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Oxygen is contrary to popular belief not flammable but toxic! To those divers doing decompression and mixed gas dives this gas is lethal if inhaled at to high a concentration.
There are two varieties of oxygen toxicity namely Pulmonary Oxygen Toxicity and Central Nervous System Toxicity (CNS).
PO toxicity occurs when the diver is exposed to elevated levels oxygen for several hours. This causes damage to the cell lining of the lungs, the lung walls, the formation of fluid in the lungs and causes a feeling of shortness of breath combined with a burning throat and chest. This causes breathing to become very painful.
PO toxicity manifests itself after the diver is exposed (12 hours or more) to relatively low levels of partial oxygen pressures (pO2), 0.6 ATA (atmospheres absolute) and up.
The lungs are lined with a viscous fluid almost acting like a surfactant which helps in gas exchange and helps to lower the surface tension of the alveoli(lung sacs). When the alveoli are exposed to elevated pO2, the oxygen starts to attack the surfactant and breaks it down. This causes a shortness of breath. Normally this surfactant is regenerated if the exposure is only a few hours but on continued exposure the alveoli walls start to swell, filling the lung sacs with fluid and and decreasing the oxygen supply to the alveoli cells.
Thus too much oxygen results in not enough oxygen reaching the blood resulting in hypoxia (oxygen shortage)! That is why diving on 100% oxygen is only safe up to 20 ft or 7m.
PO toxicity is not a serious problem since it is difficult to expose oneself to oxygen long enough for this to become a real problem.
Central Nervous System Toxicity (CNS)
This is the serious kind and needs to be tracked on every dive if doing decompression or mixed gas dives. If you breathe oxygen at very high pO2s (0.9 ATA and up) for a short period of time then problems arrive much quicker. This results in visual disturbances, ringing in the ears, dizziness, mood swings, convulsions and finally coma. According to the NOAA (a US government research group) this times are: 1.6 ATA for 45 min, 1.5 ATA for 120 min, 1.4 ATA for 150 min, 1.3 ATA for 180 min and so on. Technical divers keep track of CNS toxicity by using a CNS clock. This shows your exposure as a percentage of the total allowable exposure.
As sport divers has an ATA of about 1.5 when using compressed air at depth. Generally technical divers will use an ATA of 1.4 or 1.3. We do not know the reason behind CNS toxicity but it is speculated that it might be oxygen free radicals, that is oxygen atoms with an extra electron which makes them highly reactive. This reactivity sets of a chain reaction which proceeds and forms more free radicals and thus do macro-cellular damage which sets of a whole chain of new reactions.
A final thought: Training, knowledge, good practices, planning, configuration. Attention to these points will ensure that oxygen toxicity becomes a managed risk.
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