Re: what percentage does the oxygen in the atmosphere fall with altitude?

Howard, this question has a very simple answer if I take it literally. The 
PERCENTAGE of oxygen in the atmosphere remains exactly the same at all heights up 
to about 85 km! It is just over 21% by volume (just under 23% by mass) whether we 
measure it at sea level, or at the top of Mt Everest!

But that does not mean that there is just as much oxygen available to breathe at 
the top of Mt Everest as there is at sea level! The air as a whole gets thinner 
as we go higher -- there is less nitrogen and argon as well as less oxygen.

As a rough approximation, the AMOUNT of oxygen present goes down by a factor of 
10 for each 17 km of altitude increase, a factor of 2 for each 5000 m, or 10% for 
each 650 to 700 metres.

Tables of values for the "Standard Atmosphere" are available in the CRC Handbook 
of Physics and Chemistry, around about page F200. The quantity that will exactly 
parallel the amount of oxygen present is the "density". "Pressure" is also a 
reasonable guide, but not exact because, as well as the amount of gas present, 
changing temperature also affects the pressure .