Oxygen Toxicity

Oxygen toxicity happens when our body’s protective systems are affected by increases in oxygen partial pressure. The tissue-protective mechanisms and biochemical reactions of our bodies are tuned to life in an atmosphere containing 21 percent oxygen, or 0.21 atmospheres absolute (ATA) oxygen partial pressure. Exposure limits are given as partial pressure over time. As the partial pressure gets higher, the recommended exposure time gets shorter.

Oxygen toxicity in the lungs (pulmonary oxygen toxicity) is like a bad case of the flu, but it will rarely cause permanent damage. The most common cause of lung oxygen toxicity is very long recompression treatments.

Oxygen toxicity of the brain, commonly referred to as central nervous system (CNS) oxygen toxicity, is more serious. It can occur during diving, and when it does, it can put a diver at serious risk. The following sections are about CNS oxygen toxicity.

Signs and Symptoms

  • Flashing lights in front of your eyes
  • Tunnel vision
  • Loud ringing or roaring in the ears (tinnitus)
  • Confusion
  • Lethargy
  • Nausea and vertigo
  • Numbness or tingling
  • Muscular twitching (especially lips)
  • Grand mal convulsion


The safest practice is to pay attention to the partial pressure and the amount of exposure time. To lower your risk of CNS oxygen toxicity, consider the following recommendations.

  • The U.S. Navy uses 1.3 ATA as the maximum limit in its closed-circuit rebreathers. Very long exposures, however, may put the diver at risk for some lung toxicity symptoms.
  • The National Oceanic and Atmospheric Administration (NOAA) recommends a more conservative 180 minutes at 1.3 ATA for normal exposures and 240 minutes only for exceptional exposures.
  • The Professional Association of Diving Instructors (PADI) has proposed a limit of 1.4 ATA for open-circuit nitrox scuba diving. Because open-circuit scuba diving would not expose divers to this level continuously, it should be at least as safe as the Navy limit for continuous exposures.
  • Shallow exposure times in the 1.3 to 1.4 ATA range are mainly to avoid lung oxygen toxicity. The likelihood of CNS toxicity at these levels is very low and probably not much different over this range.
  • The Navy allows an exercising exposure at 1.7 ATA for up to four hours, but that assumes breathing 100 percent oxygen at 25 feet (7.6 meters) by trained combat swimmers. A depth excursion of only 5 feet (1.5 meters) puts a diver in a range where convulsions have occurred. Divers who tend to retain carbon dioxide during exercise may be at increased risk.
  • The NOAA limit for nitrox diving at 1.6 ATA is 45 minutes for normal diving and 120 minutes for exceptional exposure diving.
  • Breathing 100 percent oxygen during a decompression stop at 20 feet (6.1 meters) is a common practice. At this depth, the partial pressure will be about 1.6 ATA. Under resting conditions at that depth, the chance of CNS oxygen toxicity should be very low but is not absent.

Oxygen Partial Pressure Ranges

For open-circuit scuba, the “green light” region is any oxygen partial pressure of 1.4 ATA or less (about 82 feet or 25 meters on a 40 percent oxygen mix). If you don’t exceed this level, the other limitations of open-circuit scuba diving will limit your exposure time to lengths where CNS oxygen toxicity is unlikely.

Between 1.4 and 1.6 ATA (99 feet or 30 meters on a 40 percent mix) is the “yellow light” region. The possibility of oxygen toxicity at 1.6 ATA is low, but the margin of error is very slim compared to 1.4 ATA. Individual variation, an unplanned depth excursion that causes an increase in oxygen partial pressure, and the possibility of having to perform strenuous exercise in an emergency raise the possibility of oxygen toxicity to levels where you should exercise caution. Levels of 1.5 to 1.6 ATA should be only for conditions where you are entirely at rest, such as during decompression. The dive team must still prepare for the possibility of an oxygen convulsion at these levels.

