Cindy, if you know your tidal volume & breathing rate you can use this calculator to estimate blood oxygen levels. Set the units to feet & enter altitude in feet.
If you don't change breathing rate and tidal volume, the magenta curve is pretty much a dead person.

Also some partial pressure info here.

              elevation  oxygen % compared
  Location       feet     to sea level
------------    ------     ---
Fresno            450'     98%
Half Dome        8800'     73%
Lone Pine        3700'     88%
Whitney Portal   8360'     75%
Mount Whitney   14505'     57%
Everest         29035'     33%


And at 367442' (SpaceShipOne's apogee), you might see the occasional oxygen molecule drift by.

Verum audaces non gerunt indusia alba. - Ipsi dixit MCMLXXII