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> The crux of the issue that I am interested in is what is the biggest
> problem with deep air ( 200 - 300 fsw). Is it narcosis ( which although
> the tolerance idea seems to be out of the window seems to be something
> which can be 'coped' with given that the dive is planned to avoid 
> complication, both with task and equipment ), or is it oxygen toxicity
> which has the disturbing element of poor predictability ?

My subjective opinion, from LOTS of DEEP air diving experience, is that
even with "adaptation" to narcosis (if that's even a valid concept),
narcosis stays ahead of O2 toxicity as the highest probable cause of a
fatal accident.  Equipment failure (i.e., why we need redundancy), comes
in at a distant third.  From what I know of the accident record - the
numbers seem to support me.  The number one cause of accidents seems to be
"user error", follwed by several instances of O2 convulsions (most of
which seem to be a result of user error anyway), and a few instances of
equipment malfunction. 

One perspective is that O2 toxicity is nastier than narcosis because it is
a more "fickle" ailment, and comes without warning (unlike narcosis, the
severity of which, despite it's delluding effects, is for the large part
self-evident).  However, I still believe that on any given deep air dive,
mental incapacitation in most cases exceeds probability of O2 convulsion. 
Bottom line:  nitrogen-enriched air has little practical value for
anything other than saturation dives at shallow depths. 

My 2 cents....

Aloha,
Rich

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