OXTOX limits - MOD, PPO2 or CNS Clock?

[Jax]

Thanks!

I guess that makes you a duct tape fan? :blinking:

 

[PfcAJ]

LOL, 90' on 100% are we still talking Apples to Apples? Out of the 1000 views on this post, you posted 3 instances of what you HEARD. The thousands of members on here dive every week, some every day. What have you personally SEEN. If we added up the cumulative dives from every member who has posted on this thread alone, we are literally talking about tens of thousands of dives. Yet, no one here has seen an O2 hit that resulted in convulsions? C'mon, it's not voodoo guys. It's rare. Certainly don't go spend an hour at 2.3ppo2, but this isn't the big scary monster people are trying to sell it to be.

Pressure is pressure. Almost all of us working in the gulf are diving wet. Most of us wear nothing but coveralls, not even a wetsuit (unless it's a mixed gas dive)

Your sample of divers who have seen a tox is pretty low, ya. In fact, it probably is zero on this board. I really like how the examples I gave you (which were just a few, I KNOW there are more) weren't good enough. Cute. For a phenomenon that never happens, it sure did kill those folks.

Now, is that because its so exceedingly rare that it never happens, or does it never happen because divers do not expose themselves to 1.6+ on a regular basis?

I have never seen it because a) I and everyone I dive with analyzes their gas and b) I don't dive with unsafe divers. There is just no reason at all to dive high ppo2s.

You reference diving "wet". I'm not so sure that its not the head being wet that increases the likelihood of a toxic episode. To my knowledge, no one even really knows what causes it. There are some hypothesis, but no conclusive evidence on way or another.

High ppo2s is one thing in diving that will get you killed faster than anything else. There is simply no reason to push it, esp when the industry standard of 1.4ish is working pretty well. Personally, I shoot for a 1.2ish average. YMMV.

 

[Slamfire]

I don't know where that chart came from. Pulmonory Oxygen Toxicity takes longer than CNS Oxygen Toxicity as far as I know.

"The time limits for ppO2s [in the NOAA table] below 1.4 atm stem more from pulmonary oxygen toxicity concerns than from CNS (acute) toxicity" (Hamilton, 1989). I'm interpreting this as being more likely to have pulmonary O2 toxicity than getting a CNS hit when you go for more than 150 mins at 1.4 atm.

There was a thread here on SB... not too long ago, where a poster posted essentially that he was in need of getting his buddies attention, but his buddy went a little too deep, and he didn't want to exceed his MOD...

I've seem more than one thread like this in SB. This is one where I participated.

Now, is diving at 1.0, 1.3, 1.4 safer than 1.6? Of course it is. But if 100 people today make a 4 hour dive at 1.6, how many of them are going to take a hit? 99% of the time, zero people in the above scenario will take a hit. For years in the commercial field, I watched this over and over and over. I've NEVER seen an oxygen hit from 1.6ppo2. That doesn't mean that it won't happen to me tomorrow. But the chances are very very slim.

Donald (1947) reports that "The Admiralty Experimental Diving Unit was unable to demonstrate oxygen poisoning in the range of 0 to 20 fsw." This is the ppO2 range of 1.0 to 1.6 atm.

The bottom line is that nobody ever toxed in less than five minutes at any ppO2, but the variability in WHEN they toxed is absolutely chilling. The variations between text subjects, and from day to day with the same test subject, are tremendous. Seeing those results has given me a great personal respect for the unquantifiable nature of toxicity risk from elevated ppO2s.

I am not denying the variability. I am just saying that just because there was great variability when people were subjected to ppO2s of, say, 3.5 atm, does not necessarily mean that you will have the same variability at 1.6 atm. And even if you do have that same variability once you get past the threshold, hasn't that threshold been moved far enough ahead to make it impractical to cross it due to restrictions like the amount of gas available for the dive and nitrogen loading?

My replies above are copied from a post I made a couple of months ago in another thread. I'll put the post here so that the context of my replies can be understood:

Oxygen Exposure Limits
I'll try to be succinct. First of all thanks to those that pointed me in the right direction to get the information. I read some of the information last night and learned several new things I did not know before. I welcome any well-based corrections to my conclusions:

Great Degree of Variability
Kenneth Donald is often cited for a study about variability. Unfortunately I have not read Donald as a primary source yet. He made a point of the individual variation to time-quantitative symptoms. There's the classic plot of the diver who was exposed to a fixed oxygen partial pressure day after day and his time quantitative symptoms ranged from five minutes to several hours -- Figure 4 page 5.

