What is the highest elevation that a person can go and still be able to breathe?
The height to which one can go in the atmosphere and still be able to breathe depends on your physical condition and the duration of your visit, but there is a limit to the absolute height that you can go. The density of air decreases as one goes higher, and the amount of oxygen 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 actually limits the height at which we can breathe. An elevation of about 20,000 feet above sea level is the maximum height at which sufficient oxygen exists in the air to sustain us. By comparison, the summit of Mount Everest (the Earth’s highest mountain) lies at 29,141 feet.