Oxygen Toxicity risk with Nitrox?

[Fishdip]

Extreme hypoxia. Ox tox is breathing too rich of an 02 mix. This was the opposite as the O2 content was zero.

Extreme hypoxia! That is what I was looking for thank you!

 

[OMyMyOHellYes]

Racking up minutes and exposure on a LOB is IMHO no place to be deciding to push limits even further....

Just because you wouldn't have access to any medical facilities for the rest of your life?

OMMONY

 

[Cthippo]

Extreme hypoxia! That is what I was looking for thank you!

You might also be looking for anoxia, or total lack of oxygen.

 

[Subcooled]

Here are my questions:
Just how dangerous were my dives?

It depends on how long you stayed down there and how active you were!
Oxygen toxicity is a process/condition that takes time to develop in your body.
The deeper below MOD you go and the harder you work the faster it manifests itself, though.
And a random element is included.

An example: I once found myself at 30m (ppO2=2.0) with nitrox 50 (it's safe to ~21m only). It was an emergency situation and I did not have a choice. I did not stay down there a long time, so I escaped without oxygen toxicity. Good luck I guess.

Was I taking a huge risk or are there already safety margins built in?

Of course there are some safety margins. What tourist industry could tolerate e.g. 1% dead clientele?
Just remember that (e.g.) a 0.01% chance of death does not comfort you a lot would you happen, by chance, to be in that 0.01%

Historically there have been tables (I've seen) that tell how long (if young and fit) you can stay at certain ppO2. Nowadays however we usually recommend ppO2=1.4 as max for open circuit, perhaps 1.6 at rest during decompression (with pauses if prolonged time), and rebreather divers might want to keep it even lower at 1.3 or below as the exposure times are long (and pulmonary oxygen toxicity becomes an issue). On my trimix course (TDI) we used ppO2=1.5 max (21% oxygen at 62m) but the bottom time was short (a couple of minutes) and the dive was not a strenuous one.

Nitrox 30 at 41m would be ppO2=1.5. I might dive down to that depth on that gas momentarily for some good reason (not to stay though). Narcosis is very real at these depths, too.

 

[Lriemann]

Thank you all for the input.

I'm going to be more careful next time.

 

[formernuke]

So you exceeded maximum recreational depth in general and also exceeded 1.4 ppO2.

It is well known that the navy tested divers at above 1.4 and 1.6 and divers have done it including you and survived (1.4 that is). That does not make it a safe pratice. Not only do you the immediate concern of exceeding this buy you also have to worry about total O2 exposure for a given amount of time.

In short you violated several well established diving safety rules. That fact that others did as well is no excuse.

 

[MrVegas]

Can you supply some sort of an official estimate on this for NDL diving?

I know of many cases with technical diving, but I don't believe I have ever heard of a single one in NDL diving. I would like to see some examples the "way too many people" if you can find them.

In the technical diving cases I know, we are not talking about someone going to a PPO2 of 1.55 for 4-6 minutes. We are talking serious MOD violations, often caused by the diver thinking he or she had a different mix than reality. In those cases, the toxicity came after significant time. Here are some cases I know of. I am writing these from memory, and some of the details may be a tad--but only a tad--off.

• A diver broke his foot and could not dive for several months. When he was ready to dive again, he took his double tanks to a wreck (Hydro Atlantic) with a deck at 150 feet/45 meters and about 170 feet/52 meters to sand. He did not check the tanks because he was sure they had air, but they had 36%. His buddy told me it took about 20 minutes before he toxed.
• A cave diver used a tank that was clearly marked as having pure oxygen for a dive to 100 feet. He told his friends he had filled it himself, so he knew it had air instead of oxygen. He refused to test it. I am not sure how long he was diving on pure oxygen at 100 feet before he toxed.
• A cave diver was supposed to leave a decompression bottle with 50% at a 70 foot staging area and take a different one more appropriate for 200 feet. He left the deep bottle at the staging area and took the 50% to 200 feet. I don't know how long he was diving at 200 feet on 50% before he toxed.

I don't think there is much in the way of statistics out there because there seem to be very few cases - at least for standard recreational diving. This was in Alert Diver a few years back:

https://s3.amazonaws.com/books.diversalertnetwork.org/Spring_2018_flip/Spring_2018_AlertDiver/HTML/72/

Basically, a 10-year review found only one likely fatality due to oxtox, and I believe that was someone that took 32% to 160 feet. There have certainly been a few other stories discussed on ScubaBoard, though. I only did the SDI Nitrox class in 2017 and my recollection is that the book primarily focused on 1.6 being the limit with 1.4 for those wanting to be more conservative. (Though everyone sticks to 1.4 in my experience.) The book "The Last Dive" I believe mentioned someone that routinely took 39-40% deeper than 130 feet, but eventually had a convulsion and died. (Just my recollection.)

I haven't been doing this long enough to really opine on the matter, but the topic interested me and I tried to find actual numbers. As far as I can tell, the phenomenon is not all that predictable, so better to be on the safe side.

 

[boulderjohn]

There have certainly been a few other stories discussed on ScubaBoard, though.

I have only been an avid ScubaBoard participant since 2004. I don't recall any during that period. Perhaps you could link to one.

I only did the SDI Nitrox class in 2017 and my recollection is that the book primarily focused on 1.6 being the limit with 1.4 for those wanting to be more conservative.

That would be very surprising. Can you find a direct quotation?

The book "The Last Dive" I believe mentioned someone that routinely took 39-40% deeper than 130 feet, but eventually had a convulsion and died. (Just my recollection.)

It's been years since I read the book. I don't recall anything like that, but my memory is far from perfect. Perhaps you could find a quote on that for me? (I could try to contact Bernie to see if he remembers writing about it, but I really don't want to bother him.)

 

[Eric802]

I did SDI nitrox earlier this year and the written materials still say 1.6 is the limit. 1.4 is for being “more conservative”. All on-line so I can’t quote it for you.

 

[boulderjohn]

I did SDI nitrox earlier this year and the written materials still say 1.6 is the limit.

Be specific.

I took my PADi nitrox class in 2002, and they said 1.4 was the regular limit and 1.6 was the contingency limit. Looking at that, I could also say that 1.6 was the limit.

Does the SDI material say that 1.6 is the standard limit for all dives? Does it mention a contingency PPO2 that goes deeper.

I ask because I used to be a TDI instructor (same organization), and this does not being any bells for me.