Oxygen Toxicity Discussion

[DjDiverDan]

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A ScubaBoard Staff Message...

This thread has been split off from a discussion in the Accidents and Incidents forum regarding a diver lost during a deep bounce dive. Marg, ScubaBoard Senior Moderator

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Read somwhere, that rare oxtox cases happened between ppO2 1.4 and 1.6bar. 2bars are definitely toxic, the diver maybe has 90% chance to blackout in a couple of minutes.

My recollection from Nitrox training and review of DAN research is that there have been isolated cases of convulsions and/ or blackouts at ppO2 as low as 1.4, with higher frequency (though still low probability) of toxic effects at ppO2 of 1.6. My recollection is that the recommended maximum ppO2 level for Nitrox diving is 1.2. So, if diving Nitrox 32, maximum safe depth is about 90 feet. For Nitrox 36, maximum depth is about 70 feet. Using the same standard of max ppO2 of 1.2, maximum depth on air is about 160 feet. ppO2 of 1.8 would be considered VERY risky - anything more than a minute or two at that depth and there is a 50% or higher chance of adverse reaction from Oxygen toxicity. And you have to count the time that divers spent going from 160 feet (where ppO2 is 1.2) down to 250, and the time ascending back up to 160. I didn't learn about Oxygen toxicity until I took the Nitrox class. I wonder how many of these deep-diving morons never took the Nitrox class and were completely unaware of that particular risk? Is Oxygen toxicity risk covered now in the OW or AOW classes?

 

[Tigerman]

I think you may recollect wrong. 1.4 with 1.6 contingency is whats normally considered your safes. Just look at the DSAT tables and youll see where it start warning you about high po2..

 

[AfterDark]

My recollection from Nitrox training and review of DAN research is that there have been isolated cases of convulsions and/ or blackouts at ppO2 as low as 1.4, with higher frequency (though still low probability) of toxic effects at ppO2 of 1.6. My recollection is that the recommended maximum ppO2 level for Nitrox diving is 1.2. So, if diving Nitrox 32, maximum safe depth is about 90 feet. For Nitrox 36, maximum depth is about 70 feet. Using the same standard of max ppO2 of 1.2, maximum depth on air is about 160 feet. ppO2 of 1.8 would be considered VERY risky - anything more than a minute or two at that depth and there is a 50% or higher chance of adverse reaction from Oxygen toxicity. And you have to count the time that divers spent going from 160 feet (where ppO2 is 1.2) down to 250, and the time ascending back up to 160. I didn't learn about Oxygen toxicity until I took the Nitrox class. I wonder how many of these deep-diving morons never took the Nitrox class and were completely unaware of that particular risk? Is Oxygen toxicity risk covered now in the OW or AOW classes?

Chances are if the morons have grey hair they learned about O2 tox in basic scuba 40+ years ago when they taught theroy and physics before we got in the water.

 

[DandyDon]

My recollection is that the recommended maximum ppO2 level for Nitrox diving is 1.2.

If that were true, Nitrox would hardly be worth the bother...

 

[vladimir]

Read somwhere, that rare oxtox cases happened between ppO2 1.4 and 1.6bar. 2bars are definitely toxic, the diver maybe has 90% chance to blackout in a couple of minutes.

Where did you read that, please? I am skeptical.

My recollection from Nitrox training and review of DAN research is that there have been isolated cases of convulsions and/ or blackouts at ppO2 as low as 1.4, with higher frequency (though still low probability) of toxic effects at ppO2 of 1.6. My recollection is that the recommended maximum ppO2 level for Nitrox diving is 1.2. So, if diving Nitrox 32, maximum safe depth is about 90 feet. For Nitrox 36, maximum depth is about 70 feet. Using the same standard of max ppO2 of 1.2, maximum depth on air is about 160 feet. ppO2 of 1.8 would be considered VERY risky - anything more than a minute or two at that depth and there is a 50% or higher chance of adverse reaction from Oxygen toxicity. And you have to count the time that divers spent going from 160 feet (where ppO2 is 1.2) down to 250, and the time ascending back up to 160. I didn't learn about Oxygen toxicity until I took the Nitrox class. I wonder how many of these deep-diving morons never took the Nitrox class and were completely unaware of that particular risk? Is Oxygen toxicity risk covered now in the OW or AOW classes?

I am skeptical. Could you please tell me where you got that statistic from? It is strangely vague and precise at the same time.

If that were true, Nitrox would hardly be worth the bother...

It is true that 1.2 is the recommended exposure for DIR divers (on the "working" portion of their dives), based on what I have read here on ScubaBoard.

 

[DandyDon]

It is true that 1.2 is the recommended exposure for DIR divers, based on what I have read here on ScubaBoard.

I stand by my comments...

If that were true, Nitrox would hardly be worth the bother...

 

[DjDiverDan]

I stand by my comments...

I strongly disagree, Don. Most of my dives are shallower that 100 ft., and Nitrox 32 on a 90 ft. Dive significantly reduces your Nitrogen loading over the course of the dive. Your ppN at 99 feet on Nitrox 32 is only about 2.7 ATM, as opposed to the 3.2 ATM that it would be diving on air. This allows longer bottom times, deeper dives (within the limits of no O2 toxicity), and less residual N2 buildup. For anyone doing multiple dives in a day, all in 90 feet or less, like 5 dives a day off a livesboard, Nitrox significantly reduces DCS risks. Yes, I would not use Nitrox for a dive that might go below 100 ft., but I really like the advantages of Nitrox for second dives on Coz, that are typically no deeper than 60 ft.

 

[TSandM]

The GUE prescription for 1.2 for the working portion of dives was written with two things in mind: Long bottom times, and a willingness to do the required deco for using a gas mix relatively light on O2. When you think about it, the only purpose for pushing your ppO2 is to minimize deco/maximize no-deco time. (This is assuming you aren't doing it because you either don't have access to the appropriate gas for the dive, or are too cheap to pay for it.) As soon as you accept the need to do the appropriate decompression, there is no longer any need to push the ppO2 at all. GUE simply says, "Do the crime, do the time," and opts for a lower CNS risk.

 

[lamont]

unless they've changed something again very recently the GUE limits are 1.4 for rec dives, and 1.2 on the bottom on technical dives.

 

[boulderjohn]

My recollection is that the recommended maximum ppO2 level for Nitrox diving is 1.2. So, if diving Nitrox 32, maximum safe depth is about 90 feet. For Nitrox 36, maximum depth is about 70 feet. Using the same standard of max ppO2 of 1.2, maximum depth on air is about 160 feet. ppO2 of 1.8 would be considered VERY risky - anything more than a minute or two at that depth and there is a 50% or higher chance of adverse reaction from Oxygen toxicity. And you have to count the time that divers spent going from 160 feet (where ppO2 is 1.2) down to 250, and the time ascending back up to 160.

I can't tell you what every agency says, but every one I know uses 1.4 as the basic safe PPO2 for NDL dives. (Technical dives are different.) 1.6 is the contingency rate. Believe me, those rates are considered pretty darn safe, and going to 1.8 briefly is very unlikely to get you into oxygen toxicity. Again, I am not recommending that you do that, but the recommended MOD limits are very safe, and there is a pretty good cushion as things get less safe.

I am not sure what your 50% chance of adverse reaction actually means. It would be interesting for you to cite your source. It can be very deceiving. For example, if you had a 0.5 percent chance of something happening in once instance and a 0.75% chance of it happening in the second instance, that would be a 50% increase.