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  • AutorenbildMichael Mutter

Oxygen toxicity - a critical view of the "CNS-clock"

The CNS toxicity of oxygen represents a significant risk for recreational and technical diving. The "CNS-clock" is used to calculate this during a dive. However, its pathophysiological basis is very weak.

Photo: Patrick Oswald, Tauchschule H2O

The oxidation of oxygen to produce energy-rich phosphates is controlled by nature in very small steps in the respiratory chain. Nevertheless, highly reactive oxygen compounds (free oxygen radicals, peroxides, etc.) are produced as by-products, which would damage the organism. Nature has developed a large armamentarium to control them. Under hyperbaric conditions, however, they seem to overwhelm the defenses and thus damage the cells directly, although the exact mechanism of oxygen toxicity is unknown.

In diving, 2 types of oxygen toxicity play a role:

  • Pulmonary toxicity

  • CNS toxicity

Pulmonary toxicity has no significance for recreational diving, as it only occurs after very long exposure to O2. The situation is quite different with CNS toxicity. As early as the end of the 19th century, Paul Bert discovered that oxygen inhaled under pressure leads to epileptic seizures. This can occur without warning during diving as the first symptom of CNS toxicity. In SCUBA diving, it leads to certain death by drowning and must therefore be prevented at all costs.

Translated with (free version)


An epileptic seizure triggered by O2 (video) can occur without warning.

Donald published his studies on oxygen toxicity in British navy divers in 1947. He found that CNS toxicity varies greatly in the same individual and between different divers and is subject to various influencing factors, and recommended diving no deeper than 8 m with pure oxygen (O2 partial pressure max. 1.8 bar).

Factors that favor CNS toxicity are (non-exhaustive list):

  • Heavy exertion

  • Cold

  • Increased CO2

  • "Wet" dives (in contrast to "dry" dives in pressure chambers)

  • Fatigue, sleep deprivation

  • Various medications

Further research showed that it was possible to dive for up to 4 h without symptoms at an O2 partial pressure of 1.7 bar, and for less time at a higher partial pressure. The duration of exposure therefore appears to play a certain role in CNS toxicity. As mentioned, however, CNS toxicity does not necessarily develop gradually, but can occur as an "all-or-nothing event" in the form of an epileptic seizure without warning. Unfortunately, the time factor is too uncertain to reliably rule out such an event at O2 partial pressure values above 1.6 bar. Therefore, diving with oxygen is limited to partial pressure values below which CNS symptoms do not occur at any time.

Diving is safe up to 1.4 bar O2 partial pressure with regard to CNS toxicity. A maximum oxygen partial pressure of 1.6 bar is permitted on the decompression stages, provided that no exertion is made.

For compressed air this means a maximum diving depth of 56 m (max. O2 partial pressure 1.4 bar), for nitrox diving accordingly lower depths (e.g. with EAN 28 max. 40 m). This recommendation must be strictly followed.


What is the "CNS clock" for? - This question is more than legitimate.

The "units of pulmonary toxicity dose" (UPTD) are used to determine pulmonary oxygen exposure. 1 UPTD corresponds to the damage caused by breathing 100% oxygen for 1 minute at 1 bar and can be measured with lung function tests to a limited extent, but relatively easily. The concept was subsequently expanded. The "oxygen toxicity unit" or "oxygen tolerable unit" (OTU) was created, which is also used for repetitive and multi-day exposures (REPEX guidelines) and is intended to measure whole-body exposure. This is meant literally, as the concept has not been validated on divers.

The NOAA's "CNS-clock" is intended to provide a guideline for estimating the oxygen exposure of the brain and spinal cord. It is an attempt to transfer the concept of lung toxicity to the CNS, taking into account the observation that it was possible to dive with an O2 partial pressure of 1.7 bar for up to 4 h without signs of CNS toxicity. In plain language, it is a mishmash of 2 fundamentally contradictory concepts: the predictability of pulmonary toxicity and the unpredictability of CNS toxicity.

The "CNS-clock" is a synthesis of two contradictory concepts.

It therefore contains a very large safety margin (maximum allowable exposure 1.6 bar over 45 minutes). These limits are often expressed as a "CNS percentage" or "%CNS" by dividing the time during which a given partial pressure of oxygen is breathed by the NOAA limit for that pressure and adding the fractions to form a percentage.

O2 partial pressure


Dive time


Dive 1

1.4 bar

150 min.

75 min.


Dive 2

1.3 bar

180 min.

60 min.

33 %


83 %

Hypothetical example for calculating CNS toxicity in % of the upper limit (based on NOAA). Dive 1 consumes 50% of the permitted O2 exposure, dive 2 33%.

Computers calculate "CNS%" during a dive by continuously accumulating the fractions. When 100% is reached, the limit for CNS oxygen exposure is hit.

There are rules of conduct for exploiting the limits in particular and the surface intervals during repetitive dives in general, which may differ depending on the diving organization.

There is no scientific basis for the "O2 half-time" during surface intervals.


The assumption of an O2 half-life of 90 minutes during a surface interval is generally accepted. The CNS percentage at the end of a dive after a 90 minute surface interval would drop by half, i.e. from 50% to 25% in the above example. This would be added to the next dive, i.e. dive 2 would start with 25 "CNS%" instead of 50% and the cumulative dose after both dives would be 58%. However, there is no scientific basis for the oxygen half-life of 90 minutes and to speak of "half-lives" in connection with oxygen, a metabolic gas, is almost painfully unscientific.

The concept of the "CNS clock" is based on a very narrow experimental foundation. It is an "educated guess" based on expert opinions and has hardly been validated on divers, if at all. It rests on a very idealized weighting of the time factor, is derived from the prognosis of pulmonary (!) toxicity and attempts to make a prediction for the CNS based on the duration of whole body exposure. This has little or nothing to do with reality, especially in the range of oxygen partial pressure values above 1.4 bar. As mentioned above, other factors play a role here for the CNS, which invalidate the predictive power of the time factor and thus make it largely useless. One could even argue that the "CNS clock" fundamentally contradicts the unpredictability of CNS toxicity and anyone who has ever seen the results of Donald's study will wonder whether the CNS clock has any right to exist at all.

"CNS-clock": Planning tool but no physiological concept

The "CNS-clock" is therefore not to be understood as a physiological principle, but at best as a theoretical planning aid for dives with oxygen-enriched breathing gases.

In fact, accidents due to oxygen toxicity with partial pressure values in the range of 1.4 to 1.6 bar are known despite compliance with the guidelines. On the other hand, there are no documented incidents with oxygen partial pressure values below 1.3 bar. This means that avoiding too high an absolute oxygen partial pressure value is the decisive factor in preventing oxygen toxicity - regardless of the duration of exposure. In technical diving (e.g. with closed systems), values of 1.3 bar and less apply.

Only one thing applies to recreational diving: limiting the oxygen partial pressure to max. 1.4 bar.


In accordance with current NOAA guidelines for Nitrox diving, it is strongly advised not to exceed the limits of the "CNS-clock", i.e. the use of oxygen partial pressure values above 1.4 bar during the "working phase" of the dive. This does not apply to the decompression stages, where oxygen partial pressures of up to 1.6 bar may be breathed, provided that the recommendations for this are adhered to, such as no or only slight exertion and the possibility of temporarily switching to a mixture with a low O2 content during very long decompression.


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