== How much breathable air is on Earth? == 

 I have a question that seems like it would be relevant to this article.  How much breathable air is on Earth at any one given time?   

 I see that the majority of the mass of the atmosphere is below the height of Mt. Everest.  I know that most humans can breath naturally up to a height a bit lower than that, but my math is not up to calculating the volume of a spheroid and subtracting the volume the Earth.  Obviously there would be some variables, and evaporation, etc. would make it change every so often.  But is there a rough estimate of this number out there somewhere?  

 I would make rough estimateThis article talks about calculating the volume of the Earth. http://en.wikipedia.org/wiki/Volume_of_the_Earth like this: 

 The real formula is v=4/3 pi a2 b, (where π is the mathematical constant pi, and a and b are the equatorial and polar radii) and the volume of the Earth is "1,083,207,317,374 km3, or about 1.08321×1012 km3, in scientific notation.[1] " Id.  But I don't have a good way to use that figure. 

 So if instead I turn that volume into a regular sphere, it would have a radius of 6371 km, or a diameter of 12742 km (1.08321 x10 12 = 4/3 pi r 3, solved for r). Earth's actual diameter varies from 12713 km to 12752 km. http://en.wikipedia.org/wiki/Earth_radius http://hypertextbook.com/facts/1999/RicardoMartinez.shtml  So that seems like a pretty good estimate. (It looks like 6371 is actually an official number used to estimate Earth's radius, so maybe that's how they came up with the 1.08321 number).  This all seems to have something to do with the Geoid, but to be honest, I don't know enough geography about this concept, and it is difficult to correlate the geode with geological heights from "sea level." 

 So here where I get really rough.  I know that the summit of Chimborazo is 6,384.4 km (3,968 mi) from the Earth's center.  http://en.wikipedia.org/wiki/Earth_radius. 
 I also understand Chimborazo is 6,267 meters above sea level.  http://hypertextbook.com/facts/2001/BeataUnke.shtml So, it appears "sea level" is about at 6378.2 km from the center of the earth?  On the other hand, my arbitrary "max breathability" elevation is Everest Base Camp @ 5600 m above sea level, or 6,383.8 km from the center of the earth.  They probably picked Chimborazo because it's near the equator and so further from the center of the earth than other summits, so I'll just round that number down to 6383. Then I calculate the volume of that larger "breathable" sphere as 1.08934 E+12. 

 By the way, I have no idea how the differences in gravitation affect the breathability of air above the pole vs. above the equator, but someone out there must know.  It would be interesting to compare a calculation of the volume of air using that data and the v=4/3 pi a2 b formula, or even more sophisticated methods, vs. this attempt.  

 So the difference between the two spheres is 6,129,226,437 km3.  Does anyone have a real number to correct my back of the napkin figure?