How conservative is the PPO2 Limit

[tphelps]

I've given limits related to recreational use of Nitrox with FO2 up to 40 %.

I am not a technical diver but I understand PPO2 of 1.2 b is used for prolonged exposure to high concentration oxygen which would occur when using say a rebreather for decompression diving. They will switch between hyperoxic gas mixes and air to minimise CNS exposure. Commercial divers may exceed these recommendations, the justification I've seen being that they have a full face mask (which prevents them drowning in the event of a convulsion) and may have a communications link topside which means quick action if something goes wrong. None scenarios apply in the case of recreational use of Nitrox.

Right you are Foxfish! I run my P02 at 1.2 when diving my rebreather, i will run it higher for deeper dives though.

To answer the question, a P02 of 1.4, to me is a good recreational limit. I am a tec diver and have done deco stops with 1.6 but that is another ball game. Ultimately it comes down to the physiology of the individual. My advice, if you are concerned, set a personal limit of around 1.2 or however YOU think is best!

 

[Centrals]

How conservative is the PPO2 limit?
Mark Andrews dived to 155m(~490ft) on AIR back in July 1999. He was well prepared and trained.

As for me, PPO2 of 1.4 for bottom gas is acceptable while 1.6 for deco(50% at 21m and 100% at 6m).
Can't speak for the others.

 

[Foxfish]

You may have misread my post. I said that incidents of divers toxing on air are virtually non-existent. There are plenty of divers who have expired at depth on air, although most of the cases I am familiar with it was believed to CO2 buildup caused by breathing resistance rather than O2 toxicity. The Steve Feldman case was a classic example of this. If I was to take an educated guess, I would assume the diving death you refer to at Dahab was much more likely to be caused by chronic and massive narcosis rather than toxing in such a short space of time. But of course I cannot know for certain.

Anyhow, I hope you have obtained enough salient information in this thread to make your own informed decisions on relative toxicity risk. Good luck and safe diving.

Yes I think I did misread what you said. I can understand if people don't find the examples particularly compelling when talking about deep diving on air. I have become more aware from this thread that the whole question of deep diving on air is controversial with some which would explain some of the irritation seen above. That said, even within the ranks of those who call themselves deep divers, I've noted many would recommend at least respecting the PPO2 1.4 b or 1.6 b limit and highlight the problems with narcosis at those depths. Based on comments of those who did deep diving in the past on other threads, it was not so much a case of whether you would get oxygen toxicity, but more a case of gauging how deep you could go before this happened.

 

[tamas970]

I have yet to see a case, where a diver died within the 1.6bar ppO2 level AND below 80% "allowable exposure" (=36min@1.6bar ppO2).

Of course NO cold water, medical condition, overextertion, narcosis (aka no deep-air dives) AND proof, that the case was not a CO poisoning.

Any examples?

My personal view is, that 1.4bar should be fine, and for short periods (<10min) 1.6bar shouldn't kill you either. For saving a family member (NOT A COWBOY GO-TO-150' ON 40%-"instabuddy") I'd risk 2bars.

Anyway, I am trying to be more conservative with my mixes and use EANx30 for deep (120-130') dives.

 

[SailNaked]

RM very interesting, maybe start another thread, but I would think that the norcosis effect of the N2 likely inhibits the O2 Tox effect. sort of like taking asperine with a headache. or drunks in a car wreck not being hurt.

it you consider that most of the limits are set first conservatively by the Navy, then more conservatively by the agencies, and even more conservatively by the insurance companies, then adding your own conservation factor should keep you about as safe as driving to work in a tank.

If I could not push the limits a little after understanding as well as possible what is going on, I would not bother diving at all, i do not dive to be safe, I dive to live, and I am very safe. maybe that is why I sail catamarans, not safe, not stable, lots of fun within limits LOL.

 

[xdjio]

I always keep pO2 below 1.2 if at all possible. That said, I was about 1.29 last weekend, but only briefly.

 

[Foxfish]

I have yet to see a case, where a diver died within the 1.6bar ppO2 level AND below 80% "allowable exposure" (=36min@1.6bar ppO2).

Of course NO cold water, medical condition, overextertion, narcosis (aka no deep-air dives) AND proof, that the case was not a CO poisoning.

Any examples?

Did you read the first case?

The underlying motivation for starting this thread was that I've noticed divers on other threads question the validity of the established PO2 limits for recreational diving. Divers claim the limits are too conservative and ask for examples where people have suffered oxygen toxicity at certain limits. The cases at the start of the thread are my response.

---------- Post added September 12th, 2013 at 12:28 AM ----------

I always keep pO2 below 1.2 if at all possible. That said, I was about 1.29 last weekend, but only briefly.

Recreational or technical diving?

 

[tamas970]

Yes. As I understood the first case:

"...a deco gas of 73% was used"
"clipping off her deco bottle at 147ft"

Nitrox73 at 147ft is almost 4bar!

