Oxygen Masks for Aviators: Many designs of aviator's oxygen mask contain a microphone to transmit speech to other crew members and to the aircraft's radio. Military aviators' oxygen masks have face pieces that partially cover the sides of the face and protect the face against flash burns, flying particles, and effects of a high speed air stream hitting the face during emergency evacuation from the aircraft by ejection seat or parachute. They are often part of a pressure suit or intended for use with a flight helmet. An early 1919 high-altitude oxygen system used a vacuum flask of liquid oxygen to supply two people for one hour at 15,000 feet. The liquid passed through several warming stages before use, as expansion when it evaporated, and absorbed latent heat of vaporization, would make the gasified oxygen so cold that it could cause instant frostbite of the lungs. Aviation passenger masks and emergency oxygen systems. Most commercial aircraft are fitted with oxygen masks for use when cabin pressurization fails. In general, commercial aircraft are pressurized so that the cabin air is at a pressure equivalent to no more than 8,000 feet altitude (usually somewhat lower altitude), where one can breathe normally without an oxygen mask. If the oxygen pressure in the cabin drops below a safe level, risking hypoxia, compartments containing the oxygen masks will open automatically, either above or in front of the passenger and crew seats, and in the lavatories. In the early years of commercial flight, before pressurized cabins were invented, airliner passengers sometimes had to wear oxygen masks during routine flights