A pressure suit regulates the air pressure a user is exposed to. They are used by high-altitude pilots and astronauts who operate in environments with low or no air pressure. Humans who are exposed to altitudes above approximately 50,000 feet (15,000 meters) will require a pressure suit to function safely. Military pressure suits and space suits share many common features.
Humans evolved on the surface of Earth, where air pressure is relatively uniform. Higher altitudes have substantially lower air pressure; the air pressure at the top of Asia's Mount Everest, the highest mountain in the world, is roughly one third that of sea level. Though humans can survive on most surfaces with high elevations, low air pressure can hinder respiration and other bodily functions. In the environments reached by high-altitude aircraft and spacecraft, humans can not survive unprotected for more than a few minutes.
At altitudes above about 33,000 feet (10,000 meters) from sea level, supplementary breathing oxygen is usually required to prevent hypoxia. Hypoxia is a general deprivation of oxygen in the body, and it can cause a number of adverse symptoms collectively called altitude sickness. Above about 39,000 feet (12,000 meters), supplementary breathing oxygen with additional pressure is required to prevent these symptoms.
Many aircraft operate at altitudes low enough to avoid the need for a pressure suit. The pressure one feels in the ears when descending in a plane, nonetheless, is the result of higher pressure near the surface. Many military planes, on the other hand, fly at altitudes where a full pressure suit is required to maintain health. These pressure suits typically include a helmet with a microphone and speakers to facilitate communication with others. They also involve oxygen breathing equipment, thermal insulation layers and even urine collection devices.
In space, there is no air pressure at all, so a pressure suit is required to exit a vehicle. Astronauts inside the walls of a pressurized spacecraft do not need to wear a pressure suit to breathe. These pressure suits, commonly called space suits, are similar in many ways to aircraft pressure suits. They typically will involve more sophisticated thermal management systems, however. Protection from radiation in space is also an important design priority.
Divers who need protection from the higher pressure of deep water wear what is called an atmospheric diving suit. This type of suit also needs to provide oxygen to the user, but the mechanical structure is fundamentally different from pressure suits. Rather than keeping a high air pressure inside, the suit needs to resist the high external pressure of the water. This is the reason these suits somewhat resemble suits of armor.