CNS not important anymore?

[crofrog]

Here are the DCIEM tables...

 

[nadwidny]

Who sais so?

David Sawatzky and Neal Pollock are 2.

 

[Pavao]

This leeds to even more questions than answers. Most of the statments contain "it seems" or similar.

Is there no scientific approach to this topic? NOAA limits were fixed may years ago and they still seem to exist. Is it really in theory only?

Modifying your question a bit may hint you in the direction of the current thinking on the subject.

Try and answer this question instead:
What was the scientific approach used by NOAA to arrive at their limits?

 

[rjack321]

Is there no scientific approach to this topic? NOAA limits were fixed may years ago and they still seem to exist. Is it really in theory only?

No because anyone with any money (commercial or military) is decoing in a chamber.

And there's basically no way to pass an ethics review having divers risking their lives in the water

 

[crofrog]

No because anyone with any money (commercial or military) is decoing in a chamber.

And there's basically no way to pass an ethics review having divers risking their lives in the water

Both the USN and DCIEM tables above are in water o2 tables…

 

[rjack321]

Both the USN and DCIEM tables above are in water o2 tables…

So? For any significant exposure they deco in chambers, not 8+ hours in homemade habitats.

 

[crofrog]

So? For any significant exposure they deco in chambers, not 8+ hours in homemade habitats.

They’re still doing dives that are well over 3000% on the cns clock. It’s really easy to find dives in the tables that’s have over 100 minutes on o2 in water.

If it was creating an undo risk to their divers or people where toxing all the time I’m sure they would have updated it.

 

[rjack321]

They’re still doing dives that are well over 3000% on the cns clock. It’s really easy to find dives in the tables that’s have over 100 minutes on o2 in water.

If it was creating an undo risk to their divers or people where toxing all the time I’m sure they would have updated it.

Sure except... you know that folks are doing those military or commercial deco exposures in water? in 2025?

Just because they are in the tables doesn't mean the experience is there behind them. In my own jurisdiction, commercial in-water deco is basically not done regardless of what the DCIEM tables might say. At least locally, I suspect in the last decade+ 1,000s more tunnel excavators were decompressed compared to commercial in-water divers. None of those construction workers even got wet.

 

[sea_ledford]

There was a panel discussing this at the last AAUS symposium. I posted a followup on a different thread (below). I'm not sure when the official consensus statement will be coming out, but the decimation of NOAA probably isn't helping things.

TL;DR - 4 hours working + 4 hours deco/resting at PO2 of 1.3 will be the updated recommendation.

Info Post in thread 'Some History of the NOAA Oxygen Limits'

Mar 31, 2025

I did end up attending the workshop. I certainly can't definitively report what will be in the consensus statement, but I'll relay my understanding of it.

The primary emphasis was on long duration moderately high po2 levels below 1.4. The data set that established the NOAA O2 limit tables was from military combat swimmers using pure O2 units and the current limits in the 1.6 and above range, the panelists all agreed, were appropriate as they are.

The problem with all the lower limits were they were just a "well, that seems about right" seat of the pants interpolation of the higher...

• sea_ledford

• 
• 

 

[old frogman]

An experienced diver told me CNS has "no importance" anymore. You may make a dive which brings you out of water at 100 or 200 or 500% or whatever CNS. It simply does not matter anymore. Never heard about that, did you?

CNS Oxygen Toxicity is a product of many factors. O2 exposure tables which appear to be an US Navy thing, look at only two exposure variables, that is, time and depth. Also, they are designed for combat swimmers (not divers). A unique thing about combat swimmers is that they swim long distances underwater (sustained speed of 1 to 1.5 knots), in many cases wearing or towing personal weapons, explosives and submersible dry bags.

Other variables are individual susceptibility to O2, physical fitness, physical exertion, etc.

There is a paper, "Descriptive epidemiology of 153 diving injuries with rebreathers among French military divers from 1979 to 2009". Military Medicine, 176, 4:446, 2011.

In the paper the author describes 26 x CNS O2 hits where combat swimmers were predominantly using the La Spirotechnique Oxygers57 O2 rebreather. The swims were sustained fin-swimming 2 to 3 hours in duration and depth was about 6 to 7 metres (160-170kPa). The O2 hits appear to have occurred after prolonged exposure. This suggests a number of additional likely variables:
1. Extreme physical exertion contributing to possibly:
a. over-breathing the scrubber's capacity remove exhaled CO2.
b. elevated levels of CO2 in combat swimmer tissues during swimming exertion.
2. Scrubber reduction effectiveness towards end of 2-3 hr swim, resulting in elevated CO2 a suspected contributor to O2 hit.

In contrast, a recreational diver including the technical diver goes nowhere near the exertion level of combat swimmer. This probably explains why the USM or NOAA CNS tables may not reflect the true risk parameters.

I did mention that some persons may be susceptible to CNS Oxygen toxicity. In the military candidates for combat swimming undergo an oxygen tolerance test in recompression chamber (for 30 mins on O2 at 18m/60ft). In addition, they are tested for epilepsy susceptibility.