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.
