The new NOAA guideline on CNS oxygen toxicity for rebreather diving

NOAA guidelines for limiting oxygen toxicity in diving date back to the 1970s and 1980s – a time when rebreather technology, digital sensors and today's diving profiles were still a long way off. Nevertheless, these early, deliberately conservative values formed the basis of all safety guidelines for decades.

With the current NOAA revision, these limits are now being fundamentally revised for the first time. The new recommendations on CNS oxygen toxicity take into account current practical experience and modern diving procedures, thus bringing scientific theory and operational reality back into line.

This adjustment was long overdue – and it changes how oxygen exposure in rebreather diving will be understood, planned and taught in the future. The background, scientific basis and practical consequences of this revision are examined in more detail below.

Lake Marmorera. Foto: Karin Aggeler

The origins of oxygen toxicity

Modern diving safety standards are based on research conducted by the US Navy and NOAA (National Oceanic and Atmospheric Administration) in the 1970s and 1980s. These studies investigated how long humans can safely breathe under elevated partial pressure of oxygen before the risk of oxygen toxicity to the central nervous system (CNS) arises – an acute, potentially fatal complication that can lead to drowning due to epileptic seizures.

However, the roots of this research go back further into the 20th century. Physiologist John Scott Haldane and his ‘chamber divers’ conducted pioneering studies on the physiological effects of pressure and oxygen in London before and during World War II. This work, which was recently revisited in Rachel Lance's book (2024), formed the scientific basis for oxygen toxicity limits.

On this basis, the US Navy and NOAA developed quantitative exposure tables over the decades, which culminated in the NOAA Oxygen Exposure Tables in 1991. These set time limits for inspiratory oxygen partial pressures (PO₂) between 0.5 and 1.6 bar. For the target value of 1.3 bar commonly used in rebreather diving today, a limit of 180 minutes per dive and 210 minutes per day applied.

These values were derived from hyperbaric chamber tests with resting subjects and were deliberately chosen to be conservative. They combined aspects of CNS and pulmonary toxicity in a common framework. While they were considered cautious and appropriate at the time, modern diving practice has long since overtaken these assumptions.

The rise of rebreathers and the limitations of outdated data

Over the past three decades, closed-circuit rebreathers (CCRs) have fundamentally changed technical and scientific diving. By maintaining a constant PO₂ (typically 1.3 ATA), they enable significantly longer bottom times and optimised decompression.

Practical experience shows that divers regularly exceed the 1991 exposure limits – sometimes for hours – without documented cases of CNS oxygen toxicity. Thousands of diving hours, including deep-sea expeditions and extended decompression dives, prove that the original limits were too restrictive for modern CCR use.

These empirical findings ultimately led the NOAA to review the data and initiate a revision of its long-standing guidelines.

A contemporary revision of the NOAA guideline

Following a specialist workshop initiated by the NOAA, a multidisciplinary group of experts led by Hoyt et al. (2025) published an updated guideline on CNS oxygen exposure last month.

Based on historical studies and current field data, the authors propose the following new limits for the standard PO₂ target value of 1.3 bar:

• Up to 240 minutes during active or work-intensive phases, and

  
  

• an additional 240 minutes during rest or decompression phases (defined as respiratory minute volume < 22.5 l/min).

This increases the permissible total exposure to 480 minutes (8 hours) per 24 hours without any increased risk of CNS toxicity being observed. The limit is thus well above the previously valid limits for the same oxygen partial pressure (1.3 bar).

This modernisation is unanimously supported by experts from military, commercial and technical diving, confirming what operational experience has suggested for years: CNS oxygen toxicity at 1.3 ATA is extremely unlikely as long as the PO₂ value remains stable, CO₂ retention is low and physical exertion is kept moderate.

CNS and pulmonary oxygen toxicity: a differentiated approach

The original NOAA tables combined CNS and pulmonary toxicity in a single set of limits. The 2025 guideline now separates these consistently: the CNS limits are redefined, while the pulmonary guidelines remain essentially unchanged.

Since the new CNS exposure times are below the known thresholds for pulmonary toxicity, the risk of lung injury remains minimal. Nevertheless, divers who are exposed to high oxygen partial pressures for longer periods or repeatedly should continue to monitor pulmonary toxicity in order to keep track of cumulative exposure.

In summary:

• The new guidelines apply exclusively to CNS toxicity, not to pulmonary limits.

• They apply only to an oxygen partial pressure of 1.3 bar.

• The risk of pulmonary toxicity remains low, reversible and controllable through monitoring.

Strategies for minimising oxygen toxicity in the CNS

Prevention of oxygen toxicity in the central nervous system is based on precise control of PO₂ and minimisation of physiological stress factors. The updated guidelines emphasise in particular:

1. Ensuring technical reliability:

   Malfunctions of rebreathers can cause PO₂ spikes. Regular maintenance, sensor monitoring and system testing are essential.

   
   

2. Avoiding CO₂ retention:

   Elevated CO₂ levels increase CNS toxicity through increased cerebral blood flow. Fresh lime, adjusted workload and steady breathing are essential.

   
   

3. Limiting workload and thermal stress:

   Excessive exertion or exposure to cold increases metabolic stress. High PO₂ set points should be limited to rest periods whenever possible.

Comment

The revision – more precisely, the introduction of specific guidelines for rebreather diving with a standard O₂ set point of 1.3 bar – marks a milestone, as it is the first fundamental adjustment of CNS oxygen limits in over 30 years. The new limits now allow for significantly longer dive times, but in practice these are likely to be far beyond the usual range of use for the majority of rebreather divers.

It should be emphasised once again that the so-called ‘CNS clock’ of oxygen toxicity, on which the previous limits are based, has a very weak scientific foundation. In fact, the concept is based on limited data and a highly simplified model of physiological reality. In the form in which it has been propagated by the NOAA for decades, this ‘clock’ has probably never really existed. A critical analysis of the clock can be found here.

An interesting side note: the new recommendations define a resting phase (e.g. during decompression) as a respiratory minute volume of less than 22.5 l/min. Many recreational technical divers will probably not exceed this value even during the ‘working phase’ of their dive. Accordingly, the actual risk of CNS toxicity in typical rebreather practice is likely to be even lower than the guidelines suggest – which focus more on high-workload operations, such as in a military setting

However, it is important to note that this revision applies exclusively to the specified oxygen partial pressure of 1.3 bar. Extrapolation downwards – and especially upwards – is not permitted. Higher PO₂ values remain insufficiently understood in terms of their toxicity dynamics. Accordingly, it is important to refrain from deriving longer exposure times for higher oxygen partial pressures (e.g. up to 1.6 bar) from the new limit values.

The NOAA will include these guidelines in the next official diving manual. It seems that several course documents for rebreather diving will need to be adapted.