The Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low (0.0618 atmosphere) that water boils at the normal temperature of the human body: 37 °C (98.6 °F). Harry Armstrong was first to recognize that the phenomenon represented an absolute altitude beyond which humans could not survive in an unpressurized environment. The altitude is variously reported as being between 62,000–63,500 feet (18,900–19,350 meters or about 12 miles).
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."
At the nominal body temperature of 37 °C (98.6 °F), water has a vapor pressure of 47 millimetres of mercury (63 hPa); which is to say, at an ambient pressure of 47 mmHg, water’s boiling point is 37 °C. A pressure of 47 mmHg—the Armstrong limit—is one‑sixteenth that of the standard sea level atmospheric pressure of 760 millimeters of mercury (1013 hPa). Modern formulas for calculating the standard pressure at a given altitude vary—as do the precise pressures one will actually measure at a given altitude on a given day—but a common formula shows that 47 mmHg is typically found at an altitude of 63,100 feet (19,200 m).
Blood pressure is a gauge pressure, which means it is measured relative to ambient pressure and is therefore additive when determining the absolute pressure to be used in equations of state (gas laws and the formulas relating pressure-dependent phase changes between liquid and gas). This is similar to a flat automobile tire. Even with zero gage pressure, a flat tire at 63,100 feet would still have an absolute pressure (pressure relative to a perfect vacuum) of 47 mmHg surrounding it—both inside and out. If one inflates the tire to non-zero gage pressure, this internal pressure is in addition to what the tire started with. Even for an individual with a diastolic blood pressure on the low threshold of the normal range, 60 mmHg, blood pressure more than doubles the absolute pressure on the blood and is more than sufficient to prevent blood from outright boiling at 63,100 feet while the heart is still beating.