use of O2 for decompression

Question
Please let me know above which depth is safe to use 100% oxygen for decompression. Using 100% oxygen at the 5m/3min safe stop will make me surface "cleaner" from residual nitrogen.

regards

Answer
Hi Nik,

There is no absolute answer to your question.  The hazard in breathing 100% oxygen is the possibility of Central Nervous System Oxygen Toxicity (CNS Oxygen Toxicity).  This condition varies somewhat from individual to individual so the answer I'll give you is general in application and not specific to any individual diver.

I am going to assume that you're already trained to dive with enriched air (Nitrox) and are familiar with the procedures and hazards associated with it's use.  If not, I would strongly suggest that you not even consider the use of anything other than standard compressed air.

It is generally accepted that at depths below 20 feet(6m), the possibility of CNS Oxygen Toxicity is very high when breathing 100% oxygen.  The susceptibility of an individual to CNS Oxygen Toxicity also varies depending on the conditions they are exposed to.  Such things as cold water, exertion and the length of the dive are factors which can influence the likelihood of developing symptoms.

It's true that the treatment for Decompression Illness involves administering 100% oxygen to the patient at higher partial pressures (around 2.8 ATA) but the patient is under tightly controlled conditions, is warm and dry, and the oxygen concentration can be instantly adjusted as required.  If a seizure should occur, the patient is not in danger of drowning as would be the case if he were underwater.

So, Nik, while off gassing of nitrogen would be increased by using 100% oxygen, the increased risk of CNS Oxygen Toxicity offsets that advantage and could even prove fatal in the event that a seizure should occur while underwater.

I think a more prudent suggestion would be to use 50 oxygen to perform your shallow safety stops.  keep in mind that a 3-minute safety stop is only a suggested minimum time and you can always extend that time as much as your air supply will permit and as long as you don't exceed your 24-hour exposure limit.  Longer times will permit you to lose more nitrogen and, as you stated, increased oxygen concentrations will speed the process of offgassing.  I just wouldn't recommend using 100% oxygen while you're underwater.

Nik, I hope this information will help you decide how to best handle your safety stops.

Thanks for writing.

Mike Giles
Mike's Dive Center
mikescuba.com