According to my chemistry textbook

[CyranoAH]

At a certain alt, the exchange of CO2/Oxygen begins to work inversely, and with every breath you are in fact expelling oxygen, reducing more and more your O2 concentration in the blood.

Even with proper training, 39000 feet is way too much for a human being to withstand for more than 15-20 seconds (I believe the Time of Conciousness at 33000 feet was approx 30 seconds).

I took the Hypobaric (not hyper RTR, that's for diving, unless you were in a dual hyper-hypo chamber) test when I was preparing for the parabollic flight campaign (Zero-G experiments) and I concur with RTR, it can greatly impair your judgement... we were given a test with simple questions (at 25000 equivalent air pressure) and the results of the group were comical to say the least

It's a test I'd recommend to any pilot, quite illuminating.

Daniel

[jigsaw]

The air at 39,000 feet above sea level has this amount

It's not a matter of "amount" of oxygen. It's a matter of lack of pressure in the atmosphere at that level to push the oxygen into the cells in the lung. Hence why it's called "hypoxic hypoxia."

There are different types of hypoxia;
Hypoxic - lack of pressure
Hypemic - blood cells can't carry enough O2
Histotoxic - something is preventing O2 from getting to the brain (like alchohol)
Stagnant - O2 not being carried due to circulatory problems

Here's a link with more detail

http://www.mountainflying.com/hypoxia.htm

FYI, some people can get hypoxic at altitudes as low as 3-5,000 feet. Especially if they smoke.

[Scootter]

I swear to God some people are hypoxic at sea level, I know some of these people:D