# the *answer*

I don't know about the 1 atmosphere part, but for a rough approximation you can easily work it out by calculating the volume of a sphere with radius of the Earth plus 5 kilometers (a height where most healthy people can still breathe). Then subtract the volume of the Earth from that. This would be pretty close, I imagine, for your purposes. The radius (r) of the Earth is about 6378km. The volume of a sphere is found by the formula 4/3 * pi * r^3 (four thirds pi times the radius cubed). Applying this formula to 6383 (i.e. 6378+5) we get 1089339226436 cubic kilometers. The volume of the Earth is 1086851326520 cubic kilometers. Subtracting these two numbers we find the volume of breathable air in cubic kilometers is 2,487,899,916, or about two and a half billion cubic kilometers of air. Quite a lot, eh?

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