ppO2 for nitrox, why so low?

[kotik]

You could have stopped at that.

I didn't though... as sometimes happens on the internet, I plowed on ahead with saying what was on my mind

Here's what I'll say for myself, not speaking on anyone else's behalf:

• Here's an essay from Shearwater on the subject. It cites an Israeli study that logged probable CNS toxicity occurrences at PO2 ~1.6. Shearwater's article is geared toward warning about CO2 buildup for rebreather divers, but CO2 buildup can have other sources too. Anyway, low probability or not, it does look like there's documentation of O2 toxicity symptoms at PO2's as low as 1.6
  
• I have 'heard', ancecdotal only, of one or two cases of fatalities in the Florida caves where the best guess on what went down was O2 tox at relatively low PO2.
• Convulsion underwater is almost a certain death sentence. I'm not going to offer any support for that statement. If anyone cares to dispute it I'll put it to them to come up with the list of those who've convulsed and lived to tell about it.

I'm not preaching against anyone deciding to choose their max depth based on PO2 1.6. I personally think there's a possibility, very small, of some people experiencing toxicity when diving within recreational limits at that. Anyone who's somewhat aware and is okay with the odds is welcome to make their own grown-up decisions.

Higher PO2's, I think the possibility becomes more and more real. Somewhere on the spectrum it shifts from becoming a possibility to a statistical certainty that someone among the many divers out there will tox and die. There is no good reason to introduce that extra risk and I don't think it's responsible to encourage whoever in the world might be reading this thread that the risk of high PO2 is really overblown.

 

[boulderjohn]

I have 'heard', ancecdotal only, of one or two cases of fatalities in the Florida caves where the best guess on what went down was O2 tox at relatively low PO2.

It is these "best guesses" to which I was referring earlier.

I am very familiar with the details of the analysis of a particular dive fatality that remains unexplained. I have spoken at length with a friend who was on the dive who has searched and searched and searched for an explanation. There is no possibility of oxygen toxicity being involved--it was a shallow dive on air. The fatality defies all explanations.

What if the PPO2 levels had been higher, approaching 1.4? Whatever the actual cause of death, I guarantee you that this fatality would be included in someone's list of oxygen toxicity events, because that would be the best guess.

 

[tursiops]

I agree. But when someone post that O2 tox will happen @ 1.6 ata we had better think about some of the deep air record dives by ones like Hal Watts, A. J. Muns, Neal Watson, and others. Take for example Bret Gilliam’s record dive to 452 on air. If my math is correct then that would be 3.086.

My non-scientific but compelling (to me) understanding is that a OxTox occurs when the nerves/neurons "twitch" due to the O2, and that the "twitch" is damped by the N2 in the system. So, the PPO2 limits of 1.6 (or whatever) are extended if you've got enough N2 in the system. This suggests that a dive to 452 on 21% O2 heliox would NOT be successful.

 

[electric_diver]

I recall reading a paper I downloaded from Rubicon that reported on a study done by the Royal Navy that put divers on 100% O2 at 25 feet (PO2 1.76) and recorded 0 instances of O2 toxicity on both working and non working divers for durations of up to 2 hours.

I suspect (but can't prove) that the rules are different for oxygen only versus oxygen plus nitrogen versus oxygen, nitrogen and helium. Empirical evidence seems to indicate that the latter requires a lower PO2 than the former two.

 

[kotik]

What if the PPO2 levels had been higher, approaching 1.4? Whatever the actual cause of death, I guarantee you that this fatality would be included in someone's list of oxygen toxicity events, because that would be the best guess.

I agree with what you're saying, but I also sympathize with the hypothetical someone who's ticking off the thing as a possible tox. As you pointed out, the cost of getting this wrong is drastic.

To those in this thread I've addressed or quoted- I'll try to be clear about my own beliefs. Not that I know so much either, but since I've been opinionated here I figure it's respectful to be plain about where I stand, being a stranger so to speak.

I imagine concerns about running at PO2 1.6 in recreational diving are largely theoretical. Among all the potential divers in the world, there might be some out there that might get unlucky on certain dives. I formed my own habits around a lower number, but that's me. If someone puts thought into the decision and ultimately decides they're comfortable with 1.6, then I assume they just don't worry so much about the various lottery numbers that might come up on them throughout the day, rather than they must be some daredevil.

I don't think it's a good idea though to offer advice on an internet forum that diving at PO2 2.0 is okay as long as you don't move much. There are definitely documented cases of toxicity in that range, and if that were a fad that caught much traction then I think there would be a tragedy before very long. In my opinion it's not a number that should be promoted.

 

[CuzzA]

I might have missed it, but does anyone have a source from a confirmed tox at 1.6?

 

[caruso]

I might have missed it, but does anyone have a source from a confirmed tox at 1.6?

The report is on the desk. In the barn. Right next to the stall with the unicorn in it.

 

[kotik]

I might have missed it, but does anyone have a source from a confirmed tox at 1.6?

The apparent answer is yes.
CNS toxicity in closed-circuit oxygen diving: symptoms reported from 2527 dives. - Abstract - Europe PMC
Oxygen Seizures at PO2 ≤ 1.6 Bar: How Rare? - Shearwater Research

Can only get to the abstract of the original IDF report without paying, but there were cases in a study of 2527 dives, including cases of lost consciousness. Dives were on O2 rebreathers at depths up to 6m. High CO2 is thought to be a large factor. I don't find anything to say the length of time it took to experience toxicity at 6m, but the Shearwater article mentions that on a 240 minute dive, some divers even had symptoms at 1.2.

Note that the dive conditions are probably pretty extreme, but the IDF divers are probably in pretty good shape too, if that does anything toward balancing out the skew.

Again- not trying to imply this is representative of normal recreational diving, but it does admit at least the remote "theoretical" possibility.

 

[TONY CHANEY]

My non-scientific but compelling (to me) understanding is that a OxTox occurs when the nerves/neurons "twitch" due to the O2, and that the "twitch" is damped by the N2 in the system. So, the PPO2 limits of 1.6 (or whatever) are extended if you've got enough N2 in the system. This suggests that a dive to 452 on 21% O2 heliox would NOT be successful.

It was not a dive on “21% O2 heliox” but just air.

 

[tursiops]

It was not a dive on “21% O2 heliox” but just air.

That's my point. It was successful because of the N2 in the air. I was trying to say maybe you've got to look at more than just the O2 content, and see whether the rest of the gas might mitigate the O2 toxicity.