Posted for "Mark Sletten" <marknlisa@hometel.com>:

 Listers,
 
 Please be careful here... There is an altitude at which the ambient pressure
 is low enough that oxygen exchange in the blood does not occur. I don't
 believe Lorn is advocating the use of a cannula above 18,000 feet, but just
 in case someone else gets the impression he, or, here's why you shouldn't...
 
 
 As you climb, the percentage of oxygen in the air remains constant, but as
 ambient pressure is drops, there is less oxygen overall. That problem can be
 overcome by adding oxygen to the ambient air -- most supplemental oxygen
 system manufacturers do this by squirting oxygen up your nose via a cannula.
 
 But, that solution doesn't address the *OTHER* problem with altitude, lower
 pressure. Let me give an example: Gas pressure is what holds the carbon
 dioxide in the liquid of a soda -- the can is pressurized (try to squeeze
 one before opening it). When you pop the top, you reduce the pressure on the
 liquid such that the gas escapes -- you get bubbles.
 
 The same principle is at work in your lungs while you breathe, except in
 reverse. At sea level, the ambient pressure of the surrounding air is what
 forces oxygen *INTO* the blood. As you climb, eventually you reach an
 altitude where ambient pressure isn't enough to force oxygen into the blood.
 Keep going high enough and the gases in your blood will actually begin to
 bubble out, just like the carbon dioxide in the soda. That's why astronauts
 and U-2/SR-71 pilots wear pressure suits. If they loose cabin pressurization
 at the altitudes at which they operate, they must have a means of
 maintaining artificial pressure on their body to keep their blood from
 boiling!
 
 The two problems of less available oxygen and reduced partial pressure are
 additive. You may be breathing an air mixture with an oxygen content of,
 let's say 75% at 25,000 feet (you're squirting oxygen up your nose with a
 cannula, but it will still partially mix with ambient air). But, the lower
 ambient pressure may actually be forcing less oxygen into your blood than
 would occur while at sea level pressure -- even though the percentage of
 oxygen is only 21%. In other words, at a certain altitude, if you're not
 wearing a mask providing positive pressure on the interior of the lungs,
 less oxygen actually gets into the blood -- even if the gas you're breathing
 is 100% oxygen!
 
 If you plan to attempt cannula use above 18,000 ft as, say, an experiment, I
 urge the utmost caution. At the very least you should bring along a
 qualified pilot breathing on a mask. Additionally, your safety pilot wearing
 a mask should be prepared for all sorts of strange behavior should you
 become hypoxic...
 
 Each person's tolerance to high-altitude is different. One person may be
 fine with a cannula up to 35,000ft; another might not make it much past
 18,000ft -- the altitude at which the FAA requires one to wear a mask
 providing positive pressure. Everyone reacts differently to hypoxia. The
 onset of debilitating symptoms can be extremely fast in some and you may not
 regain useful consciousness in time to prevent a tragedy.
 
 As has been pointed out in a previous post, the only way to be sure you have
 enough oxygen in your blood is to use a pulse oximeter, which directly
 measures it.
 
 BTW, I did no math in my examples above; they are all just hypothetical
 examples to help everyone understand the physics. I didn't bother to do the
 math because all these experiments were done long, long ago -- most of the
 current rules are written in blood.
 
 Mark Sletten
 Legacy FG N828LM
 http://www.legacyfgbuilder.com