There is a point not more than 20km away from you right now where your normal body temperature is enough to boil the saliva off your tongue and the moisture out of your lungs.The boiling point of water depends on atmospheric pressure. The lower the pressure, the lower the temperature needed to bring water to a boil. (This, by the way, is why water comes to a boil faster when you’re on top of a mountain.) Outer space has effectively no pressure, so water boils immediately. (Then it freezes, but I’m not going to explain why because I’m running out of parentheses.) Why is that important?
At sea level water boils at 100°C. In space, water boils at temperatures below 0°C. Logically, there’s a point between the two when the atmospheric pressure is low enough for water to boil at 37°C. That point is called the Armstrong limit. It’s not named after the famous astronaut, but rather a former surgeon general of the American Air Force. Why is it important? Well, you see, 37°C is the average human body temperature.
So here’s the thing. If you reach the Armstrong limit, some 18-19km above the Earth’s surface, your own body is warm enough to boil water. That means your tears boil. The saliva boils on your tongue. If you pee, you’re peeing boiling water.
For most of these effects, you’re just going to be uncomfortable. The water is turning into vapour, but it’s not super-hot when it does so. It is a problem for your lungs. The interior of your lungs are covered in liquid that enables gas exchange. And at the Armstrong limit, that liquid boils away and you’re going to suffocate. Fun!
This, by the way, is why high altitude pilots wear pressure suits. To keep their lungs from boiling away.
I live in Auckland, New Zealand, and am curious about most things.