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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

Title: The Stratospheric Boundary: Understanding the Armstrong Limit and Its Life-Saving Implications At an astonishing altitude of approximately 19 kilometers above sea level, the distinction between an oxygen mask and...