OXTOX limits - MOD, PPO2 or CNS Clock?

[DevonDiver]

Andy, what are in the standards and instruction regarding going deeper than MOD? (rescue etc.) Perhaps nothing due to liability issues.

PADI quote the 1.4 PPO2 limit for dive planning and identification of MOD. The 1.6 PPO2 limit is stated as a "maximum contingency depth".

I would interpret that they mean 'contingency' as being the maximum PPO2/MOD you should consider for emergency purposes. However, as you correctly identify, the agencies do not ever state such things catagorically for liability reasons!

When I teach nitrox courses, I try and be very clear that the 1.4 PPO2 provides a MAXIMUM operating depth. I make it clear that going beyond 1.4/1.6 steadily increases risk. As with all my courses, this will invariably lead to some frank discussions. I like to encourage open debate about the risk assessment that divers would have to do, when deciding whether to conduct a rescue or not. What factors the diver should try and consider. I try to make the debate realistic and also share my own views and experiences.

Having a 'contingency MOD' is pointless unless you have a contingency plan to go with it. The key is to consider these things in advance, and not wait until the sh1t hits the fan before you think about it.

I certainly don't use the "...or you will die!" approach to teaching nitrox courses.

 

[DiveNav]

I was trying to edit the thread at the same time another mod was. I think I killed it as I can't find it anywhere. I apologize for the inconvenience.

bad boy .... I will tell your boss

 

[DiveNav]

..... Again, it's the whole "VIOLATION OF MOD MAY CAUSE IMMEDIATE DEATH" approach to education......

And who said that?

So, if the recommended limit for 1.5atm is 83minutes and the one for 1.6atm is 45min, I just can't see why the limit for 1.601atm should be 0min .... human body doesn't work like that ... in fact I think I saw the table below somewhere ...

 

[danvolker]

Having written a reply to the 'What role does OxTox play in recreational diving?' thread (which has just been closed?), I thought I would transfer it here for discussion, as it raises some valid points..



Technically, you do hit the nail on the head with this. However, it is important to remember why the MoD limits were introduced and, then, subsequently made progessively more and more shallower as more diving statistics became available over the decades.

MoD is calculated as the depth at which a given breathing gas reaches 1.4PPO2. When a diver exceeds that MoD (or more importantly, that PPO2) their STATISTICAL likelihood begins to increase dramatically.

It is not a case that you "WILL" convulse at 1.4 or 1.6 PPO2.... it is a case that you COULD convulse under those circumstances.

The diving community has agreed, by consensus, that divers ideally should not exceed the statistical likelihood of OXTOX that exists beyond PPO2 of 1.4. The line has to be drawn somewhere and, after decades of diving, THAT is where the line was drawn.

It may be simplistic to define OXTOX prevention in terms of adherance to max PPO2/MoD, rather than focusing on the CNS Clock, but it is also a very effective approach that goes a long way to increasing diver safety AND developing an attitude of conservatism in dive planning/conduct.

It is also important to remember that the onset of OXTOX is also heavily influenced by numerous (and mostly not yet understood) physiological and physical factors that differ with every diver and every dive. In that respect, a conservative approach to limiting PPO2/MoD as a 'baseline' to OXTOX avoidance seems, IMHO, to be wise.



I disagree that the issuance of strict MOD/PPO2 limits is about easing the burden of the dive educator... especially as we 'should' be teaching students about CNS toxicity anyway.

MODs and PPO2 limits are about saying to students "This is the line that will keep you safe. Overstep that line and things could go fatally wrong for you."

To the discussion on recreational diver issues, here is one that most on scubaboard may not have considered.......This comes from my wife and several of her friends diving for macro photography at the Blue Heron Bridge ( max depth between 10 and 15 feet).....They had been doing many 2 to 3 hour long dives at this location using around a 37 percent mix...( 80's can last a long time on a 12 foot diver at the pace of a macro photographer) , and each had been complaining of poorer vision after diving...
This is known as hyperoxic myopia. Not permanent damage to lens of eye, but not smart to be creating temporary disfunctions like this either.

