OXTOX limits - MOD, PPO2 or CNS Clock?

[DeepSeaExplorer]

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

Okay... I have to say posts like this are where a lot of the misinformation comes from. You've made a medical diagnosis and you want people to change their diving procedures based on this 'evidence' of a problem. This is why the PO2 keeps getting lowered, by the broadcast of anecdotal 'evidence' that simply isn't based on facts. Then throw in a reference to the WKPP to try and attach credibility to your claim. There's no way a 2 or 3 hour dive on 37% at 15 feet is an O2 problem. I suspect the problem is probably more a combination of eye strain and dehydration.

This reminds me of the "I got bent in the pool" thread a few years back. The idea was absurd, but a surprising number of posters believed it was possible. Now, scale that up to something a bit more complicated with more uncertainty and posts like this and the next thing you know the max PO2 will be 1.0.

 

[berk]

Refer to the chart. 12' average depth is 1.36 ATA (37%) = 0.5 ATA PO2.

Oddly enough right on the line where you can essentially stop tracking long term exposure for whole body toxicity.

-matt

 

[berk]

If still worried about the eyes:

Hyperoxic myopia.

QUOTE:
We have reported the development of 1.6 diopters of myopia in a group of patients exposed to reptitive oxygen breathing at two atmospheres absolute pressure. No significant change in corneal curvature accompanied this refractive change. During the three months following termination of the hyperoxic exposure, the myopia gradually disappeared. It is speculated that hyperoxygenation alters the metabolism of the adult lens producing an increase in its dioptric power.

The study population consisted of patients undergoing repeated hyperoxic exposure for the treatment of osteoradionecrosis. The mean age of the patients studied was 59 years with a range of 51 to 69 years. These patients were exposed on a daily basis to two atmospheres absolute pressure and breathed 98% oxygen for 120 minutes at this pressure.
END QUOTE:

Seems like they did about 40 of these 2ATA "dives" for a total of 80 hours.

Save the vitamin money for diving.

-matt

 

[DevonDiver]

Save the vitamin money for diving.

 

[D_B]

I think that if someone really looks at the PPO2/Ox exposure chart they can see that time is very much a factor in it. Time at 1.6 is less than time at 1.4 , the allowable time at 1.8 would be less, and 2.0 even more so ... but there would still be time to descend and get your buddy and return to a shallower depth, one higher than your planned MOD

Thanks for your posts on the matter Howard

 

[DevonDiver]

This reminds me of the "I got bent in the pool" thread a few years back. The idea was absurd, but a surprising number of posters believed it was possible. Now, scale that up to something a bit more complicated with more uncertainty and posts like this and the next thing you know the max PO2 will be 1.0.

I heard that Gamma-Aminobutyric Acid (G.A.B.A.), a common health supplement, could block/reduce likelihood of a tox hit.

Gamma-Aminobutyric Acid and Oxygen Poisoning
Journal of Neurochemistry volume 10, Issue 9, pages 625–633, September 1963




GABA and oxygen toxicity: A review Department of Biochemistry, University of Saskatchewan






Maybe there should be a different max PPO2 for people who took that?

 

[Superlyte27]

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.

 

[ianr33]

Of the literally thousands of dives we've done, guess how many O2 hits i've seen? Yup, you're right, Zero!


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

How do you know that if you have never seen it happen?

 

[PfcAJ]

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.

It's not ALWAYS the first symptom, but it sure is the worst. It results in a funeral. Most recreational divers don't use helmets and don't have support crews.

I can't see a reason for pushing ppo2s for dive planning (other than deco weenery, which is dumb). If you plan for a lower ppo2 (think 1.2ish), the depth range that you can use to help another diver is quite large before you're at a risky ppo2 (greater than 1.6).

I only dive at a 1.6 at rest, on deco. IIRC, there has been a pretty solid link demonstrated between exercise, co2, and oxygen toxicity.

 

[TSandM]

I believe that there is some thought that nitrogen has a neuroprotective or damping effect, which is why people successfully dove well beyond acceptable ppO2s today, when doing deep air dives.

I have some nice tables summarizing the studies on oxygen toxicity, which I will try to get scanned into some kind of format where I can post them. 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.

Given the extreme variability in the data, the concept of a universally applicable O2 clock is really pretty dubious. There is undoubtedly a relationship with the product of concentration and time, but I think the only thing we can say with confidence is what the lowest limit of that product is. At this point, it appears that exposure to ppO2s at or below 1.4 is extremely unlikely to produce oxygen toxicity, and we certainly have data for extremely long exposures to 1.2 (followed by higher concentrations on deco).

I agree that it's undesirable to give students the idea that dipping slightly below their contingency ppO2 in an emergency is going to kill them instantly. But I do think people diving Nitrox should know that toxicity is very unpredictable, and that people have been known to seize at a ppO2 of 2.0 with less than ten minutes of exposure.