The primary purpose of the altitude chamber is for the subjects to learn what their hypoxia symptoms are. The symptoms of hypoxia are different for each individual, and this training is helpful for aviators to be able to recognize these symptoms during actual flight so as to avoid in-flight oxygen emergencies. Military pilots who fly aircraft at altitudes in excess of 10,000 feet, and civilian pilots who fly unpressurized aircraft above 12,500 feet, must use oxygen equipment. Altitude chamber training is required of U.S. military aviators every five years. The FAA and some larger airlines also require their pilots to periodically take altitude chamber training. Anyone with a pilot certificate in the United States who has a current Class I or Class II medical certificate can normally sign up and receive altitude training from several commercial facilities and a very limited availability from a government facility.