Armstrong Limit
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The Armstrong Limit 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) [1].
The altitude, also sometimes referred to as Armstrong's Line, is variously reported as being between 18.9 km - 19.4 km (62,000 - 63,500 feet) (about 11.8 miles) [2] [3]. 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 [4].
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." [5]
