Oxygen is essential for life, but in certain conditions, it can become dangerously toxic. Deep sea divers, especially those using specialized breathing gases, are particularly vulnerable to a condition known as oxygen toxicity. This phenomenon can have severe, even fatal, consequences if not properly understood and managed. This article explores what causes oxygen toxicity, how it affects deep sea divers, and what can be done to mitigate the risks.
Understanding Oxygen Toxicity: The Basics
Oxygen toxicity occurs when the body is exposed to an excessive partial pressures of oxygen (PPO₂) for an extended period. Under normal atmospheric conditions, oxygen makes up about 21% of the air, and our bodies are well-adapted to this ratio. However, when a diver goes deep underwater, the pressure increases, causing more oxygen to dissolve in the blood and tissues.
There are two primary types of oxygen toxicity:
- Central Nervous System (CNS) Oxygen Toxicity – This occurs with short exposures to high PPO₂, usually above 1.4 ATA (atmospheres absolute). It affects the brain and can lead to symptoms such as seizures and unconsciousness.
- Pulmonary Oxygen Toxicity – This results from prolonged exposure (typically over hours) to elevated oxygen levels (above 0.5 ATA). It primarily damages lung tissue, causing inflammation and breathing difficulties.
Both types are dangerous, but CNS toxicity poses a more immediate risk, especially for deep divers who use enriched oxygen mixtures like nitrox or pure oxygen during specific phases of their dive.
Why Deep Sea Divers Are at Risk
Deep sea diving involves breathing gases under increased ambient pressure. The deeper a diver goes, the higher the pressure, and consequently, the greater the partial pressure of each gas in the breathing mixture. Oxygen becomes toxic when its partial pressure exceeds safe thresholds, regardless of its percentage in the gas mix.
For instance, at 66 feet (20 meters) below sea level, the pressure is 3 ATA. If a diver breathes a gas with 100% oxygen at that depth, the PPO₂ would be 3.0 — far above the recommended limit of 1.4 to 1.6 ATA. This drastically increases the risk of CNS oxygen toxicity.
Divers using rebreathers, nitrox (enriched air with higher oxygen content), or other mixed gases must carefully calculate their oxygen exposure. Even slight miscalculations or equipment malfunctions can lead to a dangerous overdose of oxygen.
Signs and Symptoms of Oxygen Toxicity
Recognizing the symptoms of oxygen toxicity is critical, especially underwater, where a sudden seizure can be fatal. Symptoms depend on the type of toxicity but may include:
CNS Oxygen Toxicity:
- Visual disturbances (tunnel vision, blurred vision)
- Ringing in the ears (tinnitus)
- Nausea
- Twitching, especially around the mouth
- Irritability or anxiety
- Dizziness or loss of coordination
- Seizures (the most dangerous symptom, often occurring without warning)
Pulmonary Oxygen Toxicity:
- Burning sensation in the chest
- Persistent cough
- Shortness of breath during exertion
- Chest tightness
- Reduced lung capacity over time
Because a seizure underwater almost always results in drowning if the diver is not rescued immediately, CNS toxicity is considered an acute emergency in diving medicine.
Preventing Oxygen Toxicity in Diving
The key to avoiding oxygen toxicity is strict dive planning and adherence to safety protocols. Divers use dive computers and tables to calculate safe exposure levels, based on depth, duration, and oxygen concentration. These calculations help them stay within established limits for PPO₂.
Here are some standard practices to prevent oxygen toxicity:
- Monitor PPO₂ Closely: Never exceed 1.4 ATA during the working phase of the dive, and 1.6 ATA during decompression stops, which are less physically demanding.
- Use Proper Gas Mixes: Avoid pure oxygen at depth unless you’re using it at very shallow levels for decompression.
- Limit Exposure Time: The longer the exposure to elevated oxygen pressures, the greater the risk — even if the PPO₂ is only moderately elevated.
- Equipment Checks: Ensure that oxygen sensors in rebreathers are functioning properly. A faulty sensor can give false readings and lead to inadvertent overexposure.
- Dive Training and Emergency Drills: Divers should be trained to recognize early signs of toxicity and respond appropriately, including how to help a diver experiencing a seizure.
Recreational divers rarely experience oxygen toxicity because they typically use air (21% oxygen) and stay within no-decompression limits. However, technical divers, military divers, and commercial divers face a higher risk due to the use of mixed gases and extended bottom times.
Long-Term Effects and Treatment
While CNS toxicity is usually acute and resolves quickly once the diver is brought to the surface and breathing normal air, pulmonary oxygen toxicity can have lasting effects, especially with repeated exposure. Chronic damage may include:
- Fibrosis (scarring) of lung tissue
- Reduced lung elasticity and volume
- Decreased exercise tolerance
- Susceptibility to respiratory infections
There is no specific antidote for oxygen toxicity. Immediate removal from the high-oxygen environment is the first and most important treatment step. For CNS symptoms, this means ascending to shallower depths (if safe) and switching to a lower oxygen gas mix. In the case of seizures, the priority is to keep the diver’s airway clear and get them to the surface safely.
For pulmonary toxicity, rest and breathing normal air (or even lower oxygen mixtures) can help the lungs recover. In severe cases, hospitalization and oxygen therapy (ironically, at controlled levels) may be required to support lung healing.
