Oxygen Toxicity: CNS and Pulmonary

Oxygen keeps you alive, but under pressure it can also kill you. Understanding oxygen toxicity is fundamental to safe diving with any breathing gas — especially nitrox and oxygen-rich decompression mixes.

Two types of oxygen toxicity

Oxygen toxicity comes in two forms, each with different mechanisms, timelines, and risks:

CNS (Central Nervous System) toxicity

CNS toxicity is the acute form. It occurs when the partial pressure of oxygen exceeds approximately 1.4-1.6 bar and can strike without warning. The primary danger is convulsions, which underwater almost certainly cause drowning.

This is the type that sets your Maximum Operating Depth.

Pulmonary (whole-body) toxicity

Pulmonary toxicity is the chronic form. It results from prolonged exposure to elevated oxygen partial pressures — typically above 0.5 bar over many hours. It causes inflammation and damage to lung tissue. It is primarily a concern for saturation divers, hyperbaric oxygen therapy patients, and technical divers doing extended decompression on high-oxygen mixes.

CNS toxicity in detail

The VENTID-C mnemonic

The signs and symptoms of CNS oxygen toxicity are classically remembered as VENTID-C:

• Visual disturbances — tunnel vision, blurred vision, flashing lights
• Ear symptoms — tinnitus (ringing or roaring)
• Nausea — sometimes with vomiting
• Twitching — involuntary muscle twitches, especially in the face and lips
• Irritability — anxiety, restlessness, mood changes
• Dizziness — vertigo, disorientation
• Convulsions — full tonic-clonic seizures

The problem is that convulsions can occur without any preceding warning signs. You cannot rely on catching early symptoms.

When it happens

CNS toxicity risk increases with:

• Higher ppO2: The risk rises sharply above 1.6 bar. At 2.0 bar, the risk is significant even for short exposures
• Longer exposure: Time at elevated ppO2 is cumulative within a dive
• Exercise: Physical exertion increases metabolic oxygen consumption and CO2 production, which appears to increase susceptibility
• Cold water: Cold stress may increase susceptibility
• CO2 retention: Elevated carbon dioxide levels (from skip-breathing, high gas density, or poor ventilation) amplify the risk

Why convulsions are fatal underwater

A convulsing diver typically:

1. Loses consciousness
2. Clenches their jaw (which may displace the regulator)
3. Experiences violent muscle contractions
4. Cannot maintain airway protection

Even with a buddy present, managing a convulsing diver at depth is extremely difficult. Ascending with a convulsing diver risks pulmonary barotrauma if the diver’s airway is not clear. This is why prevention — staying within ppO2 limits — is the primary strategy.

Pulmonary toxicity in detail

Pulmonary oxygen toxicity develops over hours of exposure to ppO2 above approximately 0.5 bar. Symptoms include:

• Substernal burning (chest tightness)
• Cough
• Reduced vital capacity (you cannot take as deep a breath)
• Eventually, pulmonary edema

It is tracked using the Unit Pulmonary Toxicity Dose (UPTD) or the Oxygen Tolerance Unit (OTU) system. Most recreational and technical divers never approach dangerous pulmonary oxygen exposure limits in a single dive, but multi-day technical diving with extended oxygen decompression can accumulate significant doses.

Safe ppO2 limits

These limits are empirical — derived from decades of operational experience and incident analysis rather than from a precise physiological threshold. Individual susceptibility varies, and the same diver may be more or less susceptible on different days.

CNS oxygen clock

Prolonged exposure to elevated ppO2 is tracked using the NOAA CNS oxygen toxicity clock. This assigns a percentage based on time spent at each ppO2 level:

The CNS clock is cumulative — 50% on one gas plus 50% on another equals 100%. Most dive computers track this automatically.

Safety considerations

• Know your MOD: Calculate the Maximum Operating Depth for every gas you carry and never exceed it
• Use conservative limits: If conditions are demanding (cold, current, exertion), consider using 1.2 or 1.3 bar instead of 1.4 bar
• Monitor your CNS clock: On long dives with multiple gases, track cumulative oxygen exposure. Dive computers do this automatically
• Never breathe pure O2 deeper than 6 meters: Pure oxygen at 6 meters gives a ppO2 of 1.6 bar — the absolute limit
• Manage CO2: Breathe normally, never skip-breathe, and be aware of gas density effects at depth

Sources

• NOAA Diving Manual, 6th Edition
• US Navy Diving Manual, Revision 7
• Arieli, R. (2019). “Calculated risk of pulmonary and CNS oxygen toxicity.” Respiratory Physiology & Neurobiology
• DAN (Divers Alert Network) Oxygen Toxicity Resources