the instrument strip below, **O₂ CNS** carries a bar as well as a number. The
fill is the fraction of the 100 % limit used, and it turns amber at 80 % and red
at 100 %, matching the two warning thresholds. **OTU** is a plain count.

Both figures are for the whole plan, surface to surface, decompression included.
On this dive the deco gas is 50 % taken at 21 m, the shallowest depth at which
that mix reaches 1.6 bar; take it deeper and the ppO₂ passes 1.6 bar, which the
planner raises as an error rather than an advisory. See
[Switch depths](/gases/switch-depths).

## What the two numbers do not measure

Neither clock decays. Within a plan they only accumulate: there is no
surface-interval recovery, no CNS half-time, and no credit for time spent
breathing air. Both are single-dive totals, and a repetitive dive is not
modelled by adding them to a previous plan's.

Neither is a personal threshold. The NOAA limits and the 300 OTU figure are
population recommendations with a large individual spread, and CNS toxicity is
influenced by CO₂ retention, exertion, cold and immersion, none of which enter
the calculation. Divers have seized well below 100 % and dived well past it
without incident: a clock at 60 % is a fraction of a tabulated allowance, not a
probability of anything.

## Next

- [Gas density and narcosis](/theory/gas-density) covers the other numbers a mix
  is judged on.
- [Plan warnings](/planning/warnings) shows how these four messages appear.

Source: https://docs.divelogic.ai/theory/oxygen-exposure/index.mdx