Taking the issue further, even on a 60 foot dive with a 36% mix, you could easily say that you are causing a massive increase in the creation of "free radicals" in your body....as many of us take large amounts of vitamin C and E , etc., in an attempt to limit concentrations of free radicals in our bodies, the idea that extensive nitrox use is actually quite unhealthy ( metabolically), may be worth considering.
The WKPP direction for this issue was always using normoxic trimix, as the "healthier" alternative to nitrox.

Regards,
Dan V

 

[DevonDiver]

Was it medically diagnosed as hyperoxic myopia.... or could it just be eyestrain from long hours of macro photography??

 

[danvolker]

Was it medically diagnosed as hyperoxic myopia.... or could it just be eyestrain from long hours of macro photography??

If it was just Sandra ( she was the photographer), I would think it could have been eye strain...although the use of the view finder is for only a few seconds, every few minutes--so you would imagine the eyes would not strain all that much from this....but she also had some dive slaves with her, just swimming along with her, looking at each creature Sandra would locate.....

She did NOT seek medical counsel or analysis, instead, the next 3 hour trip at the bridge will be on pure air....and I will post the results of this

Regards,
Dan

 

[cruiser]

Is the "Time" on your chart below really in Hours?

And who said that?

So, if the recommended limit for 1.5atm is 83minutes and the one for 1.6atm is 45min, I just can't see why the limit for 1.601atm should be 0min .... human body doesn't work like that ... in fact I think I saw the table below somewhere ...

 

[HowardE]

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

 

[DiveNav]

I don't know where that chart came from. .....

found it "down under" ..... www.divingmedicine.info

Online book comes with a nice intro from Glen H. Egstrom, Ph.D., Professor Emeritus, Department of Physiological Sciences, University of California, Los Angeles.

I assume he knows way more than me on this subject (it doesn't take that much :blinking

Alberto (aka eDiver)

 

[HowardE]

The oxygen clock is a mechanism for monitoring oxygen exposure over time. When diving at oxygen partial pressures above 0.5 Bar for long periods of time it becomes as important to monitor your oxygen exposure as it is to monitor your nitrogen exposure, although for quite different reasons. Whereas there is a saturation level for nitrogen after which you incur no additional decompression penalty and can remain underwater almost indefinitely given adequate facilities, with oxygen this is not the case. Over time, exposure to elevated partial pressures of oxygen is detrimental to the pulmonary system.

The theory behind the oxygen clock has been around for about 30 years and concerns pulmonary oxygen toxicity (aka whole body toxicity or the Lorrain Smith Effect). It is measured in units of pulmonary toxic dose (UPTD). There are various other names; the oxygen tolerance unit (OTU) and the cumulative pulmonary toxic dose (CPTD). Dr Bill Hamilton has suggested that we use the term OTU as he feels it gives more positive vibes. The OTU is based on empirical data from which the following best fit formula has been derived:

OTU = t * (PO2 - 0.5) 083
0.5
where:

t is the exposure time in minutes
PO2 is the partial pressure of oxygen in Bar
0.5 is the threshold below which no significant pulmonary oxygen toxicity has been observed.
0.83 is the exponent which gives the best fit to experimental observations.
However, very roughly, 1 OTU is equivalent to 1 Bar exposure per minute.

Period (days) Dose/day (units) Total (units)
1 850 850
2 700 1400
3 620 1860
4 525 2100
5 460 2300
6 420 2520
7 380 2660
The thing to remember, however, is that the values are not exact, hard limits, they are only guidelines. For most sport divers the oxygen clock is not a concern. However, for those who dive to partial pressures in excess of 0.5 Bar for long periods, especially if they are doing repetitive diving. It would be in their interest to track the OTU build up.

The NOAA Table for CNS Exposure is

Again... This is not "THE LAW" - but a guideline. 45 minutes at 1.6 doesn't mean that 46 minutes will cause tox.