Dear Tom,
The air gets thinner the higher up one goes. How high in the atmosphere can one go and still be able to breathe?
James Concker, Morris, Illinois

Dear James,
The density of air decreases as one goes higher and the amount of oxygen in the air also decreases, although its proportion in the mix of gases in the atmosphere remains the same. It is the lack of oxygen rather than the reduced air pressure that limits the height at which we can breathe.

An elevation of approximately 20,000 feet above sea level is the maximum height at which sufficient oxygen exists in the air to sustain us. As a matter of comparison, the summit of Mount Everest in Asia (the Earth’s highest mountain) stands at 29,141 feet. Mount Denali (North America’s highest mountain): 20,322 feet. (Both heights are approximate.)

The height to which one can go and still be able to breath depends of your physical condition.