have a friend who one day, while flying at Flight Level 210 (FL210), rolled the empty copilot seat next to him back for some extra room. He would do this often, but this time, he accidentally crimped the oxygen tubing to his mask. The ATC tapes of his subsequent radio transmissions were beyond ugly. Fortunately, he got this oxygen situation straightened out before a disaster occurred. He was lucky.

No one would deny that the top of Mount Everest, above 29,000 feet, is a hostile environment for humans. Indeed, the summit area above 26,000 feet on the mountain is called “The Death Zone.” There is no difference to that danger in an aircraft cabin when we fly over the top of the mountains and even higher. And our bodies may not be ready without preparation, support, and a dose of caution.

The most significant issue at the flight levels in an unpressurized aircraft is oxygen. The most significant issue in pressurized aircraft is oxygen too! Since there are now many production turbocharged aircraft taking advantage of the better performance available in the flight levels, more and more pilots are setting their altimeter to 29.92 as they climb through FL180. As such, it’s important that these pilots have a thorough understanding of the physiology of this harsh environment.

This means that when we breathe at higher altitudes, each quart of air we take in has fewer molecules of air, which contains about 19% oxygen molecules and 80% nitrogen molecules (the other 1% is minor stuff). The percentages of each type of gas stay basically the same at all altitudes, but each breath (also a fairly constant volume) contains less oxygen molecules and thus, there is less oxygen to be absorbed into our blood stream.

First, let’s review a little physics. At sea level, a quart of air has a specific number of oxygen molecules. Have you ever flown with a bag of potato chips? If you take a bag up in an aircraft, the amount of atmospheric pressure on the air inside the bag decreases. The space between the air molecules increases, and the bag expands. That expanding pressure may actually burst the bag. Thus, the quart container mentioned above holds fewer air molecules per volume because the air molecules are spilling out of the container.