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18,000 to 30,000 Ft. An unacclimatized man must have oxygen or lose consciousness in a maximum of 45 minutes, perhaps as little as 1½ minutes. Around 25,000, many a man has trouble with the expansion of gases trapped in his intestines, especially if he has drunk beer or pop or eaten beans, corn or fried foods.
30,000 to 43,000 Ft. A flyer must wear a tight-fitting mask over nose and mouth, breathe oxygen under pressure. In effect, this turns his windpipe and lungs into an internal pressurized cabin. The natural breathing process is reversed: the gadget forces the oxygen into the flyer, and he must make a positive effort to exhale. One major effect is to make communication more difficult. A pilot cannot utter a whole sentence, can gasp only a few words, perhaps mere syllables at a time. Even with oxygen, pilots may get the bends or the chokes.*Also: a man cannot whistle, and he is likely to suffer from formication —the feeling that ants are marching over his body.
43,000 Ft. Pressure breathing becomes impractical for long periods because of the strain on the chest. But up to 50,000 ft. an experienced pilot might remain conscious for one to ten minutes. Without oxygen he would pass out cold in 15 seconds.
50,000 Ft. The heart could no longer tolerate, even for a minute, the strain of an internally pressurized chest. So the whole body must be kept under pressure, in either a suit or a cabin. Most likely the pilot is now deep in the stratosphere (reached at a mere 24,000 ft. over the poles, at 60,000 ft. over the equator). With the clouds and the earth far below him, he has no points of reference for depth perception (judgment of distances) or focusing. He tends to keep his eyes in focus a short distance from the plane, will not see a distant enemy. This is the "pseudo-myopia" of altitude. At supersonic speeds, if he sees another plane approaching half a mile away, it will have passed him before the message from the retina registers on the perception center in his brain. This is "distance scotoma." The quality of light itself is changed: there are not enough dust particles to diffuse it. Even with sunshine all around, the pilot cannot see instruments in the shadows unless they are lighted.
63,000 Ft. "The Armstrong line," named for General Armstrong, who forecast it on a theoretical basis, later proved it with animals. Here, without protection, the blood boils, because the air pressure (57 mm. of mercury) equals the vapor pressure of water at body temperature.
80,000 Ft. Oxygen in the outside air now becomes poisonous because ionizing rays turn some of it into ozone, with three atoms to the molecule instead of two. Ozone rots rubber, corrodes metal and ruins a man's lungs. From here on up, only a sealed cabin with a built-in climate including its own air supply can sustain life.