Thread split: CNS toxicity limits

[Superlyte27]

This is the same argument as people saying don’t overpressure LP tanks because the man said so, but don’t understand why the man said so, and also can’t math that there has never been a single fatality from a properly maintained LP steel tank, despite 10s of thousands of over pressured tanks every year, just like I’m certain there are 10s of thousands of dives vastly exceeding 100% CNS without incident.

I ask again, when does anecdotal evidence become empirical data? Certainly a hundred thousand dives over the course of 20 years, right?

 

[boulderjohn]

This is the same argument as people saying don’t overpressure LP tanks because the man said so, but don’t understand why the man said so, and also can’t math that there has never been a single fatality from a properly maintained LP steel tank, despite 10s of thousands of over pressured tanks every year, just like I’m certain there are 10s of thousands of dives vastly exceeding 100% CNS without incident.

I ask again, when does anecdotal evidence become empirical data? Certainly a hundred thousand dives over the course of 20 years, right?

Science will want to have some numbers, and to get those numbers, you need something more than people offering vague beliefs in social media. I know about dives that exceed those numbers, but I cannot begin to guess how many or point to specific cases. We were told that there are many cases where people succumbed within those limits--I don't know anything about those.

The big issue is that we are starting with some hard and fast numbers that were put out long ago, and the starting point is the belief that they have some validity. As I said, my job as an instructor was to teach my students to calculate all that to a couple decimal points. So really, where did those numbers come from? Where is the research? How were they determined?

 

[PfcAJ]

I am getting exactly what you are saying. You are choosing to ignore what I am saying. And making up ridiculously extreme examples to try and support your argument.

Yes, if the tables said a 60' dive had an NDL of 2 minutes, I would ignore it. But, that is an example of a table putting such a huge limitation on a dive that it's not worth doing.

And that is not what the NOAA CNS tables do. Diving within the limits prescribed by the CNS tables still afford us adequate bottom time to have very enjoyable dives.

I am approaching it from the standpoint of "what gives the vast majority of the people the ability to dive without getting hurt the vast majority of the time?"

You are approaching it from the opposite perspective. You are taking some examples of some extreme violations of the table that have not produced CNS hits and saying that just because there are occasions where you can commit a major violation without a CNS hit means that the tables have no value.

And that is just rubbish. Millions of dives happen within the bounds of the tables and without CNS hits. Millions of very fun dives with very enjoyable amounts of bottom time. Those tables give a very safe baseline that MOST divers can and do use for enjoyable dives without incident.

The fact that there are extreme examples of table violation with no "hit" is one way we know that we really do not understand all the parameters of a CNS hit. Those examples do NOT mean that the tables are useless. There are also plenty of examples of divers taking a CNS hit even though they were within the limits prescribed by the tables. That is one way we know that we are not at the "2 minute NDL" level of ridiculous overcautiousness.

There is a percentage of dives that happen and get "undeserved" CNS OxTox hits. When that percentage is at some level that the experts have deemed "safe" - which seems to usually run somewhere around 2%-ish or less - then we know that the tables are conservative in the "right" amount (based on our current level of understanding of the issue).

Those tables are the basis of a LOT of dives that happen without incident - and that is their value.

You may as well also preach that recreational dive computers are useless because some get bent anyway and some people surface with 45 minutes of omitted deco and don't get bent.

I just got out of the water less than an hour ago and my CNS% was well past 100%.

This isn’t “on occaision”. It’s routine. On a random Friday evening. Not special. Just normal.

How? Because the “limits” aren’t reflective of reality.

 

[Imla]

Don't you mean "every other tec diver would drown from a CNS toxicity seizure"? Am I still confused or is it very common to conflate CNS and pulmonary toxicity? This has confused me a lot in the past, and it has happened several times in this thread. Is there something I'm not getting? Are you implying a link between the CNS limits and OTUs and making a shortcut? Or is somebody saying the CNS limits are helpful to limit pulmonary toxicity because both OTUs and CNS clock tracks PPO2 exposure over time?

O2 can influence both lungs and brain negatively. In the brain, it causes ox tox with seizures. In the lungs, the oxidative stress causes damage to the tissue with following degradation in gas exchange over the alveoli.
As a nice anectode. I currently have the dubious pleasure of being treated in chamber for a wound.
3x30min O2 at 14m (2,4ATA), 5 days a week going on 8 weeks now...
That is 8888 OTUS pr DAY, 5 days a week....
(The good thing... at least running my GUE rebreather on PO2 of 1,3 wont kill me...)

 

[KevinNM]

We know, from treatment in hospitals, that extended high O2 levels (like the nearly 100% you get at 15l/min) will eventually cause lung damage, and the longer you continue the worse it gets. We know this because people have been on 100% in hospitals long enough for pulmonary toxicity to show up. AFAIK, CNS toxicity isn’t something that happens outside of a hyperbaric chamber or while diving, no matter how many days you are on 100% or how much lung damage you get.

Is there any real science behind the CNS clock?

 

[nadwidny]

We know, from treatment in hospitals, that extended high O2 levels (like the nearly 100% you get at 15l/min) will eventually cause lung damage, and the longer you continue the worse it gets. We know this because people have been on 100% in hospitals long enough for pulmonary toxicity to show up. AFAIK, CNS toxicity isn’t something that happens outside of a hyperbaric chamber or while diving, no matter how many days you are on 100% or how much lung damage you get.

Is there any real science behind the CNS clock?

No

 

[steinbil]

O2 can influence both lungs and brain negatively. In the brain, it causes ox tox with seizures. In the lungs, the oxidative stress causes damage to the tissue with following degradation in gas exchange over the alveoli.
As a nice anectode. I currently have the dubious pleasure of being treated in chamber for a wound.
3x30min O2 at 14m (2,4ATA), 5 days a week going on 8 weeks now...
That is 8888 OTUS pr DAY, 5 days a week....
(The good thing... at least running my GUE rebreather on PO2 of 1,3 wont kill me...)

That is a lot more than the daily 850 OTU limit

I have a question about the long term effects of pulmonary toxicity, but I think I'll start a new thread. These poor moderators...

 

[boulderjohn]

So on September 11 I asked DAN for the research behind the limits on oxygen exposure, and I got a response yesterday from Emmanuel Dugrenot. Here is what he wrote:

You are right, the CNS clock is conservative and many technical divers just ignore it… We do not have perfect means to prevent CNS toxicity, but some empirical studies can give you a better idea of the limits.​

I attached a short text I wrote with some references and articles on oxygen exposure!​

​

Reading through the articles he sent, parts of which are over my non-engineering head, makes it clear that there really isn't a lot of firm science behind the limits. What I saw clearly referenced was that studies showed tremendous variability between individuals and even with individuals on different occasions.

Here is one of the files he sent. It looks like I can only attach one per post.

 

[boulderjohn]

Here is another article. The third is too large.

 

[Capt Jim Wyatt]

Thanks for those articles @boulderjohn