Everything about The Armstrong Limit totally explained
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.
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)
The altitude is variously reported as being between 18.9–19.4 km (62,000–63,500
feet or about 11.8 miles)
(External Link). 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
(External Link).
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."
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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.
(External Link)(External Link) However, if a person has been breathing a
nitrogen-containing atmosphere, bubbles can form due to
decompression sickness ("the bends").
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