Oxygen toxicity in scuba diving occurs when the body is exposed to an excessively high partial pressure of oxygen (PO2). Breathing compressed gas with an elevated concentration under the increased pressure of depth can become harmful to the central nervous system and lungs. This hazard primarily concerns divers who use enriched air nitrox (EANx) or other oxygen-rich gas mixtures. Understanding the mechanisms and limits of oxygen exposure is paramount for divers to safely plan and execute dives, especially when using enriched gases.
The Two Forms of Oxygen Toxicity
Oxygen toxicity manifests in two ways: Central Nervous System (CNS) toxicity and Pulmonary toxicity. For the typical scuba diver, CNS oxygen toxicity is the immediate and most serious threat, occurring from short exposures to very high partial pressures of oxygen. This acute form targets the brain and spinal cord, leading to seizures that are catastrophic underwater because they can cause the diver to lose their regulator and drown.
Pulmonary oxygen toxicity is a chronic condition that affects the lungs. It generally results from breathing elevated oxygen partial pressures for much longer periods, often days rather than hours. This form is typically a concern for commercial saturation divers or technical divers conducting very long exposures, making it less relevant to standard recreational dive profiles. The effects of pulmonary toxicity, which include chest pain and a persistent cough, are usually reversible once the oxygen exposure is reduced.
The Mechanics of Partial Pressure
The true driver of oxygen toxicity is the partial pressure of oxygen (PO2), which is the absolute pressure exerted by the oxygen component of the breathing gas. As a diver descends, the ambient pressure increases by one atmosphere absolute (ATA) approximately every 33 feet (10 meters). This increased ambient pressure acts as a multiplier, causing the PO2 to rise proportionally, even if the percentage of oxygen in the tank remains constant.
The formula for calculating this is straightforward: PO2 equals the ambient pressure multiplied by the fraction of oxygen in the gas mixture. For instance, a gas mix containing 36% oxygen at the surface (1 ATA) has a PO2 of 0.36 ATA, but at 99 feet (4 ATA), the PO2 dramatically increases to 1.44 ATA. The diving community has established clear safety limits for PO2 to manage this risk.
The generally accepted operational limit for PO2 during a working dive is 1.4 ATA, which provides a comfortable margin of safety against CNS toxicity. A higher contingency limit of 1.6 ATA is typically reserved for short-duration exposure, such as during decompression stops when the diver is resting and stationary. Exceeding these limits significantly increases the risk of acute toxicity.
Identifying the Immediate Signs
The onset of CNS oxygen toxicity can be sudden, sometimes progressing to a seizure with little to no warning. However, there are potential precursor symptoms that a diver or their buddy may notice. These warning signs are often summarized by the acronym CONVENTID.
While these symptoms can sometimes overlap with other diving issues like nitrogen narcosis, any unusual sensation should be treated as a possible indication of impending oxygen toxicity. The ultimate and most life-threatening manifestation is the convulsion, which is a tonic-clonic seizure that can lead to the loss of the regulator and subsequent drowning.
The CONVENTID acronym stands for:
- Convulsions
- Visual disturbances (such as tunnel vision or flickering lights)
- Ears (ringing or auditory disturbances)
- Nausea
- Twitching (often in the facial muscles, particularly the lips)
- Irritability
- Dizziness
Safety Strategies and Emergency Protocol
Preventing oxygen toxicity begins with careful dive planning and adherence to exposure limits. Divers using oxygen-enriched gases must calculate their Maximum Operating Depth (MOD) for the specific gas mix to ensure the PO2 never exceeds the 1.4 ATA operational limit. This calculation must be strictly followed, as factors like cold, exertion, and elevated carbon dioxide levels can lower an individual’s seizure threshold.
If a diver or their buddy observes any of the signs of CNS oxygen toxicity, the immediate priority is to reduce the partial pressure of oxygen. The emergency protocol requires an immediate, controlled ascent to a shallower depth to decrease the ambient pressure and thus the PO2. If the diver is using an enriched gas, switching to a gas mixture with a lower oxygen percentage, such as air, is also a necessary step. The dive must be aborted safely, and the diver should be monitored closely for any further neurological symptoms.