Up to what altitude can humans normally breathe without the use of special instruments?

Answer 1

The question is one of "for how long". The US Federal Aviation Administration (FAA) allows pilots to fly at up to 12000 feet (about 3600m) above mean sea level (MSL) for however long their plane will stay up there without cabin pressurization or pilot oxygen. There are rules for how long you can stay at altitude between 12000 feet MSL and 15000 feet MSL (4500m) in an unpressurized cabin (about 30 minutes). At 15000 feet MSL, the FAA requires that the crew of an unpressurized cabin be on oxygen and that a lone crew member of a pressurized cabin be on oxygen with no exceptions.

These numbers do not reflect the limits of human ability, but are as good of an answer to what a competent regulating authority believes humans who have to think and complete complicated tasks should do.

I am pretty sure that military pilots have to go unpressurized to the equivalent partial atmospheric pressure of 25000 feet in order to be qualified, but I am equally sure that they don't stay there for very long. I think that the idea is to give you enough training so that you can think clearly for long enough to grab your oxygen mask, or at the very most to dive the plane to a more breath friendly altitude.

You may want to know that 10000 feet MSL (3000m) is used as the emergency altitude for jets who have lost cabin pressure. The usual rule, if you lose cabin pressure quickly, is to drop down to 10000 MSL as fast as you can do so without tearing your wings off.

People, however, who climb high mountains (over 8000m), routinely go up and down (sometimes several times) to make an effort to get their blood to absorb more oxygen and then finally dash to the summits from a lower base camp without oxygen for their final ascent.

One of the theraputic uses of cocaine is to give you a better chance to think at high altitude. This, in fact, is how the drug was dicovered (people in high Andean villages would chew on Coca leaves to help them breath at high altitude). I suppose Cocaine would be considered "Special Equipment".

Answer 2

Heavily depends on medical condition and the speed of ascent itself. 15000 ft is a chalenge for a normal person living on sea level or a bit higher.
For me (living at 300 ft) a one day climb to 15000 ft (half car, half on foot) was a challenge and I felt slightly discomforted (grasping for air). Nothing serious, though. Spending a night at that altitude and returning down the next day was ok.

Answer 3

At about 15,000 Feet most people tend to have difficulty breathing due to the thin atmosphere. Those with medical
problems may experience difficulties long before that
height, however.

Answer 4

About 13000 to 15000 feet if they are in good health. People went to Mt.Everest without oxygen.

Answer 5

15000 feet not very easy to breath but possible