When oxygen is breathed at partial pressures greater than 1.6 ATA it rapidly produces toxicity and affects the central nervous system (CNS). The likelihood of central nervous system oxygen toxicity increases with oxygen partial pressure, exposure time, exercise, cold, anxiety, increasing carbon dioxide levels in the blood and steroid therapy.

Activities associated with increased risk of CNS oxygen toxicity include diving on air approaching and exceeding 60 meters or using gas mixtures with an increased percentage of oxygen. Recompression treatment with oxygen for decompression illness or carbon monoxide toxicity occasionally results in oxygen toxicity.

Signs & Symptoms of Oxygen Toxicity:
(occur when underwater or shortly after surfacing)
• Dizziness
• Nausea
• Twitching of muscles, particularly of the face
• Difficulty in concentration, agitation
• Disorientation
• Light-headedness
• Visual abnormalities
• Hearing sounds (e.g. ringing bells, roaring, machine sounds)
• Unconsciousness (often the first indication)
• Convulsions

First Aid Treatment for Oxygen Toxicity:
• Bring the diver to the surface
• Monitor the consciousness, airway and breathing and begin CPR if necessary

Special Considerations for Pulmonary Oxygen Toxicity:
When oxygen is breathed for very long periods at partial pressures greater than about 0.6 ATA, pulmonary oxygen toxicity can result. The symptoms can include coughing, wheezing, shortness of breath and pain behind the breastbone. It does not generally threaten the air-breathing recreational diver unless the diver requires recompression, in which case the oxygen level is reduced and monitored. However, divers breathing enriched air and technical divers need to monitor their use of high oxygen mixtures.