Altitude means different things to different pilots. In crop dusting or banner towing, it’s not something we need. When striving for efficiency and economy over a distance, it can be, since the air is thinner and cooler the higher we climb. True airspeed increases and so do tailwinds, if you’re headed in the right direction. Higher altitudes also bring with them some physiological changes we should already know about, like hypoxia—the lack of oxygen—which demands we use supplemental oxygen in unpressurized aircraft. The altitudes and hypoxic effects vary with the individual and duration, but it’s a good idea to start thinking about supplemental oxygen at and above 10,000 feet msl.
In fact, the proportion of oxygen in the atmosphere at, say, 25,000 feet is the same as it is on the surface—there’s just less of it because the air is thinner. The lower atmospheric pressure at altitude, which is one way we measure altitude, can affect the human body in ways we may not realize. For example, very few people know that flying can cause something called decompression sickness. It’s important for pilots to know when it’s likely, how it feels (so you can recognize it), what to do about it and—most important—what risks it entails.
