Above 63,000 feet, the air pressure is so low that your body's moisture boils at your own body temperature.

The Armstrong Limit is named after Harry George Armstrong, who founded the U.S. Air Force Department of Space Medicine in 1947. He was the first to recognize that at a specific altitude, roughly between 62,000 and 65,000 feet, atmospheric pressure becomes so low that water boils at the normal human body temperature of 98.6°F. At sea level, water boils at 212°F, but as altitude increases, atmospheric pressure decreases, lowering the boiling point.When a human is exposed to this environment without a pressure suit, a phenomenon called ebullism occurs. This is the formation of gas bubbles in bodily fluids due to reduced ambient pressure. While the heart continues to pump and the skin provides enough elastic tension to keep blood from boiling, the moisture on the tongue and the lining of the lungs evaporates immediately. This process causes rapid cooling and can lead to severe tissue damage and hypoxia within seconds.NASA and other space agencies use this limit as a critical physiological threshold for high-altitude flight and extravehicular activity. Any pilot or astronaut traveling above this line must be enclosed in a pressurized environment or a full-pressure suit to survive. Modern data from the Standard Atmosphere model places this limit precisely at an altitude where the ambient pressure is 6.3 kilopascals. This physical reality remains one of the greatest challenges for high-altitude exploration and commercial space travel.