Authors: Joseph Hoyt¹, F. Gregory Murphy², Neal W. Pollock^3,4, Dawn Kernagis⁵, Nicholas Bird¹, Michael Menduno², John Bright³, Simon J. Mitchell^4,5,6

• ¹ NOAA Diving Program, National Oceanic and Atmospheric Administration, Seattle, Washington, USA
• ² US Navy Experimental Diving Unit (NEDU), Panama City, Florida, USA
• ³ Department of Kinesiology, Faculty of Medicine, Laval University, Quebec, Canada
• ⁴ Hyperbaric Medicine Service, Quebec Diving Medicine Center, Lévis, Quebec, Canada
• ⁵ DEEP Research Inc., Miami, Florida, USA
• ⁶ Department of Anesthesiology, University of Auckland / Department of Anesthesia, Auckland City Hospital, Auckland, New Zealand

Corresponding Author: Professor Simon J. Mitchell, Department of Anesthesiology, University of Auckland, Private Bag 92019, Auckland, New Zealand. E-mail: sj.mitchell@auckland.ac.nz

Keywords: Oxygen clock; Rebreathers; Risk; Seizures; Technical diving.

Abstract

(Hoyt J, Murphy G, Pollock NW, Kernagis D, Bird N, Menduno M, Bright J, Mitchell SJ. Revised guideline for central nervous system oxygen toxicity exposure limits when using an inspired PO₂ of 1.3 atmospheres. Diving Hyperb Med. 2025;55(3):262–267. doi: 10.28920/dhm55.3.262-267. PMID: 40992383.)

Technical and scientific divers using closed-circuit rebreathers (CCRs) regularly dive with an inspired oxygen partial pressure (PO₂) setpoint maintained at 1.3 atmospheres (ATA). Existing National Oceanic and Atmospheric Administration (NOAA) guidelines for central nervous system (CNS) oxygen toxicity exposure limits define the maximum single-exposure duration at PO₂ = 1.3 ATA as 180 minutes, with a 24-hour limit of 210 minutes. This limit was historically extrapolated from trials conducted at higher PO₂ levels, without considering specific clinical trials conducted by the US Navy Experimental Diving Unit (NEDU). As part of these NEDU trials, hundreds of diver-hours were successfully accumulated under continuous PO₂ = 1.3 ATA exposure lasting up to 240 minutes of exercise, followed by up to 240 minutes of resting decompression at the same PO₂ (totaling up to 480 minutes). No seizures or severe symptomatic oxygen toxicity were recorded.

An expert working group comprising NOAA, NEDU, and diving medicine experts reviewed the current standard. This consensus guideline recommends increasing the allowable exposure limit at PO₂ = 1.3 ATA to 240 minutes of bottom time (under moderate exercise) plus 240 minutes of decompression (at rest), provided there is strict control over carbon dioxide (CO₂) retention, work of breathing, and implementation of risk mitigation measures.

Introduction

CNS oxygen toxicity is one of the most unpredictable and dangerous underwater threats to a diver's life. Acute toxicity manifesting as generalized tonic-clonic seizures can lead to loss of the mouthpiece, water aspiration, and immediate drowning. Historically, exposure time limits for managing CNS toxicity risks were developed by the US Navy and the National Oceanic and Atmospheric Administration (NOAA), establishing the "oxygen clock" concept. To minimize symptom onset risk, each PO₂ level was assigned a time limit for a single dive and a 24-hour period.

For oxygen partial pressure PO₂ = 1.3 ATA — the standard setpoint during the bottom phase in many modern closed-circuit rebreathers (CCR) — the NOAA table sets the following limits:

• Maximum single exposure: 180 minutes;
• Maximum 24-hour exposure: 210 minutes.

Over the past three decades, the duration of deep technical and scientific dives has increased significantly. The 180-minute limit became a major barrier to extended research and exploration expeditions. Consequently, a scientific revision of this parameter based on current experimental evidence was required.

Origin of Historical NOAA Limits

The initial exposure limits published in the NOAA Diving Manual were developed primarily based on research conducted at PO₂ ranges between 1.4 and 1.6 ATA and higher. For PO₂ ≤ 1.3 ATA, no direct systematic human endurance studies prior to symptom onset were conducted in the 1970s and 1980s. Limits for PO₂ = 1.3 ATA were derived by extrapolating the pressure-time curve using a conservative safety margin.

