Oxygen Toxicity and Scuba Diving

What Scuba Divers Need to Know About Oxygen Toxicity

Divers diving in a cenote
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Oxygen toxicity is a risk for scuba divers who expose themselves to high concentrations of oxygen by diving deep or by using mixed gases. This risk is easily managed by complying with safety guidelines. Recreational divers who dive on air have almost no chance of experiencing oxygen toxicity provided they follow the rules and dive within recreational limits. The risk of oxygen toxicity is yet another reason to dive within the limits of your training.

When Is Oxygen Dangerous for Scuba Divers?

Oxygen is a good thing – up to a point. The human body metabolizes oxygen to perform basic cell functions. The metabolism of oxygen for these necessary functions, as well as collisions between oxygen molecules in the cells, creates a small number of oxygen “free radicals” (molecules with at least one extra electron). Free radicals can cause major damage or even kill cells. Cells normally inactivate free radicals as soon as they are formed, but when a person breathes high concentrations of oxygen, free radicals build up in the cells more quickly than they can be eliminated. This is when oxygen becomes toxic.

In What Situations Do Scuba Divers Risk Oxygen Toxicity?

Scuba divers risk oxygen toxicity if they breathe an excessively high partial pressure (concentration) of oxygen or if they are exposed to elevated partial pressures of oxygen for extended periods of time. Situations in which the risk of oxygen toxicity must bemanaged include diving beyond recreational depth limits on air, diving on enriched air nitrox or another gas mixture with a high percentage of oxygen, and using oxygen or enriched air for decompression stops.

Central Nervous System (CNS) Oxygen Toxicity:

Central nervous system (CNS) oxygen toxicity occurs when cells in a diver's central nervous system (primarily in the brain) are damaged or experience cell death. This most commonly happens when a diver breathes partial pressures of oxygen greater than 1.6 ata, such as breathing EANx32 beyond 130 feet. Most training organizations recommend a maximum oxygen partial pressure of 1.4 ata for this reason.

  • Convulsions and Unconsciousness: In scuba diving, CNS oxygen toxicity usually manifests as uncontrollable convulsions and unconsciousness. There is frequently no warning of the impending convulsions - a diver is perfectly fine one moment and convulsing the next. Underwater, a diver who experiences CNS oxygen toxicity risks losing his regulator and drowning or pulmonary barotrauma if the convulsions begin while his airway is closed.


  • Recognizing CNS Oxygen Toxicity: While a common effect of CNS oxygen toxicity is sudden convulsions, an alert diver may sometimes notice other warning signs and symptoms. These include visual disturbances such as tunnel vision; auditory anomalies such as ringing ears; nausea and dizziness; twitching - especially of facial muscles; and mood changes such as irritability or euphoria.
  • Dealing With CNS Oxygen Toxicity - Go Up: A diver who experiences CNS oxygen toxicity must immediately ascend to a shallower depth to reduce the partial pressure of oxygen. A conscious, non-convulsing diver can do this on his own, but a diver experiencing the more severe effects of oxygen toxicity must rely on his buddy. One training organization recommends holding a convulsing diver with his regulator in place for up to 15 seconds while waiting for the convulsions to stop (a convulsing diver who ascends may risk a pulmonary barotrauma). After 15 seconds, or when the convulsions stop, the victim should be slowly taken to the surface.

Pulmonary Oxygen Toxicity:

Pulmonary oxygen toxicity occurs when the cells in a diver's lungs are damaged or experience cell death. It is primarily a risk for technical divers, as the condition occurs when divers breathe elevated partial pressures of oxygen for extended periods of time, such as breathing pure oxygen on a series of decompression stops. Most divers can breathe a partial pressure of oxygen of 1.4 - 1.5 ata for 8 - 14 hours before feeling the effects of pulmonary oxygen toxicity.

  • Recognizing Pulmonary Oxygen Toxicity: Divers effected by pulmonary oxygen toxicity experience a progression of symptoms, beginning with a burning sensation in the trachea, and progressing to difficulty breathing, shortness of breath, tightness in the chest, and uncontrollable coughing. If no action is taken, a diver's lungs eventually cease to work, and the diver dies (ironically) from a lack of oxygen.


  • Dealing With Pulmonary Oxygen Toxicity - Breathe Air: Technical divers who plan to decompress with high partial pressures of oxygen for long periods of time avoid pulmonary oxygen toxicity by taking air breaks. For every 20 - 25 minutes that the diver breathes the decompression gas, he breathes air for at least 5 minutes. This allows his lung cells to eliminate any accumulated oxygen free radicals before they become a problem.


The Longer The Exposure, The Greater The Risk

When training for deep, enriched air, or decompression diving, divers must learn to track their exposure to elevated partial pressures of oxygen. The longer and more intense a diver's exposure to elevated partial pressures of oxygen, the more susceptible he will be to oxygen toxicity. There is a point at which the diver must stop his exposure to high partial pressures of oxygen or run an unacceptable risk of oxygen toxicity. There are three main ways to track a diver's oxygen exposure:

  • Oxygen Toxicity Units: A diver can track his exposure to oxygen using "oxygen toxicity units" (OTUs). One OTU is equivalent to breathing pure oxygen on the surface for one minute. A diver uses a chart or mathematical formula to determine his OTUs for a given dive. A diver's OTUs should not exceed about 615 in a day. The number of acceptable OTUs per a day decreases with progressive days of diving.


  • Oxygen Clock: A diver uses a chart to determine what percentage of his total allowable oxygen exposure he has used for a given dive. For example, a dive with an oxygen partial pressure of 1.4 ata for 60 minutes uses about 33% of a diver's allowable oxygen exposure for the day.
  • Dive Computer: A nitrox or mixed gas dive computer is by far the easiest way to track for a diver to keep track of her total oxygen exposure. However, she should employ one of the above methods as a back-up in case of a computer malfunction.

Avoiding Oxygen Toxicity

Recreational divers can avoid or reduce the risk of oxygen toxicity by diving on air within the recreational depth limit of 130 feet. The use of enriched air nitrox and other mixed gases and diving deeper than 130 feet require additional training. In general:

  • Stay Within Depth Limits: Recreational divers using air have a maximum depth limit of 130 feet, much shallower than the depth at which oxygen will become toxic. However, divers using enriched air nitrox or other gases must calculate their depth limits and be sure to stay within them. Most training organization recommend a maximum partial pressure of oxygen of 1.4 ata.


  • Maintain Buoyancy Control and Awareness: Proper buoyancy control allows divers to maintain a safe depth.


  • Take Air Breaks: If decompressing with high partial pressures of oxygen for long periods of time, be sure to take appropriate air breaks to reduce the risk of pulmonary oxygen toxicity.


  • Track Your Total Oxygen Exposure: If diving with nitrox or mixed gases, use a dive computer, oxygen toxicity units, or oxygen clock calculations to track your total oxygen exposure.


  • Keep Your Carbon Dioxide Levels Low: Strenuous exercise and poorly functioning regulators can elevate a diver's carbon dioxide levels, which causes him to retain oxygen and increase his risk of oxygen toxicity.
  • Avoid Oxygen Exciters: Some medications, such as decongestants containing Psuedoephedrine HCl, act as oxygen exciters, accelerating the onset of oxygen toxicity at unusually low partial pressures or shortened exposure times. Be sure to check with a doctor before using any medication when scuba diving.

Oxygen toxicity, like most other potential dangers in scuba diving, can be avoided to avoid – simply understand the risks and dive within the limits of your training!