Did you read the first case?

 

[Foxfish]

it you consider that most of the limits are set first conservatively by the Navy, then more conservatively by the agencies, and even more conservatively by the insurance companies, then adding your own conservation factor should keep you about as safe as driving to work in a tank.

The examples I posted indicate the established PPO2 limits are not as conservative as you might think. Did you read them? That was the point of the exercise.

---------- Post added September 12th, 2013 at 01:11 AM ----------

Yes. As I understood the first case:

"...a deco gas of 73% was used"
"clipping off her deco bottle at 147ft"

Nitrox73 at 147ft is almost 4bar!

The dive was planned to a ppO2 of 1.4 using a mix of 24/26. Because of the cave profile, a deco gas of 73% was used, since deco at 20ft was next to impossible.

Divers4Life | Liz & Tim's Adventures

She was breathing the 24% O2 mix for the main dive. The gas mix was checked and found to be 24%. Deco gas would be used at the end of the dive for decompression.

Gas was analyzed by four separate analyzers multiple times. Three were O2 only, one also included helium. All four analyzers read an O2 percentage of between 24.0% and 24.6%. The helium analyzer read 26% for the helium percent. There were no apparent gear malfunctions, everything was in working order. A gas sample was sent to Trace Analytics and all parameters were well within specifications (except for oxygen percentage, but that is because their instruments aren&#8217;t set up to measure anything but air).

Divers4Life | Liz & Tim's Adventures

---------- Post added September 12th, 2013 at 01:15 AM ----------

The bottom line is that, while 1.4 ppO₂ may be safe for most divers most days, it is not always safe. People have seized with no advance warning whatsoever on 1.4, 1.3, and in some cases even as low as 1.25, but to the best of my knowledge and the knowledge of the doctors whom I have involved in this evaluation, there have been no seizures reported at 1.2. So before you consider "pushing" the ppO₂, especially on a Trimix dive, ask yourself -- is the additional risk worth the slightly shorter deco?

Divers4Life | Liz & Tim's Adventures

---------- Post added September 12th, 2013 at 01:48 AM ----------

This is an interesting study done on rebreather divers at a PP02 of 1.19 b.

No indication of exposure time is given in the summary. Given that it was done on divers using rebreathers, it seems likely that exposure times would be longer than normal for a recreational diver.

The result showed on 3 dives out of 1000 (0.3 %) a diver experienced loss of consciousness.

METHOD:

We collected 2527 dive reports from 473 closed-circuit oxygen divers (a mean of 5.2 reports per diver), and analyzed the relationships between various symptoms and their dependence on depth and diving time.
...
RESULTS:

No CNS oxygen toxicity-related symptoms were reported at a depth of 2 m seawater (msw), but their proportion increased at depths from 3 to 6 msw. We found that CNS oxygen toxicity-related symptoms appeared in 2.5% of dives conducted at a Po2 of 119 kPa. The main symptoms and signs reported were headache: 4.5%; nausea: 2.6%; hyperventilation: 2.6%; heavy breathing: 2.4%; dizziness: 1.6%; hiccups: 1.5%; bloody sputum: 1.4%; cold shivering: 1.1%; tinnitus: 0.9%; difficulty maintaining a steady depth: 0.9%; disorientation: 0.6%; tiredness: 0.5%; tingling in the limbs: 0.4%; hearing disturbances: 0.4%; a choking sensation: 0.4%; extreme effort: 0.4%; and loss of consciousness: 0.3%.

CNS toxicity in closed-circuit oxyge... [Aviat Space Environ Med. 2006] - PubMed - NCBI

 

[tamas970]

Divers4Life | Liz & Tim's Adventures

Okay, I misunderstood it that she used the deco gas already at 147ft.
I (and maybe no one) don't know how gases
exactly interact in the brain, maybe trimix divers might be
in a greater risk than simple nitrox divers.

Besides, she took Oxymetazoline regularly as well before
the dive, which might have affected her CNS:
Oxymetazoline - Wikipedia, the free encyclopedia

Also:

...a sign I knew from prior dives that indicated she was dizzy...

... She also struggled taking the 100% O₂ on the shallow stops of the (chamber) ride, with tingling, dizziness, and blurry vision.

Multiple sings for enhanced individual susceptibility, maybe because of her
medication. Very sad, that this was ignored and no additional precaution was done KNOWING that high O2 levels cause issues while resting in a chamber I would immediately 1. buy a full face mask, 2. limit my ppO2 to 0.8bar, where the CNS clock ticks around 10x slower.

One more reason that I'll take the chamber ride my club is offering as a
Christmas program and another reason for avoiding ANY kind of medication
before the dive. In case of congestion the most I do is a hot shower and
inhaling the steam of a hot tea.