Armstrong Limit - Effect On Bodily Liquids

Effect On Bodily Liquids

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. To calculate blood pressure it has to be summed to ambient pressure when calculating when blood will boil. This is similar to a flat automobile tire: even with zero gauge pressure, a flat tire at altitude of the Armstrong limit would still have an absolute pressure (pressure relative to a perfect vacuum) of 47 mmHg, that is, it will have the ambient pressure at 19.000 meters, both inside and out of it. If one inflates the tire to non-zero gauge pressure, this internal pressure is in addition to those 47 mmHg of ambient pressure. This means that for an individual with a diastolic low blood pressure, typically 60 mmHg, its blood pressure would be 107 mmHg (the sum of the blood pressure and the ambient pressure). This pressure is more than twice the ambient pressure at the Armstrong limit. This extra pressure is more than sufficient to prevent blood from outright boiling at 19,000 meters while the heart is still beating.