Above 1.6 ATA is the “red light” area. Recreational divers should not exceed this level. Even mild exercise may put divers breathing high-density nitrox mixes at increased risk. Open-circuit scuba divers can achieve durations likely to get them into trouble at these levels. Diving using these high partial pressures of oxygen should be for trained professionals who can weigh the risks and benefits and have the necessary training and support structure in place if an oxygen convulsion occurs.

Responding to Oxygen Toxicity

Nonconvulsive Symptoms

Alert your dive buddy and make a controlled ascent to the surface. Inflate your life preserver if necessary. A buddy should watch closely for the progression of symptoms. While you are not at immediate risk of injury from these symptoms, you should change to a breathing gas with a lower oxygen partial pressure as soon as possible.

Underwater Convulsion

• Get behind the convulsing diver. Release their weight belt. If they are wearing a drysuit, leave the weight belt in place to prevent the diver from assuming a face-down position on the surface.
• Leave their mouthpiece in their mouth. If it is not, do not attempt to replace it. If you can, ensure that the mouthpiece is in the surface position.
• Grasp the diver around the chest above the underwater breathing apparatus (UBA) or between the UBA and the body. If you can’t gain control of the victim in this manner, use the best method possible to obtain control. Grasp the UBA waist or neck strap if necessary.
• Make a controlled ascent to the surface, maintaining a slight pressure on the diver’s chest to assist exhalation.
• If you need additional buoyancy, activate the diver’s life preserver. Do not release your weight belt or inflate your life preserver.
• Inflate the victim’s life preserver upon reaching the surface if you haven’t yet.
• Remove the diver’s mouthpiece after surfacing and switch the valve to surface position to prevent the possibility of the rig flooding, which could weigh them down.
• Signal for emergency pick-up.
• Tilt the diver’s head back to open their airway once the convulsion has subsided.
• Ensure they are breathing. A trained provider can initiate mouth-to-mouth breathing if necessary.
• Monitor the diver for signs of other decompression illness and respond appropriately.

Other Considerations

The main goal while the injured diver is in the water is to keep them from drowning. Next is to ensure that the airway is open after the convulsion stops by keeping the neck extended.

Look for foreign bodies in the trachea. A diver can bite off parts of the mouthpiece during a convulsion, which can find their way into the trachea and block the airway. In these cases, the injured diver will begin coughing as they return to consciousness or may try to breathe but not get any air into the lungs. A trained provider should respond to this foreign-body obstruction of the trachea.

Convulsions are rare, but the symptoms noted above do not always warn of an impending convulsion. The convulsion itself is not harmful. The danger is in the diver losing their mouthpiece underwater or banging their head while thrashing, which could result in trauma or drowning.

Additional Recommendations

• Be aware that oxygen toxicity is unpredictable. Divers have experienced convulsions at shallow depths under conditions where most experts would not have expected them to occur.
• Whenever you breathe a gas with an oxygen fraction above 21 percent, oxygen toxicity is a possibility. Ensure you have appropriate training. Always have a buddy visible. Make sure you and your buddy know what to do if oxygen toxicity occurs.
• Using equipment designed to compress high-oxygen mixtures can be hazardous and requires special training.
• What you get in your cylinder may not be what you expect. Make sure you have a method of analyzing the amount of oxygen in the tank independent of the filling station.
• Remember that rebreathers are intricate pieces of life-support gear. They require more care than a standard scuba regulator.
• Consider your risks when using nitrox. When an oxygen convulsion occurs, it’s almost always underwater, which makes responding more complicated and increases the risk of severe injury or death. Experience, good training and thorough knowledge of nitrox diving by everyone involved are essential.
• Remember that CNS oxygen toxicity symptoms are functions of both time and duration. They will not suddenly occur the moment you exceed a partial pressure threshold — it takes time. As the inspired oxygen partial pressure increases, the exposure time decreases.

Ed Thalmann, M.D.