wrote:

Unless you are moderately active in excercise and live in the rockies at 5000
feet or higher, or are a particularly unusual person, you WILL be impaired at 10k for
any time longer than a few minutes. Hypoxia is insideous.

Pilots in WWI routinely flew patrols at altitudes up to 20,000' without oxygen.
Basically, being physically able to perform at 6,000 meters was part of the
criteria for being an airman. If you couldn't hack it, you washed out (or died).

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.