Does anybody know what was the ppO₂ that Donald was using in this experiment? According to Shreeves and Richardson (2000), most of the published body of testing oxygen exposure involves ppO₂s greater than 1.6 atm. This makes extrapolating to lower ppO₂s difficult. SB Medical Moderator, Saturation (2003) says that several O₂ convulsion studies have been made "across a range of pp02 with most at pp02 > 3". I am not denying the variability. I am just saying that just because there was great variability when people were subjected to ppO₂s of, say, 3.5 atm, does not necessarily mean that you will have the same variability at 1.6 atm. And even if you do have that same variability once you get past the threshold, hasn't that threshold been moved far enough ahead to make it impractical to cross it due to restrictions like the amount of gas available for the dive and nitrogen loading?


1.6 vs 1.4 vs 1.2
Donald (1947) reports that "The Admiralty Experimental Diving Unit was unable to demonstrate oxygen poisoning in the range of 0 to 20 fsw." This is the ppO₂ range of 1.0 to 1.6 atm. Shreeves and Richardson (2000) say that "against this data set, a shorter time limit of 45 minutes at the lower O₂S limit of 1.6 atm, certainly seems reasonable." I am guessing that one reason why so many O2 convulsion studies are done at such high ppO2s is because they cannot get subjects to easily convulse in controlled lab situations when you keep the ppO2 to 1.6 atm or below.

The time limits for ppO₂s [in the NOAA table] below 1.4 atm stem more from pulmonary oxygen toxicity concerns than from CNS (acute) toxicity (Hamilton, 1989). I'm interpreting this as being more likely to have pulmonary O₂ toxicity than getting a CNS hit when you go for more than 150 mins at 1.4 atm.

So 1.6 atm is very conservative and 1.4 atm even more so. Why, then, did PADI choose 1.4? D. Richardson (2000) from DSAT/PADI says, "We should look at the database and known outcomes to answer that question. For this whole discussion, if you look at Kenneth Donald's work from 1942 to 1945, working with the Royal Navy, they did about 2,000 test dives using oxygen breathing. If you compare that database to NOAA's dose duration, it falls right in between 1.4 and 1.6 atm. PADI had no objection to 1.6 atm. We set it at 1.4 atm for recreational divers as a margin of safety for diver error. However, 1.6 atm fits very nicely under the Donald data set as well."

Bjorkmann (2000) from EnviroDive says that DCIEM allowed them 1.5 atm, but they moved it to 1.2 atm because it "allowed us to put it all on a simple table that was very easy for people to track... if you have to go beyond a 1.2 atm, we have to start getting back into the formulas and this is where we're running into problems" [with divers remembering how to use formulas].

Wells (2000) from NOAA says they created their table "thanks to the fine work of Butler and Thalmann that had been published a couple of years ago...". He goes on to say, "I've been asked many times why the Manual doesn't match the numbers in the Butler and Thalmann paper. It was considerably less than they had recommended in their paper. As you remember, the [old] Navy [manual] cut back from 1.6 atm for 30 minutes. We wound up increasing it to 1.6 atm for 45 minutes based on Thalmann's work... The 1.6 atm for 45 minutes fell within what we considered a quite acceptable risk."

Thalmann (2000) comments on why 1.3 atm was chosen for the Navy. "Because it can be dived all day long. It doesn't impact Navy operations all that much and they don't want any O2 hits in the water. The Navy has within the last month rewritten its helium tables to eliminate 100 percent oxygen in the water. Why? In the last two years there have been two O2 incidents. They don't want any of them, they've eliminated O2 in the water and everybody's happy with that."

In the 2000 DAN Nitrox Workshop, the participants made it clear that they were not aware of a single case of CNS convulsions for single exposure dives at 1.6 ppO2 or less. Single exposure dives refer to a one gas, usually within the NDL dives. Multi level exposure dives refer to staged decompression dives where the diver gets exposed to higher O2 levels several times throughout the dive. Apparently the participants where aware of 2 cases of Multi level exposure dive hits. It seems that this was taking into consideration about 10 years (1990-2000) of dives and about 250,000 dives.

Vann (2000) from DAN says, "When you're talking about a multi-level oxygen exposure as you are after a long decompression dive, it's a whole new ballgame. You can't compare that with a single limit."

Even though a quarter of a million non-controlled anecdotal dives without a hit (or with 2 hits if you include tech diving) is hardly scientific data, it does hint that 1.6atm for 45 mins may be adequate.

Risk
So why go through the big deal of trying to eliminate all risk of a O2 hit when it seems that the likelihood of a DCS hit is much higher? An underwater O2 hit can carry much more dire consequences than a light DCS hit like skin bends, or pain in your knees. It seems that'll be easier to survive a small DCS hit than an O2 hit, particularly if you're soloing.

