O2 toxicity on EANx

[JPENDERGRAST]

I was talking with a dive buddy the other day, and ran into a question. I can't seem to find my EANx booklet anymore, so figured I would ask here.

Because Nitrox has an increased percentage of O2, it seems that you would be more susceptible to oxygen toxicity at APPROPRIATE depth. Is this correct, or does this only apply when you pass depth x ? *Obviously, you would be more prone to it if you chose not to adhere to the max allowable depth correlating with the blend you're diving, but if you ARE following the correct profile, are you still more susceptible to O2 toxicity?

 

[H2O 70]

The most current studies show that there aren't hard and fast rules to Oxtox.

If you dove a profile that had a ppO₂ of 1.4, you could do that profile hundreds of times. All of a sudden for no real known reason you could suffer Oxtox.

Some agencies are pushing for a max ppO₂ of 1.4.

There is another Nitrox tread that I'll try to link back to arguing this exact questions.

http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/327663-nitrox-question.html

 

[charlesml3]

Well yea. Keep in mind that all of the tables and clocks are based on the best information we know right now. It's entirely possible that someone could go into OxTox staying above the MOD and short of the 24 hour clock. These aren't absolutes.

That said, the depths and times are pretty conservative. I always get a good chuckle out of the myriad of threads on "What should I put in my pony?" There's huge fear of taking even a single breath off of a pony full of Nitrox at 1 foot below the MOD.

-Charles

 

[PfcAJ]

Diving nitrox in and of its self does not make you more susceptible to o2 toxicity. After all, air can be called EAN 21.

However, if talk about ppo2, the closer you get to 1.6, the higher your chances of having an issue. Oxygen toxicity is a very complicated thing that is affected by time, repetitive doses, co2, exertion, breathing gas density, temperature, individual physiology, and how you are at the given time of exposure.

Air at 100ft has a ppo2 of .84. Pretty low. 32%, on the other hand, has a ppo2 of 1.28. Your chances of oxygen toxicity at that ppo2 is extremely low, bordering on nonexistent. However, move up to a 1.6 , and now you are running a higher risk of toxing. Thats not to say you will or won't tox, its just the chances increase.

 

[Tortuga68]

Because Nitrox has an increased percentage of O2, it seems that you would be more susceptible to oxygen toxicity

Short answer, yes

Longer answer, there are different types of oxtox (eg CNS, pulmomary etc) and some are 'worse' than others - plus it depends on length of exposure, rate of work etc

So it's not all about PP, there are other factors

I wouldn't worry about increased oxtox risk when using appropriate mix at appropriate depths as per the OP

 

[reefduffer]

Because Nitrox has an increased percentage of O2, it seems that you would be more susceptible to oxygen toxicity at APPROPRIATE depth. Is this correct, or does this only apply when you pass depth x ? *Obviously, you would be more prone to it if you chose not to adhere to the max allowable depth correlating with the blend you're diving, but if you ARE following the correct profile, are you still more susceptible to O2 toxicity?

... It's entirely possible that someone could go into OxTox staying above the MOD and short of the 24 hour clock. ...

charlesml3 is implying the missing piece of your answer without saying it explicitly. The training materials and computers track two routes to oxygen toxicity.

One is an instantaneous exposure to a ppO2 greater than 1.4 or 1.6. This is expressed as a MOD for the %02 you are diving.

The other is a time-integrated exposure to any ppO2 greater than 0.5, and looks a good bit like the way N2 exposure is tracked wrt DCS. This is captured in table form in the DSAT O2 exposure table. As an example, if you are exposed to a ppO2 of 1.0, you cannot do that for more than 300 minutes (5 hours) in any 24-hour period. Even at a ppO2 of 0.6, you only get 12 hours in every 24, in case you were thinking about some Guinness record or something.

I agree with all the other posters about how the O2 rules are approximations and the physiology is incompletely understood, etc. etc. But don't confuse that with the issue that the rules themselves predict O2 toxicity at depths well less than MOD if you stay there long enough. So the answer to your question is yes, greater depths but still less than MOD increase the chance of O2 toxicity.

 

[SkimFisher]

There have been cases where people have tox'd at PO2's lower than 1.40 (just heard of one the other day in fact). On the other side of that, there are many more instances where divers have dived well beyond the limit of 1.6 and not tox'd. Some old salts will tell you of diving above 2.

The limits (1.4 and 1.6) were established based on a "well, no one's dying, so we'll set the limit here" kind of thought process.

 

[TSandM]

I think the heart of your question is whether oxygen toxicity is a binary function -- in other words, are you fine until "boom", or is there some kind of toxicity going on before you get that far. It's likely that the latter is true, but absent any symptoms or any evidence of long term problems (of which, in the central nervous system, there are none) we don't know about it.

Breathing elevated oxygen tensions creates a risk of seizures -- we know this. We also know that adverse events have not been recorded at certain ppO2s, so Nitrox divers are taught to keep their ppO2s below that level. 1.4 has previously been generally recognized as safe, but there has been at least one death at that level, so some people are recommending going down to 1.2 for the working part of the dive.

If you are diving at a ppO2 of 1.0, are you at risk for symptomatic oxygen toxicity? The statistics are very strong that you are not.

 

[Meng_Tze]

On moderate depth dives I keep my PO2 around 1.1- 1.3. On deep dives, I bring that all the way down to 0.9 for the working part.

 

[scubadiver888]

I was talking with a dive buddy the other day, and ran into a question. I can't seem to find my EANx booklet anymore, so figured I would ask here.

Because Nitrox has an increased percentage of O2, it seems that you would be more susceptible to oxygen toxicity at APPROPRIATE depth. Is this correct, or does this only apply when you pass depth x ? *Obviously, you would be more prone to it if you chose not to adhere to the max allowable depth correlating with the blend you're diving, but if you ARE following the correct profile, are you still more susceptible to O2 toxicity?

Oxygen toxicity related to depth is all about the partial pressure of the oxygen in your mix. You can get oxygen toxicity with air if you go deep enough. For recreational divers, you should be staying well above the depths air CAN be toxic.

DAN had a workshop back in 2000 where recreational diving agencies decided to start supporting Nitrox for recreational diving. See DAN Divers Alert Network.

I've read the transcript from the workshop and remember reading that only a few people had oxtox with less than 1.6 ppo2. It was decided that 1.4 is a good safety margin. There is nothing saying you will not oxtox at 1.3 ppo2 (actually one person in the data did oxtox at 1.3).

I have meet people who have a certain attitude. They smoke because no one in their family died of lung cancer. They believe that they don't fit in the statistics, i.e. they won't get cancer. They also believe that YOU might get oxtox at 1.6 ppo2 but they won't. They have gone to 2.0 ppo2 and were fine so they can continue to go to 2.0 ppo2 and be fine. There is no scientific evidence to support their belief but they'll do it until they have an oxtox. Maybe luck is on their side and they will die of something else before they oxtox.

Basically, you can roll a pair of dice one hundred times and get 2 every time. It is physically possible but highly unlikely. How much are you willing to gamble that you will get 2 hundred times? Are you willing to bet your life? The odds of rolling 2 hundred times is better understood then oxygen toxicity. Maybe some day we will be able to better predict oxtox in each individual but right now, I'm staying about 1.4 ppo2.