Oxygen toxicity

The human body is adapted to breathe air containing approximately 21% oxygen at surface pressure, resulting in a partial pressure of oxygen (PO₂) of about 0.21 bar (ata). When descending underwater, the ambient pressure increases, and with it, the partial pressure of all gases in the breathing mixture, including oxygen. If the PO₂ exceeds certain thresholds, oxygen can become toxic.

Central Nervous System (CNS) oxygen toxicity, also known as acute oxygen toxicity or the Paul Bert effect, primarily affects the brain and manifests rapidly. High PO₂ levels interfere with neuronal function, potentially leading to symptoms like tunnel vision, ringing in the ears (tinnitus), nausea, twitching (especially of the lips), irritability, and dizziness, which can escalate to convulsions and unconsciousness. A convulsion underwater is extremely dangerous as it can lead to regulator loss and drowning.

Pulmonary oxygen toxicity, or the Lorrain Smith effect, affects the lungs and typically develops after prolonged exposure to moderately elevated PO₂ levels (e.g., above 0.5 bar (ata)). It causes inflammation and damage to the lung tissues, leading to symptoms like a burning sensation in the chest, persistent cough, and shortness of breath. This form is more common in medical hyperbaric oxygen therapy or during very long technical diving decompression schedules.

For liveaboard divers, managing oxygen exposure is a primary safety consideration, especially with the prevalence of enriched air nitrox and the potential for multiple, repetitive dives. Liveaboards often offer nitrox fills, allowing divers to extend bottom times or reduce surface intervals compared to air, but this benefit comes with the responsibility of calculating and respecting oxygen limits.

Divers must accurately analyse their nitrox blend before each dive and use a dive computer or dive tables to determine their maximum operating depth (MOD) for that specific gas. Exceeding the MOD, even by a small margin, can push the partial pressure of oxygen to unsafe levels. Additionally, many dive computers track a 'CNS clock' or 'oxygen toxicity units' (OTUs), which helps divers manage cumulative oxygen exposure over multiple dives and days.

Liveaboard diving typically involves a rigorous schedule with several dives per day. Maintaining a conservative approach to oxygen limits, avoiding excessive exertion, and ensuring adequate hydration can help reduce the risk. Dive guides and instructors on liveaboards are generally vigilant about oxygen management and provide briefings on safe diving practices, but ultimate responsibility lies with the individual diver.

Myth: Oxygen toxicity only affects technical divers. Fact: While technical divers manage higher oxygen exposures, recreational divers using enriched air nitrox (EANx) are also at risk if they exceed their maximum operating depth (MOD) for the gas blend they are using. Any diver breathing an elevated partial pressure of oxygen is susceptible.

Myth: If I'm feeling fine, I don't need to worry about oxygen toxicity. Fact: CNS oxygen toxicity can have a sudden onset, with little to no warning. Symptoms can progress rapidly from mild twitches or visual disturbances to a full-blown convulsion. Relying solely on subjective feeling is dangerous; adherence to MODs and CNS limits is essential.

Myth: More oxygen is always better for diving because it reduces nitrogen. Fact: While higher oxygen content reduces nitrogen, it directly increases the partial pressure of oxygen at depth, thereby reducing the maximum allowable depth. There is a trade-off, and the optimal blend depends on the planned dive profile, not simply 'more oxygen'.

Myth: Oxygen toxicity is the same as nitrogen narcosis. Fact: These are distinct conditions. Nitrogen narcosis is caused by the narcotic effect of nitrogen at depth, manifesting as impaired judgment and motor skills, and it typically dissipates upon ascent. Oxygen toxicity is caused by excessive oxygen, affects the nervous system, and can lead to convulsions regardless of nitrogen levels.