Conclusions
Max ppO₂.- I think that the 1.6 or 1.4 atm limit is perfectly suitable for single exposure dives. Think about it in the context of the limitations of an average recreational diver with a single Al 80 trying to stay within the NDL. For shallower depths, the long air NDL doesn't make it very worthwhile to use Nitrox. For deeper depths, you run out of gas before you hit the NDL or the Oxygen clock limit.

The Oxygen Clock .- The clock may have been created using what seems arbitrary rationale -- at the 1.6 atm level -- and pulmonary toxicity rationale that does not address CNS at the 1.4 atm and lower levels. But the reason for doing this seems to be simply because it is very hard to get people to convulse at 1.6 or below.

Does that make the clock useless? I don't think so. It provides a measurement parameter that may be correlated to risk. I prefer to measure %CNS and log it for every dive and have a relative idea of where I am standing. Relative to what? I don't know, but something is better than nothing.

If I were to go on a staged deco dive using nothing but ratio deco, you better believe I'll want to put my max ppO2s as low as possible (1.2 and lower) simply because I have no way of tracking my %CNS and this is a multi level exposure dive -- which, the experts say, does wonky things. If on the other hand I have V-planner giving me a %CNS figure I'll have more peace of mind because I can refer to previous dives like this and their respective %CNS. I am able to benchmark against myself. I know there could be the variability issue. But something is better than nothing, and I'm still not fully convinced that variability will play a crucial factor at the lower ppO2 levels. If it had, we should have seen much, much more O2 hits at 1.6 or below.[/QUOTE]

 

[Tortuga68]

In my admittedly uneducated mind, I couldn't perceive how one could on-gas enough O2 to break the PPO2 limit in just a few seconds to a minute

The PPO2 limit - like the MOD limit - is determined by depth & gas mix only, not time or gas consumption; that's pretty much the point of this thread

 

[cruiser]

When you are jetting, basically you are shooting a high pressure 3" water hose underwater to dig a big trench. Sand, sea shells, jelly fish, everything in this jet stream is blown everywhere. So, to keep this crap out of you a little, you tape up every opening in your cover-alls. I wear cheap nylon mesh gloves to protect my hands, but tape them to my sleeves. I wear steel toed rubber boots (like what cement workers wear when working in cement) then tape my coveralls to them so they don't fill with crap.

Oh, and still love the new avatar.

How do you keep the crap out of your neck opening?

 

[Jax]

The PPO2 limit - like the MOD limit - is determined by depth & gas mix only, not time or gas consumption; that's pretty much the point of this thread

Ugh . . . my misstatement -- should have said "I don't see how one could on-gas enough O2 in a minute or so to be dangerous . . . ." :blush:

 

[Tortuga68]

Indeed. But people still freak out about it, which lends creedence to the contention that the training on this topic is inadequate. When I posted a thread about a 74m air dive, people flipped over the PPO2 exceeding 1.6 without even asking what the exposure was ( <5 minutes)

 

[TSandM]

If I were to go on a staged deco dive using nothing but ratio deco, you better believe I'll want to put my max ppO2s as low as possible (1.2 and lower) simply because I have no way of tracking my %CNS and this is a multi level exposure dive

You know, I'll have to go back and look at the material, but I'm quite sure we used some basic calculations to compute %CNS time in both Rec Triox and my RD seminar, so I think the statement that you have no way of tracking your %CNS is untrue.

Slamfire, your post is generally excellent, and highlights the lack of solid data for making the specific decisions about acceptable ppO2s. I think the bottom line is that oxygen toxicity seizures underwater on recreational or standard technical scuba gear are almost uniformly fatal, so the guidelines have been written to attempt to reduce the risk to zero. There are at least two recorded cases of seizure and death at 1.4, but in general, prolonged exposure to that level of oxygen appears to be very safe.

GUE, which prefers 1.2 for the working portion of the dive, reasons quite rationally, I think, that you are better of planning for and doing the decompression required, rather than attempting to minimize it with high ppO2s over long exposures. That's the tradeoff -- shorter bottom time or longer deco, versus increased risk of CNS (or with very long dives, pulmonary) toxicity. We have to remember that the benefit gained here really has a very small value when compared to someone's life.

"I don't see how one could on-gas enough O2 in a minute or so to be dangerous . . . ."

There is a difference between equilibration and ongassing. When we talk about ongassing nitrogen, we are talking about the relatively slow diffusion of gas molecules OUT of the circulation and into tissues. Equilibration of partial pressures between blood and the lungs occurs far more rapidly -- truly in the range of seconds. And the brain is one of the most highly perfused tissues in the body, so that mass of oxygen which is dissolved and carried on hemoglobin goes there first and in large quantity. That's why it takes such an extremely short time for inhaled anesthetics to render someone unconscious.

 

[Akimbo]

Let's back up 30 years ago when the limit for air was 320'.

I know about 100 people who were diving 320'ish 30+ years ago on air. None of them ever took a hit, albeit a couple of them got bent.

