At about 19 kilometres up the air pressure falls low enough that water boils at human body temperature, so an unprotected person there would feel the saliva on their tongue turn to vapour, which is why aircrew above that altitude wear sealed pressure suits rather than an oxygen mask
At sea level, an oxygen mask and a pressure suit can look like two versions of the same idea. Both seem to give a pilot air. At about 19 kilometres above Earth, the difference becomes the difference between breathing and remaining physically intact. This is an account of aerospace physiology and flight protection, not guidance … Read more