Armstrong limit in the context of Boiling

A space suit (or spacesuit) is an environmental suit used for protection from the harsh environment of outer space, mainly from its vacuum as a highly specialized pressure suit, but also its temperature extremes, as well as radiation and micrometeoroids. Basic space suits are worn as a safety precaution inside spacecrafts in case of loss of cabin pressure. For extravehicular activity (EVA) more complex space suits are worn, featuring a portable life support system.

Pressure suits are in general needed at low pressure environments above the Armstrong limit, at around 19,000 m (62,000 ft) above Earth. Space suits augment pressure suits with complex system of equipment and environmental systems designed to keep the wearer comfortable, and to minimize the effort required to bend the limbs, resisting a soft pressure garment's natural tendency to stiffen against the vacuum. A self-contained oxygen supply and environmental control system is frequently employed to allow complete freedom of movement, independent of the spacecraft.

Harry George Armstrong (February 17, 1899 – February 5, 1983) was a major general in the United States Air Force, a physician, and an airman. He is widely recognized as a pioneer in the field of aviation medicine. The Armstrong limit, the altitude above which water boils at the temperature of the human body, is named after him.

Armstrong served in the Marines during World War I and the Army and Air Force from 1930 to 1957. As director of the United States Aeromedical Research Laboratory, he applied his medical and aviation knowledge to the improvement of aircrew protection from temperature extremes and the lack of oxygen at high altitude.

The effects of high altitude on humans are mostly the consequences of reduced partial pressure of oxygen in the atmosphere. The medical problems that are direct consequence of high altitude are caused by the low inspired partial pressure of oxygen, which is caused by the reduced atmospheric pressure, and the constant gas fraction of oxygen in atmospheric air over the range in which humans can survive. The other major effect of altitude is due to lower ambient temperature.

The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 metres (6,900 ft) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the death zone, where it is generally believed that no human body can acclimatize. At extreme altitudes, the ambient pressure can drop below the vapor pressure of water at body temperature, but at such altitudes even pure oxygen at ambient pressure cannot support human life, and a pressure suit is necessary. A rapid depressurisation to the low pressures of high altitudes can trigger altitude decompression sickness.