Now, lets go to the commercial field (my field) where we spend lots of time at 1.6ppo2. Of the literally thousands of dives we've done, guess how many O2 hits i've seen? Yup, you're right, Zero!

Now, is diving at 1.0, 1.3, 1.4 safer than 1.6? Of course it is. But if 100 people today make a 4 hour dive at 1.6, how many of them are going to take a hit? 99% of the time, zero people in the above scenario will take a hit. For years in the commercial field, I watched this over and over and over. I've NEVER seen an oxygen hit from 1.6ppo2. That doesn't mean that it won't happen to me tomorrow. But the chances are very very slim.

By the way, the absurd idea that convulsions is the first symptom, is just dumb and untrue.

I have never seen Oxygen toxicity symptoms either. The Navy air limit was 297' on air, or a 2.1 PPO2 in 1970 when I was in First Class Diving School. In fact, all First Class Divers when to 297' on air in a wet pot just to graduate. I can’t recall even hearing about O2 hits in commercial diving in over 40 years. I know of a few O2 hits on non-diving medical hyperbaric treatments on highly compromised individuals.

I have worked with several ex-Navy SEALs and O2 hits were of minor concern even though circumstances often drove them deeper than 33' on pure 02 rebreathers — heightened awareness for symptoms yes, but little reservation. One guy told me he and his buddy had to swim at about 55' for 20 minutes (2.66 PPO2). Granted these people are superb physical specimens; but I doubt any of us ever see their stress levels either.

My take on this thread is that nobody is advocating a less conservative approach to PPO2 limits for recreational diving, just a more appropriate perspective. All of the different O2 guidelines are still evolving and are just that… guidelines not laws.

It amazes me that people get so heated up over the remote probability of O2 toxicity, yet make open sea decompression dives that should at least have a chamber onboard. In fact, these dives should probably be on the end of a hose with comms if safety is a real concern.

 

[Slamfire]

You know, I'll have to go back and look at the material, but I'm quite sure we used some basic calculations to compute %CNS time in both Rec Triox and my RD seminar, so I think the statement that you have no way of tracking your %CNS is untrue.

Thanks for the correction. I never completed the RD seminar -- only went through half of it. In the literature I've read about it I don't recall coming across a way to track %CNS. I have limited knowledge of RD. I know about it, but I don't use it. I dive with people that do use it. In the past, when we plan the dives, I've usually emailed them my tables, cut from V-Planner/Z-Planner, and ask them if they are ok with doing said profile.

Slamfire, your post is generally excellent, and highlights the lack of solid data for making the specific decisions about acceptable ppO2s. I think the bottom line is that oxygen toxicity seizures underwater on recreational or standard technical scuba gear are almost uniformly fatal, so the guidelines have been written to attempt to reduce the risk to zero. There are at least two recorded cases of seizure and death at 1.4, but in general, prolonged exposure to that level of oxygen appears to be very safe.

Thanks. I agree. I also agree with what Howard brings up about OW, AOW divers thinking they'll be in imminent, dire, mortal danger if they cross beyond 1.6 for a few seconds. Operating under those assumptions is not good. It is actually dangerous in the sense that people could be left out to die when there was somebody around that could have done something to prevent the death. Instructors may not be actually saying that you will die if you go beyond 1.6, but what we've been seeing here in SB is that some OW/AOW's operate under that fear.

By all means, I advice all divers to plan and execute their dives to 1.4 or less. I do so myself. But if you find yourself diving EAN36 at 95' and your buddy just dropped, catatonic without a reg in his mouth, to a ledge 20' below, for goodness sake, go fetch him up.

I think knowledge is a good antidote against operating under those unfounded fears. It would benefit OW/AOW's to know where those 1.6, 1.4, and 1.2 numbers came from. Know the risk associated with going beyond those figures. Personally, I like conservatism. It is my friend. But I think the risk of going below 1.6 for a few mins to make a rescue is more than well justified.

GUE, which prefers 1.2 for the working portion of the dive, reasons quite rationally, I think, that you are better of planning for and doing the decompression required, rather than attempting to minimize it with high ppO2s over long exposures. That's the tradeoff -- shorter bottom time or longer deco, versus increased risk of CNS (or with very long dives, pulmonary) toxicity. We have to remember that the benefit gained here really has a very small value when compared to someone's life.

Again, I agree. I also want to note that these GUE dives, more than often, involve multilevel O₂ exposures. It seems that multi-level exposures brings an additional risk factor in itself. Diving 1.4 vs 1.2 is not going to give you a huge deco advantage, but I think the rationale is more about a secondary "justification" for not deviating from standard gases and training people to dive from the beginning as if they're going to do these huge high risk dives (but that's just me ).