Armstrong Limit
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The Armstrong Limit, often called Armstrong's Line, is the altitude that produces an atmospheric pressure so low (.0618 atmospheres), that water boils at the normal temperature of the human body: 37°C (98.6°F). It is named after U.S. Air Force surgeon Harry George Armstrong, who first described it. [1]
The altitude is variously reported as being between 18.9–19.4 km (62,000–63,500 feet or about 11.8 miles)[2]. At or above this point, exposed human fluids will boil without a pressure suit, and no amount of breathable oxygen, delivered by any means, will sustain life for more than a few minutes. A human would, eventually, boil in their own body fluids (a process known as ebullism), though death from asphyxiation would occur first, as the barrier of the skin and control of blood pressure would prevent blood from boiling immediately [3].
A NASA technical report, Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects, discussing 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."[4]
It is important to note that this applies to unconfined water, such as saliva and tears. Normal human diastolic blood pressure is higher than this, so blood never experiences a pressure this low, even if a person is exposed to a complete vacuum. Thus, contrary to oft-repeated myth, a person's blood will not boil in space.[5][6] However, if a person has been breathing a nitrogen-containing atmosphere, bubbles can form due to decompression sickness ("the bends").
