Oxygen Toxicity

[Scubahagel]

I'm sure this question has been asked several times, although I could not find a thread for it so I'll just go ahead and ask.

Why does oxygen become toxic when the percentage reaches and exceeds its depth?
I understand that different gas mixtures have to be set to certain depths, and that those depths cannot be exceeded because O2 becomes toxic at depth, but why does this happen? What is it about O2 that causes it to become toxic at depth?

I'm not a Tech Diver, I haven't taken my Enriched Air course yet, but I thought I would try and get some back ground info to help me along.

Thanks in Advance for the help.

 

[Thalassamania]

Why? That's the real question and there is no definitive answer. All we have is the empirical observation that it happens. PPO2s and their depths are guidelines, some people get into trouble with less, some can be exposed to a lot more without issues ... you pays your money and takes your chances. Sorry to be so inexact, but that's the real nature of the beast.

 

[TSandM]

To paraphrase Thal, the precise mechanism of oxygen toxicity, whether in the central nervous system or elsewhere, is not understood and there has been a fair amount of interesting research done to try to identify the mechanism. You can pull up a lot of papers if you search on oxygen toxicity in the Rubicon Research archives (see my sig line).

 

[Mr Carcharodon]

Try Googling this:
Current Thoughts on Mechanisms of Hyperoxic Seizures, Johnny E. Brian, Jr., M.D.

The readers digest version of which is that elevated partial pressures of oxygen lead to elevated levels of hydrogen peroxide in the brain (this first part is known). Then the speculation is that increased hydrogen peroxide levels exhaust one or more of the enzymes that are involved managing free radicals which in turn compromises neurotransmitters leading to a seizure.

But an interesting observation is that seizures often do not occur when hydrogen peroxide levels are at their peaks. But more commonly after decreasing ppO2 after a period of hyperoxic exposure. That has lead to speculation that CO2 and not hydrogen peroxide is the culprit or that both the oxidative stress of the hydrogen peroxide and the CO2 is needed to trigger a seizure.

 

[wedivebc]

But an interesting observation is that seizures often do not occur when hydrogen peroxide levels are at their peaks. But more commonly after decreasing ppO2 after a period of hyperoxic exposure. That has lead to speculation that CO2 and not hydrogen peroxide is the culprit or that both the oxidative stress of the hydrogen peroxide and the CO2 is needed to trigger a seizure.

The off-oxygen effect has been witnessed in several diving incidents.

 

[acwest]

The off-oxygen effect has been witnessed in several diving incidents.

An example that springs immediately to mind is the incident in this thread http://www.scubaboard.com/forums/accidents-incidents/316830-i-need-help-finding-more-info-about-christmas-accident-philippines.html
It appears that through some mix-up of tanks, one diver had both 100% O2 deco tanks and his buddy had both 50% O2 tanks. They completed their 21m deco stop, went to 16m for their next stop and he went into convulsions. Sounds like exactly the same off-oxygen effect.

 

[ianr33]

They completed their 21m deco stop, went to 16m for their next stop and he went into convulsions. Sounds like exactly the same off-oxygen effect.

Or his O2 clock just caught up with him.

Does anyone have some links showing the "off-oxygen" affect is real ?

 

[Indian Valley Scuba]

So many variables, so little hard knowledge!

 

[Gene_Hobbs]

Does anyone have some links showing the "off-oxygen" affect is real ?

This was first described by Ken Donald in 1947.

Kenneth W. Donald. Oxygen Poisoning in Man. Part II. Br Med J. 1947 May 24; 1(4507): 712–717

Here is a re-post of something Simon Mitchell wrote on Rebreather World a while back that sums up the current thinking on the "off oxygen effect" quite nicely.

 

[Rhone Man]

Good summary for lay people on Wikipedia (it is a "featured article" quality article - top internal rating on Wikipedia).

Oxygen toxicity - Wikipedia, the free encyclopedia