Exercising at Altitude

Here’s a good overview of the challenge of altitude.

“Ever wondered why you breathe harder and faster when you’re working out at a higher altitude than that to which you’re accustomed?  In colloquial terms, we often say that at elevation,the air is thin, or that there’s less oxygen in the air.  What does this mean, though, particularly in light of the fact that whether you’re at sea level or on top of Mount Everest, every breath you take is 21% oxygen?”

http://trailandultrarunning.com/the-chemistry-of-high-altitude-running/