How high can you go without oxygen?

We pass out when the pressure drops below 57 percent of atmospheric pressure — equivalent to that at an altitude of 15,000 feet (4,572 meters).

Climbers can push higher because they gradually acclimate their bodies to the drop in oxygen, but no one survives long without an oxygen tank above 26,000 feet (8,000 m, less than 356 millibars of atmospheric pressure).