Breathing 100% Oxygen at about 30 feet

[Thalassamania]

CNS oxygen toxicity has a time dose relationship and the onset of symptoms is highly individualized.

That's really the point, in the old days candidates for O₂ Rebreathers did an oxygen tolerance run in a chamber breathing pure O₂ at 60 FSW (2.81ATA). Those who passed were cleared to dive an O₂ Rebreather to 20 feet (1.6ATA). Pure O₂ is sometime used (with care) for decompression as deep as 30FSW (1.9ATA). While this exposures are a bit extreme and should not be undertaken without training and a good reason, they do no automatically result in convulsions.

 

[limeyx]

CNS oxygen toxicity has a time dose relationship and the onset of symptoms is highly individualized. If you go to 33 ft breathing 100% O2, which, as spectrum stated, would equal 2ata ppO2, you would not necessarily begin to have convulsions. The acceptable limit for O2 is 1.4ata ppO2. For deco purposes it is acceptable for the ppO2 to be 1.6 ata because you are at rest. Breathing 100% at 20 ft equals 1.6ata ppO2.

From my understanding, there is research published that indicates that for PPO2 the onset of CNS symptoms varies so much (between different divers and even between the same diver on different dives) that the time component definitely cannot be relied on.

You could tox in 5 mins one day, and the next day at 50 mins for instance.

This is why I try to keep the PPO2 on the working part of the dive below 1.2, and then go to 1.6 for deco.

 

[DavidPT40]

I think the original poster wanted the actual mechanism that caused the convulsions.

 

[Thalassamania]

I think the original poster wanted the actual mechanism that caused the convulsions.

Wouldn't we all.

 

[Mako Mark]

everything in moderation, too much of anything is bad for you, including O2.

I have never heard an adequate explanation of the actual mechanism that O2 creates CNS convulsions, we know that O2 has a narcotic effect that interferes with the sheaths that cover the nerves just like nitrogen does. It would be logical to think that it could interfere with proper nerve function in other ways too, but exactly how, I dont know.

 

[cwilson]

"wouldn't we all"

Exactly. The mechanism for O2 toxicity is not known. It probably involves free radicals (reactive oxygen species), it probably involves intracellular nitric oxide signaling, and the risk of it definitely increases with increasing partial pressure of inspired oxygen. But the fact is the exact mechanism is not known.

This much is known:
The same diver on different days of a 3 month long period can have seizures after 5 minutes of a given O2 dose or 150 minutes of the same O2 dose. No one knows why. In other words, getting away with it once is no guarantee that this means you're "proven" at that ppO2.

Because of this, a diver must take care to treat the phenomenon of O2 induced convulsions conservatively. To push the boundaries of this physiology is not wise.

There is increasing interest in the mechanism of O2 toxicity not just from the diving world, but also from the medical world, where hyperbaric oxygen is used to treat ailments as diverse as infection and carbon monoxide poisoning. Understanding of this mechanism will ultimately be expanded. With that greater understanding the barriers now existing may be expanded, just as new gas mixtures (trimix, nitrox) have already done. In the mean-time observing the existing recommendations on max ppO2 isn't just safer, it's just good diving.

cwilson

**This statement is intended only to be educational in nature. No medical advise is intended or implied.

 

[Charlie99]

More likely than not, you would have no problems at 30'.

Recompression chambers usually supply 100% O2 at 60', for ppO2 of 2.8ata, with periodic air breaks. The Australian method of in-water recompression uses a full face mask, underwater, at 9 meters -- about 30'. Typical treatment times at depth are 30 to 90 minutes.

While seizures are not unheard of, most divers manage to sustain treatments of a few hours without having a seizure.

The problem is that underwater, a seizure has a high likelyhood of being fatal, so we set very conservative limits for O2 exposure for scuba divers. That's also why a proper IWR setup is with full face mask.