The PHAK (A US governmet publication, so it can't be wrong!!!) pg. 2-1
states:

http://www.faa.gov/library/manuals/a...ilot_handbook/

The atmosphere is composed of 78 percent nitrogen, 21
percent oxygen, and 1 percent other gases, such as
argon or helium. As some of these elements are heavier
than others, there is a natural tendency of these heavier
elements, such as oxygen, to settle to the surface of the
earth, while the lighter elements are lifted up to the
region of higher altitude. This explains why most of the
oxygen is contained below 35,000 feet altitude.

Is this correct??

I always thought that only the density changes until a very high altitude.

Best,
Karl
"Curator" N185KG