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>I didn't realize your phisiology was so predictable that you know exactly
>how much oxygen is the maximum limit your body can withstand.  When do you
>convulse?  At a PO2 of 1.6ATA, or 1.62ATA? Or is it 1.58ATA?  Would that
>be at 15 minutes into the dive, or 17 minutes? 31 and a half minutes?
>
>People have convulsed at 1.2ATA oxygen.  People have also functioned
>completely normally at 7+ATA oxygen (yes, that's pure oxygen at SEVEN
>atmospheres)
>As far as I'm concerned, any inspired PO2 between about 1.2ATA and 1.8ATA
>is a crap-shoot.
>The only reason I'm making such a big fuss about this is that one of the
>aspect of "technical diving" that concerns me is this "number-worship"
>that's going on all over the place.  People keep drawing crisp lines for
>limits of PO2, and OTUs, and time, and a whole bunch of other factors, much
>like Skin Diver mag has done with the "go below 130 feet and you
>instantly die" depth limit. But they forget these are, at BEST, totally
>rough estimations.

I would like to add something in support of Richards point of view about 
"number-worship".  From my understanding of CNS oxygen toxicity,the line 
which describes the 10% probability of occurance of convulsions on a graph 
of PIO2 versus exposure time asymptotes at 2bar.  Based on this information, 
there is no way of describing the relationship between CNS O2 toxicity and 
PIO2 or exposure time below 2bar PIO2.  In support of this, in Donald's 
extensive trials of O2 toxicity in humans, convulsions never occured at PIO2 
less than 1.8bar.  Nevertheless, as Richard points out, convulsions have 
occured at trivial PIO2 exposures.  Indeed, vast numbers of people have 
convulsions at PIO2 of 0.21bar, and who is to say these are not O2 
convulsions; at some level, the mechanism is probably the same!!!  But back 
to hyperbaric O2 exposures, I am not convinced that there is a definite 
relationship between PIO2 <1.8bar and CNS O2 toxicity, except that based on 
proposed mechanisms, less is probably not worse than more.  

regards,

David Doolette
ddoolett@me*.ad*.ed*.au*

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