Scott, I think you need to consider a fundamental issue with oxygen
exposure.  The NOAA "limits" are for a diver in the water, with a
pressure differential across the lungs.  Toxicity tolerances are much
greater for a person in a gas space, with equal ambient pressure at all
points.  This is why you can tolerate PPO2's above 2.0 in a chamber,
but on average, only 1.6 in the water.  As I understand it, the
relationship to the CNS "clock" is similar.  Once I'm out of the water,
I stop worrying about it.  I don't breathe O2 on the way to my car, but
that is only because I don't need spectators thinking I am dying or
something.  Whatever turns your crank.  Pulmonary tox, on the other
hand, would be an issue here if you were talking about that sort of
exposure, but as you pointed out, we don't.

-Sean


On Sun, 20 Aug 2000 20:23:36 EDT, ScottBonis@ao*.co* wrote:

>Hi Brock,
>
>You ask ...
><< A couple of my favorite sites (Oronogo, Lake Elmer Thomas) and some
I've 
>yet
>to visit (Cannonball Springs, Ribidoux Springs) require a hefty walk/climb
>to a parking lot- easily 300 ft on a steep incline with no shade. I've
>adopted Scott's recommendations and they seem to help. Over a
>multi day visit (I've some training coming up)- do I place myself at some
>danger doing this? >>
>
>From what I understand, in most cases I see no real problem with breathing 
>oxygen or a high oxygen mix on the surface if it makes you feel better.  The 
>max PP(O2) you'll get is 1.0 ATA from the pure oxygen and 0.5 ATA from EAN 
>50.  So you use up the oxygen clock at around 0.3 %/min. at 1.0 ATA and 0.1 
>%/min. at 0.5 ATA.  So if it takes you 30 minutes to get your stuff to the 
>car, you have added around an extra 10% to the CNS oxygen clock by breathing 
>pure oxygen and around 3% for EAN50.  You need to decide if that's important.
>
>By the way, the previous numbers are based on the NOAA limit of 5 hours 
>allowed for the CNS oxygen clock for a PP(O2) of 1.0 ATA.  But have you ever 
>been in the burn ward of a hospital?  You may see people in oxygen tents 
>breathing pure oxygen for two weeks in a row.  You make up your own mind on 
>the level of conservatism that you want to accept.
>
>Pulminary oxygen toxicity almost never comes into play for the type of diving 
>you're doing.
>
>And always remember, that the advice you recieve from me is worth what you're 
>paying for it.
>
>Take care and dive safe,         Scott


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