How to Manage Oxygen Toxicity in Diving

Oxygen keeps every diver alive, but it can also turn against the body under pressure. When divers descend, the surrounding water pressure increases and changes how gases behave inside the lungs and blood. What is perfectly safe to breathe on the surface can become harmful at depth.

This article explains the theory behind oxygen toxicity—how it develops, how different breathing gases affect it, and the scientific principles used to control it. Understanding these concepts is essential for safe diving because oxygen toxicity gives no warning until it happens.

Understanding Oxygen Toxicity and Its Mechanism

Oxygen toxicity occurs when the partial pressure of oxygen (PPO₂) becomes higher than what the body can tolerate. As pressure increases with depth, more oxygen molecules enter the lungs with each breath. While oxygen is necessary for metabolism, an excess disrupts the body’s chemical balance.

At high PPO₂, oxygen forms reactive oxygen species (ROS)—unstable molecules that damage cell membranes and interfere with nerve function. When these molecules accumulate, the nervous system can react violently, leading to CNS (central nervous system) oxygen toxicity, often expressed as convulsions or visual disturbances.

If the exposure lasts longer, the lungs may also suffer slow oxidative damage,which is called pulmonary oxygen toxicity. These two forms differ in how quickly they develop, but both occur when oxygen pressure exceeds what the body can safely manage at the cellular level.

The Relationship Between PPO₂, Depth, and Gas Composition

How oxygen behaves underwater follows one clear rule of physics: as depth increases, so does oxygen pressure. PPO₂ depends on two things—the oxygen fraction in the breathing gas and the pressure surrounding the diver.

At the surface, air produces a PPO₂ of about 0.21 ATA, which is completely safe. At 30 meters, the same air reaches roughly 0.84 ATA , which is still well below danger.But once PPO₂ rises beyond about 1.4 ATA, oxygen begins to behave differently at the molecular level.

Scientific research identifies 1.4 ATA as the recommended working limit, while 1.6 ATA is considered a short-term maximum.Beyond that point, oxygen starts producing oxidative stress faster than the body can neutralize it. What makes oxygen toxicity particularly risky is that it develops without sensation—the diver feels normal until symptoms appear suddenly.

How Different Breathing Gases Affect Oxygen Toxicity

The same physics governs all gases, but the rate at which PPO₂ increases depends on the oxygen content of the mixture.

Air (21% Oxygen)

In normal air, PPO₂ increases slowly with depth. Even at 40 meters, it is about 1.05 ATA, well below the threshold for CNS effects. For this reason, oxygen toxicity is almost never an issue in recreational air diving. It only becomes a concern beyond 55–60 meters, where PPO₂ approaches 1.4 ATA.

The KOSPET Scuba Diving Mode is operated under this same principle. It uses the standard 21% oxygen air mixture to calculate pressure and safety data, ensuring that oxygen exposure remains within the safe range defined for recreational depths. By tracking depth, time, and PPO₂ in real time, the mode helps divers maintain safe oxygen levels during every dive.

Nitrox (Oxygen-Enriched Gas, 32–40% Oxygen)

Adding more oxygen to breathing gas reduces nitrogen uptake but raises PPO₂ faster. For example, Nitrox 36 reaches a PPO₂ of 1.4 ATA at around 29 meters, which becomes its maximum operating depth (MOD). Beyond that, oxygen pressure becomes unsafe for the nervous system.

The principle is simple: higher oxygen percentage means a shallower safe depth, while lower oxygen allows for a deeper one. Nitrox extends safe bottom time only when oxygen exposure is carefully calculated.

Pure Oxygen (100% O₂)

Pure oxygen behaves differently because it has no inert gases to dilute it. At the surface, PPO₂ is already 1.0 ATA, and reaches 1.6 ATA at 6 meters—the upper physiological limit.But CNS toxicity can occur within minutes at a few meters deeper.

That is why pure oxygen is never used for normal diving; it is reserved for shallow decompression or medical therapy, where time and pressure can be tightly controlled.