While a conservative approach is justified in the absence of data, accumulating rebreather experience has shown that actual human tolerance to PO₂ = 1.3 ATA is significantly higher than previously assumed.

Pathophysiological Factors and the Role of Carbon Dioxide (CO₂)

The primary factor triggering CNS toxicity at relatively low PO₂ values (such as 1.3 ATA) is hypercapnia (CO₂ retention).

1. Cerebral Blood Flow: Carbon dioxide is a potent cerebral vasodilator. Elevated PaCO₂ causes cerebral arteriolar dilation and a dramatic increase in brain perfusion. As a result, substantially more oxygen dissolved under high pressure is delivered to brain tissue, accelerating the formation of reactive oxygen species (ROS) and exhausting antioxidant defenses.
2. Causes of CO₂ Retention Underwater:
   • Gas Density: Increasing depth increases breathing gas density. High density (exceeding 6.0–6.2 g/L) sharply increases flow turbulence and work of breathing (WOB), causing hypoventilation and CO₂ retention.
   • Physical Exercise: Intense swimming or heavy underwater work leads to high CO₂ production.
   • Scrubber Efficiency: Partial exhaustion or breakthrough of carbon dioxide through the rebreather absorbent.

US Navy Experimental Evidence (NEDU Studies)

During the development and validation of the Thalmann decompression algorithms (XVAL) for closed-circuit heliox and nitrox systems (specifically the US Navy MK 16 MOD 1 UBA, which operates at a constant PO₂ setpoint of 1.3 ATA), the US Navy Experimental Diving Unit (NEDU) conducted extensive experimental dives (Gerth & Johnson, 2002; Doolette et al., 2019).

NEDU Protocol Parameters:

• Subjects: US Navy divers performing dives in hyperbaric chambers and pools.
• Bottom Phase: Cyclic physical work on underwater ergometers at PO₂ = 1.3 ATA for up to 240 minutes.
• Decompression Phase: Resting decompression stops at PO₂ = 1.3 ATA for up to 240 minutes.
• Total Exposure: Up to 480 minutes (8 hours) of continuous PO₂ = 1.3 ATA breathing mix during a single dive.

Results:

• CNS Oxygen Toxicity: Across hundreds of test dives under this regimen, no cases of seizures or overt prodromal CNS toxicity symptoms were observed.
• Pulmonary Oxygen Toxicity: Despite the high total oxygen dose (up to 480 minutes at 1.3 ATA), no clinically significant or life-threatening reduction in vital capacity (VC) was detected for single dives (Shykoff, 2011).

Risk Mitigation Strategies

The consensus panel emphasizes that increasing allowable exposure time to 240 minutes of work plus 240 minutes of decompression is safe only when the following risk mitigation factors are observed:

1. Gas Density and Work of Breathing (WOB) Control

Breathing gas density should not exceed 5.2 g/L to prevent elevated work of breathing. A value of 6.2 g/L is recognized as the hard limit, beyond which the risk of CO₂ retention rises significantly.

2. Minimizing Physical Exercise

Using diver propulsion vehicles (DPVs) or diver transport systems during the bottom phase eliminates excess CO₂ production from heavy exertion.

3. Airway Protection in Case of Unconsciousness

Using mouthpiece retaining devices (retainers), full-face masks (FFMs), or bail-out valves (BOVs) protects the diver's airway from water aspiration should an involuntary seizure occur. Strict adherence to the buddy system is essential to ensure immediate diver extraction to the surface.

4. Exposure Interruption ("Air Breaks")

In hyperbaric medicine, brief interruptions in high PO₂ delivery (switching to a lower oxygen mixture, e.g., air for 5 minutes every 60–75 minutes) reduce seizure probability by over 50%. In diving, reducing PO₂ during decompression stops or brief gas switches provides an extra safety margin.

5. Use of Dry Decompression Habitats

Conducting extended decompression phases in dry underwater habitats or hyperbaric chambers completely eliminates drowning risk should complications arise.

New Consensus Guideline (Updated Standards)

Based on NEDU data analysis and expert consensus, revised exposure limits for a partial pressure of PO₂ = 1.3 ATA have been established.