Oxygen Toxicity Discussion

[vladimir]

I do understand exponential functions, holding an engineering phd... However, not all non-linears are the same, some accelerate "faster" some "slower".



I am not an expert on hyperbaric medicine, but these numbers seem to be on a different units...
(OTU's maybe?) And I am sceptical that a tec diver can take 10x more CNS oxygen just because of training.

OTUs are used to track pulmonary oxygen toxicity risk. The units in the chart you linked are, explicitly, percentage of the NOAA allowable limit for a single dive.

 

[mdax]

Each time I read debates over how to dive deep on air I cringe. It usually happens on this board due to the number of new folks and complete morons. New divers should pay attention to the fact this conversation is happening in a death thread.

"In reality" there is ZERO reason to dive to 250 on air.

 

[boulderjohn]

When I first took nitrox, the exam was much more complex than it is today, largely because of all the time spent on pulmonary oxygen toxicity, which is what OTUs track. We had to do a lot of math to determine which was the controlling factor on a dive late in a multiple dive day, the accumulation of oxygen or the NDL. What I learned from doing those problems was that it is really, really hard to do enough diving at a high enough PPO2 to get into trouble with pulmonary oxygen toxicity. I assume that is the reason that the current teaching has nearly eliminated that from the course. I would have to be planning a very seriously high PPO2 day before I would even think about it.

 

[Tortuga68]

I am sceptical that a tec diver can take 10x more CNS oxygen just because of training.

Good, because nobody is saying that

 

[Ron Lee]

What I learned from doing those problems was that it is really, really hard to do enough diving at a high enough PPO2 to get into trouble with pulmonary oxygen toxicity. I assume that is the reason that the current teaching has nearly eliminated that from the course. I would have to be planning a very seriously high PPO2 day before I would even think about it.

So it appears that a pulmonary hit from a high ppO2 is unlikely. The concern then is a CNS hit which would probably be fatal. That is my interest should it be necessary to rescue someone. Knowing the relative risks and how you react to depths below TBD feet is far better resolved in advance.

Scenario: You are last in line at Devil's Throat. After everyone is through the DM starts into the area that used to contain the cross sponge. You turn away from the group a few seconds (10-20 or so) to look at something and when you turn back to follow the group you notice a diver is 20-40 feet below you and still sinking. The DM is gone (in swim throughs) and in no position to see the problem or respond to it.

Do you go after the person? You have rescue dive training. At what point does the risk/reward change from low risk/successful rescue to high risk/likely double fatality? How is that impacted by previous exposure to depths below 130' on air?

Gas is air.

This is an answer to a likely response: Correct, I have no experience with nitrogen narcosis on air at 250'.

 

[boulderjohn]

Do you go after the person? You have rescue dive training. At what point does the risk/reward change from low risk/successful rescue to high risk/likely double fatality? How is that impacted by previous exposure to depths below 130' on air?

You did not mention what you are breathing. Let's say you are breathing EAN 30, which would be appropriate for a dive to that depth. The risk of toxicity in the situation you describe is still very remote. I would not hesitate to go after such a diver.

Despite what some people have said in this thread, the time it takes to have an oxygen toxicity hit at reasonably high PPO2s is much longer than you are led to believe in training. You can be sure that the recommended limits for MOD have a very comfortable buffer built in. Note:

1. The WKPP had a rare fatality a couple of years ago when a group of divers mysteriously broke with their meticulous safety protocols, with the result that a diver left a tank with breathing gas designed for the bottom portion of the dive at the depth of a decompression stop (70 feet) and took the decompression gas with him instead. He was breathing EAN 50 at about 200 feet (3.5 PPO2) for a surprisingly long time before he had the toxicity hit.

2. A diver in Florida thought he had air in his doubles, but it was really EAN 36. He, too, was breathing that for quite a while at more than 150 feet (roughly 2.0 PPO2) before he toxed.​

I am not saying that you should not pay attention to MODs. I am saying that the margin for error is just not that close.

 

[finnks]

When I first took nitrox, the exam was much more complex than it is today, largely because of all the time spent on pulmonary oxygen toxicity, which is what OTUs track. We had to do a lot of math to determine which was the controlling factor on a dive late in a multiple dive day, the accumulation of oxygen or the NDL. What I learned from doing those problems was that it is really, really hard to do enough diving at a high enough PPO2 to get into trouble with pulmonary oxygen toxicity. I assume that is the reason that the current teaching has nearly eliminated that from the course. I would have to be planning a very seriously high PPO2 day before I would even think about it.

Believe you have this backwards. It's not the high PPO2 you have to worry about. As someone mentioned earlier in this thread there are different nonlinear equations with different sensitivities.

If you are using 32% and spending most of your time at ~90' (>~ 1.2 atm PPO2) you have a rather low risk of a CNS hit. However if you are aggressively getting as much bottom time as possible, managing say five - two hour dives a day for several (5+?) days in a row - I think you would be beginning to approach the pulmonary limits. Don't have tables or my model with me at the time, so I could be way off...

Other risk is if someone is injured and put on 100% O2 on the surface at a remote location. My recollection is that with no prior elevated O2 exposure, the pulmonary limit is around 1 day at 1 atm O2. If you've been pushing the nitrox limits diving, then put on surface O2 your window is smaller. Personally I've only been one place remote from professional medical care and that was probably within range for CG helo evacuation within an hour of calling for assistance. Don't plan to ever make such a call!

 

[boulderjohn]

Believe you have this backwards. It's not the high PPO2 you have to worry about.

No, I don't have it backwards. It's a combination of the PPO2 and the time on it that leads to pulmonary O2 toxicity, and as I said, it's pretty darn tough to get a dive schedule that will get you there.

 

[raftingtigger]

If you are using 32% and spending most of your time at ~90' (>~ 1.2 atm PPO2) you have a rather low risk of a CNS hit. However if you are aggressively getting as much bottom time as possible, managing say five - two hour dives a day for several (5+?) days in a row - I think you would be beginning to approach the pulmonary limits. Don't have tables or my model with me at the time, so I could be way off...

Pulmonary limits aren't likely the concern here. Without looking at tables... 5 - 2 hour 90' dives a day for 5 days seems to me the limiting factor is nitrogen loading and DCS prevention (ie decompression). (that's 50 hours of diving time in 5 days...)

This whole discussion is getting way off the original thread. Maybe it should be it's own (fantasy) thread instead.

Deep Air = significantly increased risks
How close can you cut it? Personally I don't ever want to know.

Anyone know any more details about the accident that started this thread?

 

[vladimir]

Pulmonary limits aren't likely the concern here. Without looking at tables... 5 - 2 hour 90' dives a day for 5 days seems to me the limiting factor is nitrogen loading and DCS prevention (ie decompression). (that's 50 hours of diving time in 5 days...)

This whole discussion is getting way off the original thread. Maybe it should be it's own (fantasy) thread instead.

Deep Air = significantly increased risks
How close can you cut it? Personally I don't ever want to know.

Anyone know any more details about the accident that started this thread?

I think you are confused about the purpose of this forum. The sort of useful discussion we are having is the only valid reason to dissect an incident like this.