In summary, air provides a wide safety margin, Nitrox narrows it, and pure oxygen almost eliminates it, which significantly increases the risk of oxygen toxicity. Managing oxygen toxicity requires understanding how oxygen concentration and pressure interact to approach or exceed the body’s safe threshold.

Theoretical Methods to Manage Oxygen Toxicity

Oxygen toxicity can be managed only by understanding how physics and physiology interact. Each safety limit in diving originates from how oxygen behaves under pressure inside the human body.

Calculate and Monitor PPO₂

Oxygen toxicity begins with pressure. As pressure increases, oxygen dissolves into blood plasma more efficiently, but this also accelerates the formation of reactive oxygen species.

Once PPO₂ rises above safe limits, these molecules accumulate faster than the body can remove them. Monitoring PPO₂ helps control this balance. Keeping it below 1.4 ATA for CNS safety and below 0.5 ATA for long-term exposure prevent oxidative stress that damages nerve and lung tissues.

Respect the Maximum Operating Depth (MOD)

The MOD is not a convenience but a physiological boundary. When a diver exceeds it, oxygen pressure in the lungs surpasses what the bloodstream and tissues can handle.

Excess oxygen dissolves faster than cells can use it, creating an imbalance that overwhelms the body’s chemical defenses. Staying within the MOD keeps oxygen uptake and metabolism in equilibrium, ensuring that oxygen remains beneficial rather than toxic.

Limit Exposure Time

Oxygen toxicity, often called oxygen dose, depends on both pressure and duration.Even moderate PPO₂ can become harmful with prolonged exposure because oxidative molecules accumulate in tissues over time.

Models such as CNS% and OTU (Oxygen Tolerance Units) translate this relationship into measurable limits. Limiting time at depth ensures the body’s antioxidant systems can keep pace with oxidative stress.

Control Carbon Dioxide (CO₂)

Carbon dioxide changes how the body reacts to oxygen. When a diver overexerts, CO₂ levels rise, the blood becomes more acidic, and nerve cells become more sensitive to oxygen. This phenomenon, called CO₂ potentiation, lowers the threshold for CNS oxygen toxicity.

Physiologically, high CO₂ increases blood flow to the brain, accelerating oxygen delivery and amplifying toxicity risk. Controlling CO₂ through calm, steady breathing maintains chemical balance and reduces sensitivity to high PPO₂.

Recognize the Invisible Nature of Risk

Oxygen toxicity gives no early warning. A diver may feel fine until the moment the body crosses its limit. The reactions begin at the cellular level, invisible to human sensation.

That is why safe diving depends on measurable parameters—PPO₂, depth, and time—not intuition. Theoretical understanding replaces guesswork with quantifiable safety margins.

All theoretical control methods aim to maintain a single balance: oxygen pressure multiplied by exposure time must stay within what the body can tolerate. When this balance is respected, oxygen remains a source of life; when it is exceeded, it becomes a toxin.

Safety Awareness in Oxygen Exposure

Oxygen toxicity is one of the few diving hazards that can appear suddenly and without warning. True safety depends on understanding three measurable variables—oxygen concentration, depth, and exposure time.

These factors interact constantly, and a small increase in any one of them can raise oxygen pressure to dangerous levels.

The danger does not come from oxygen itself but from exceeding its safe pressure range. Since the body cannot detect this shift, prevention relies entirely on knowledge and calculation.

The diver who understands how pressure, time, and gas composition interact is the one who remains within safe limits. Awareness, not instinct, is the foundation of safe diving.

Conclusion

Oxygen toxicity is a predictable effect of pressure on the human body. When oxygen pressure rises beyond what cells can handle, it shifts from supporting life to threatening it.

In air diving, the risk is minimal; in Nitrox diving, it requires calculated control; in pure oxygen, it demands strict limits. The key to safety lies in understanding how oxygen, depth, and time interact—and never exceeding that balance.

Oxygen is not the danger. Misjudging its limits is.