Answer: Aviation

Cross the Armstrong limit unprepared and you’re in for a very nasty surprise: the very water within your lungs will begin to boil and you’ll asphyxiate as a result of the damage to your lungs and the displacement of breathable air via water vapor.

So when does the Armstrong limit take effect? When you’re above roughly 62,000 feet (18,900 meters) in altitude and the drop in atmospheric pressure has a particularly nasty side effect: water will boil at 98.6 degrees Fahrenheit (37 degrees Celsius), or, the same temperature as the human body which means all the surface water in your body (on your tongue, mouth, throat, lungs, and so on) will begin boiling.

As such, it’s critical that high altitude pilots and astronauts in unpressurized environments wear specially pressurized suits in order to keep the pressure balanced enough to protect them from the effects of the Armstrong limit.