Oxygen Toxicity risk with Nitrox?

[Lriemann]

Hi folks!

I recently came back from a one week Liveaboard in the red sea.

Everyone dived with Nitrox, usually at 30%.
According to procedure, maximum depth should be 36.6 meters to avoid oxygen toxicity.
This is also what we signed on in the log for every Nitrox tank.

However, in reality, many if not all divers went to 40 meters and slightly below on several dives.
My maximum depth was 42 meters, and I went below 36.6 on 5 dives.
This happened to me because of diving in the blue (lack of orientation) and seeing some nice hammerheads below, plus noticing other divers were doing the same.

A few days after returning, I met a Divemaster I'm friends with and mentioned it.
He told me I was very foolish to go below 36 meters for, and I quote, "a photo of a fish".
He said insurance won't cover me and he would not go to that depth with Nitrox to save a drowning diver.

Here are my questions:
Just how dangerous were my dives?
Was I taking a huge risk or are there already safety margins built in?
Is it true that there are no warning signs before toxicity kicks in and you experience the effects?
Is the signing of the log a way for the Liveaboard (and insurance company) to avoid responsibility in case of an accident at a restricted depth?
Where can I find statistics on oxygen poisoning from Nitrox?

Thanks a bunch.

 

[Marie13]

First rule of diving nitrox is to know your MOD and obey it. Distractions of sharks and everyone else going below their MOD are no reason to ignore MOD.

How long were you below your MOD?

 

[Eric802]

Well, if you use a PPO2 of 1.6, your MOD becomes 43 meters. 1.4 is definitely safer and more conservative.

 

[grf88]

You were approaching a PP02 of 1.6. This used to be considered safe when I first got certified for Nitrox but today a maximum of 1.4 or even 1.3 is recommended for the active portion of the dive. Helium mixes are less forgiving as nitrogen does have some effect on reducing toxicity.

 

[L13]

My maximum depth was 42 meters, and I went below 36.6 on 5 dives.

42 meters is less than 1.6 PPO2. People use that all the time for DECO. It's not a sharp line, but the accepted limit for the bottom of the dive is usually 1.4 PPO2. Unless you hang out there, you are likely (but not guaranteed) to be ok even a little higher than 1.6. Very rarely people have issues below 1.4, which is why many CCR divers use 1.2 or less for the bulk of their dive.

 

[L13]

First rule of diving nitrox is to know your MOD and obey it. Distractions of sharks and everyone else going below their MOD is no reason to ignore MOD.

How long were you below your MOD?

@Marie13 is right though, YOU are responsible for YOUR diving. Exceeding MOD should not have happened if it wasn't part of YOUR dive plan, or at least well understood by YOU.

 

[rhwestfall]

The old exposure limits were developed in research using young athletic military personnel (like it was for the dive tables). As it has been adopted universally to the recreational public, you notice conservatism applied over time. There isn't enough known so those limits have been padded for statistical outcomes in an acceptable range of error for the "general public". You have to decide your own tolerance of risk....

How many rounds will you play with in the cylinder?

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

 

[inquis]

Helium mixes are less forgiving as nitrogen does have some effect on reducing toxicity

I know N2 helps for HPNS, but hadn't heard that for oxtox. Any links to further details I could explore?

 

[PfcAJ]

Narced. Likely on a single tank. Past your nitrox MOD.

Oxygen toxicity is a huge concern given the 1.6ish PO2, but that’s assuming a single dive. Repetitive exposures aren’t well modeled.

But your other risks are more concerning to me.

 

[azstinger11]

The old exposure limits were developed in research using young athletic military personnel (like it was for the dive tables). As it has been adopted universally to the recreational public, you notice conservatism applied over time. There isn't enough known so those limits have been padded for statistical outcomes in an acceptable range of error for the "general public". You have to decide your own tolerance of risk....

How many rounds will you play with in the cylinder?

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

Considering most divers are much closer to meal team six than seal team six that's a very valid point. Also the military has a much higher risk tolerance for mission completion than any recreational diver should for seeing the pretty fishes/reefs.