Oxygen toxicity is perhaps the newest of diving emergencies. As technology has allowed us to go deeper and longer underwater in the recreational and technical diving setting, we have had to confront the issues of excessive oxygen exposure.
As you will recall from the last post, every 10 metres of water subjects the diver and gear to another bar of pressure. We are at 1 bar of pressure at the surface, 2 bar of pressure at 10 metres, 3 bar of pressure at 20 metres and so on.
What is oxygen toxicity?
There are two systems affected relevant to dive safety, pulmonary and central nervous system. The central nervous system being affected is the most dangerous of the two as it leads to seizures.
In short, oxygen toxicity is just too much oxygen. It is usually only seen if you attempt to give high flow oxygen to a premature neonatal (it leads them to become blind) and is almost unheard of in adults. It is however very common in SCUBA diving.
It may be an unpopular thing to say, but only a reckless diver will experience oxygen toxicity. Provided you stay within the limits of your training and the MOD (Maximum Operating Depth) of the gas you are breathing, you should be fine.
How does oxygen toxicity occur?
Oxygen toxicity occurs in the pulmonary system from a lengthy exposure to a PO2 (partial pressure of oxygen) above 0.5 bar.
Oxygen toxicity occurs in the Central Nervous System from an exposure to a PO2 above 1.6 bar.
If you remember from last week, at 1 bar, the partial pressure of oxygen in air is 0.21
At 30 metres (the maximum dive limit for ADAS Occupational SCUBA workers) the PO2 is 0.84, this is equivalent to breathing 84% oxygen at the surface.
At 40 metres (the maximum dive limit for recreational PADI/SSI divers) the PO2 is 1.05.
At 50 metres (the maximum dive limit for most Technical divers on SCUBA) the PO2 is 1.26
Staying within diving limits, you should never approach the 1.6 PO2 required to cause a CNS emergency, you also shouldn't experience pulmonary oxygen toxicity as that would require a length of exposure longer than would be permitted.
Avoiding oxygen toxicity
Just stay within your limits. At 30 metres, according to the US National Oceanic and Atmospheric Administration you would need to spend 360 minutes at depth to be at risk of pulmonary exposure.
For recreational divers breathing air, according to PADI rules, you cannot spend more than 20 minutes at 30 metres. To get Pulmonary Oxygen Toxicity at that depth, you would have to be arrogant enough to spend 18 times the maximum limit at depth.
At the end of the day, adhere to the 40 metre maximum rule for recreational dives on air, the MOD written on your tank for Nitrox and the limits of your training for commercial and technical diving and you will be fine. Oxygen toxicity should not affect any skilled diver.