Armstrong limit

The Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low (0.0618 atmosphere or 6.3 kPa (47 mmHg)) that water boils at the normal temperature of the human body: 98.6 °F (37.0 °C). It is named after Harry George Armstrong, who founded the U.S. Air Force’s Department of Space Medicine in 1947 at Randolph Field, Texas. Armstrong was the first to recognize this phenomenon, which occurs at an altitude beyond which humans absolutely cannot survive in an unpressurized environment. On Earth, this begins at an altitude of approximately 60,000 feet (18,000 m), or about 12 miles (19 km), depending on the individual's level of physical fitness.

Effect on bodily liquids

At or above the Armstrong limit, exposed bodily liquids such as saliva, tears, and the liquids wetting the alveoli within the lungs—but not vascular blood (blood within the circulatory system)—will boil away without a pressure suit, and no amount of breathable oxygen delivered by any means will sustain life for more than a few minutes. The NASA technical report Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects, which discusses the brief accidental exposure of a human to near vacuum, notes the likely result of exposure to pressure below that associated with the Armstrong limit: "The subject later reported that ... his last conscious memory was of the water on his tongue beginning to boil."