Oxygen use is one of many factors in aviation where it’s a good idea to set your own limits, which should also be much lower than those required by the FAA. Legal requirements for oxygen aren’t applicable until you fly above 12,500 feet, but in some cases you may want to start inhaling supplemental oxygen as low as 5,000 feet.
As you climb higher into the atmosphere, the partial pressure of oxygen decreases. This means that the number of oxygen molecules in each volume of breath decreases and many of the hemoglobin molecules that normally deliver oxygen to the cells travel empty. Gradually, you’ll experience increasing levels of hypoxia -- a word of Greek origin that means a lower-than-normal amount of oxygen.
There are no set guidelines when it comes to recognizing the symptoms of hypoxia. Everyone reacts differently. The altitude at which your body begins to experience hypoxia depends much on your past exposure to high altitude. If you live in Denver, your limits will be much higher than your friends in Washington, D.C. If you fly or hike regularly at high altitude, your body will adjust over time. The only way to know when your body starts exhibiting symptoms is by exposing yourself and paying attention to the changes.
The first symptoms of hypoxia are most likely very subtle. You could begin to experience a slight headache or pressure behind the eyes. But it’s important to recognize those first signs because advanced symptoms of hypoxia impair functions critical to safe flight. Examples of those symptoms include loss of judgment, inability to make calculations, euphoria and diminished vision.
The lack of adequate vision and other hypoxic symptoms are particularly prevalent at night when the eyes need more oxygen to produce a special night vision protein in the rods of the eyes called rhodopsin. Your personal altitude limit for oxygen use should therefore be lower at night, perhaps as low as 5,000 feet if you live at sea level.
Fortunately hypoxia is a condition that is easy to cure. You simply need to introduce more oxygen to the body by descending to a lower altitude, inhaling supplemental oxygen or pressurizing the cabin. If your airplane is neither pressurized nor equipped with oxygen, you can purchase a portable system that is easy to bring along.
Learn what your first signs of hypoxia are by getting some high altitude training. Whether you train in a high-altitude chamber or in an airplane with an instructor, you won’t regret learning how to recognize your symptoms. Once you know how your body reacts, set your own individual altitude limits for when you need to use supplemental oxygen.