Oxygen is essential for life, but it can become harmful under the increased pressures experienced during underwater diving. This condition, known as oxygen toxicity, occurs when the body is exposed to elevated oxygen levels, particularly at depth. Too much oxygen under pressure can disrupt normal bodily functions and pose significant risks to divers.
Pressure’s Effect on Oxygen
Underwater, pressure increases substantially with depth, directly influencing the partial pressure of gases divers breathe. Partial pressure is the individual pressure exerted by a specific gas within a mixture, such as the air divers breathe. At the surface, oxygen is about 21% of air, with a partial pressure of approximately 0.21 atmospheres absolute (ATA).
As a diver descends, total pressure increases, elevating the partial pressure of each gas in the breathing mixture, even if the percentage of oxygen remains constant. For example, at 33 feet (10 meters) of saltwater, total pressure doubles to 2 ATA, causing oxygen’s partial pressure to also double to 0.42 ATA. This forces more oxygen molecules into the body’s tissues, increasing toxicity risk.
How Oxygen Damages Cells
Elevated oxygen partial pressures trigger cellular damage. A primary mechanism is the overproduction of reactive oxygen species (ROS), also known as free radicals. These highly reactive molecules are byproducts of normal metabolic processes, but become problematic when their levels exceed the body’s natural antioxidant defenses.
Free radicals are unstable and seek to stabilize themselves by “stealing” electrons from other molecules. This process, known as oxidative stress, damages cellular components like DNA, proteins, and lipids. This damage disrupts normal cellular function and impairs tissue integrity.
Recognizing Oxygen Toxicity
Oxygen toxicity manifests in two main forms: Central Nervous System (CNS) oxygen toxicity and Pulmonary oxygen toxicity.
Central Nervous System (CNS) Oxygen Toxicity
CNS toxicity is the more immediate danger, affecting the brain and spinal cord. Symptoms can appear suddenly, including visual disturbances like tunnel vision or flashing lights, ringing in the ears (tinnitus), and nausea. Other signs are muscular twitching, particularly around the face and lips, irritability, confusion, and dizziness. Convulsions are the most severe manifestation, posing a life-threatening risk underwater due to drowning or regulator loss. Convulsions may be preceded by subtle symptoms, though onset can be rapid.
Pulmonary Oxygen Toxicity
Pulmonary oxygen toxicity is a chronic issue from prolonged exposure to elevated oxygen partial pressures. It primarily affects the lungs, causing a burning sensation in the chest, persistent coughing, and reduced lung capacity. Though reversible, severe cases can cause significant lung damage. This form is less common for recreational or technical dives due to shorter exposure times.
Avoiding Oxygen Toxicity
Preventing oxygen toxicity requires careful dive planning and adherence to established limits. Divers monitor the partial pressure of oxygen (PO2) in their breathing gas at different depths. Recreational guidelines recommend a maximum PO2 of 1.4 ATA during the bottom phase of a dive, with 1.6 ATA allowed for decompression stops.
Divers manage oxygen exposure using specialized breathing gas mixtures. Nitrox, or enriched air nitrox, has more oxygen and less nitrogen than air. This allows for longer bottom times at shallower depths by reducing nitrogen absorption, but results in a shallower maximum operating depth (MOD). For very deep dives, technical divers use trimix, which replaces some nitrogen and oxygen with helium. Helium reduces nitrogen’s narcotic effects and oxygen’s partial pressure, allowing safer deep dives within acceptable oxygen limits.