increases such that the partial pressure of oxygen is roughly constant. Diluter-demand oxygen systems can be used up to . In a pressure-demand system, oxygen in the mask is above ambient pressure, permitting breathing above . Because the pressure inside the mask is greater than the pressure around the user's torso, inhalation is easy, but exhalation requires more effort. Aviators are trained in pressure-demand breathing in altitude chambers. Because they seal tightly, pressure-demand-type oxygen masks are also used in hyperbaric oxygen chambers and for oxygen breathing research projects with standard oxygen regulators. Supplemental oxygen is needed for flying more than 30 minutes at cabin pressure altitudes of or higher, pilots must use oxygen at all times above and each occupant must be provided supplemental oxygen above . 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 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. Self-contained breathing apparatus (SCBA) Firefighters and emergency service workers use full face masks that provide breathing air as well as eye and face protection. These masks are typically attached to a tank carried upon the back of the wearer and are called self-contained breathing apparatuses (SCBA). Open circuit SCBAs do not normally supply oxygen, as it is not necessary and constitutes a fire hazard. Rebreather SCBAs usually supply oxygen as this is the lightest and most compact option, and uses a simpler mechanism than other types of rebreather. Specialized masks for astronauts Specialized full-face masks that supply oxygen or other breathing gases are used by astronauts to remove nitrogen from their blood before space walks (EVA). Specialized masks for pets Specialized snout masks which supply oxygen to revive family pets have been donated to fire departments. Oxygen delivery to divers Divers only use pure oxygen for accelerated decompression, or from oxygen rebreathers at shallow depths where the risk