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This subject has been put foward a number of times recently and appears to 
have been lost due to the recent poor signal to noise ratio.

I would be interested in hearing about the occurance of CNS O2 toxicity in 
divers at PO2 less than 1.8 bar.  Allow me to introduce the subject, and I 
am working from memory so my figures may not be entirely accurate. 

There seems to be too little data on CNS O2 toxicity for exposures below 1.8 
bar to accurately ascribe a risk to such exposures.  Human data suggests 
that there is a PO2 threshold for convulsions, and the current hypothesized 
mechanism involving reactive oxygen species implies a PO2 threshold at which 
cellular defense mechanisms are overwhelmed.  It would seem prudent to limit 
PO2 exposure to below this threshold, but how much below?  The data I have 
seen suggests that convulsions occuring at PO2 less than 2 bar are 
out-liers, and therefore, for instance, a 1.4 bar and a 1.5 bar PO2 exposure 
may carry an identical risk of convulsion.

In published work, the time to onset of convulsions is often used as an 
index of CNS O2 toxicity, and the curve which describes the 10% probability 
of convulsions, relating time to onset with PO2 asymptotes at 2 bar.  In 
Kenneth Donald's work with the RN during WWII, no convulsions were 
experienced with a maximum PO2 exposure of 1.8 bar, I cannot remember the 
number of exposures.  Apparently, in earlier studies, the USN had two 
convulsions at lower PO2, one I think at 1.12 bar, although I haven't seen 
these studies, Donald was skeptical of the accuracy of PO2 measurement for 
the lower exposure.  Butler and Thalmann showed in 1986 showed 12 occurances 
of "probable" symptoms, but no definate symptoms, out of 153 exposures to 
1.61 bar (20fsw).  I am not aware of any published studies conducted at PO2 
< 1.6 bar, although talk of convulsions occuring at low PO2's circulate.

I am interested in two things.  Citations of any scientific studies on 
humans that have shown convulsions due to trivial PO2 exposures, and first 
hand (or reliable second hand, let's protect the innocent) experience of 
people on these lists.  For the first hand accounts, for want of 
consistency, perhaps we should use the classification of Butler and Thalmann 
as I remember it (accurately or not):
1. convulsions
2. definate symptoms = nausea, dizziness, muscle twitching (indicate which)
3. probably symptoms = anything else (indicate what symptoms)
also indicate the PO2 (depth and gas mixture), level of exertion, 
possibility of CO2 buildup and anything else I have forgotten. 

regards,

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

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