Air temperature drops by 5°C for each 1km you rise. By the edge of the stratosphere (around 15km), it’s below -50°C and the wind blows at over 100km/h. Exposed skin will suffer frostbite in less than five minutes and hypothermia will soon follow. The temperature starts rising again after 20km, but not quickly enough to save you.
Well before you reach that altitude, though, you will have blacked out from oxygen deprivation. Passenger aircraft automatically release oxygen masks if the plane depressurises above 4.5km. If your balloon doesn’t have its own air supply, you will be unconscious by the time you reach 10km, and dead a few minutes after that.
Even with an oxygen mask, you won’t survive past 18km. This is the Armstrong Limit, and is the altitude where water boils at body temperature. Above this point, the moisture in the lining of your lungs evaporates and they lose their ability to absorb oxygen. The only way to prevent this is to breathe pressurised oxygen in a sealed flight suit.
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