Following Felix Baumgartner’s record-setting skydive, Space Safety Magazine has published an article on how to survive supersonic freefall. The article is generally accurate, but unfortunately, it does perpetuate one of the myths about aerospace physiology:
At 18,900-19,350 meters, the point known as “Armstrong’s Line,” the pressure experienced is enough to make fluids within the body boil at 37 °C, the temperature of the human body.
This is not correct. Fluids in an open container will indeed boil at that temperature, above the Armstrong Line. The human body is not a closed container, however. It is enclosed by an elastic integument (skin), which prevents bodily fluids from boiling. Blood is additionally enclosed within the blood vessels. Saliva in the mouth will boil above the Armstrong line, but blood in the veins, arteries, and capillaries will not. Death from vacuum exposure will occur within minutes, but the cause of death will be hypoxia – lack of oxygen – not boiling blood.
Proof of this fact comes from laboratory experiments with animals and also from NASA astronauts who have suffered rips in their pressure suits during EVA, resulting in parts of their body being exposed to hard vacuum. The result was some local swelling and discomfort, but no boiling blood, just as physics predicts.