I'm aware.

The elevations I'm talking about are related to people becoming hypoxic when not on supplemental oxygen.

At around ~10-12,000ft the partial pressure of oxygen is low enough that if your not used to it, you can become hypoxic. O2 @ ~20%

If you are on supplimental oxygen (which if you are climbing Everest, you are), in the "death zone" (~26,000ft) even if your are on 100% supplemental oxygen, the partial pressure is low enough that you can become hypoxic. 02 @ 100%

In both cases, if your lungs don't work good (read; if you smoke), you'll become hypoxic at lower levels.