Answer: altitude without protective equipment
If your high school physics course has been several years old, we will update the basic concept needed to detect the Armstrong boundary. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure surrounding the liquid. As you increase the atmospheric pressure, the temperature at which the liquid boils increases. If you lower the atmospheric pressure, the temperature at which the liquid boils will drop. For example, water boils at 21
Once you have reached this limit or higher, all body fluids are exposed: the saliva in your mouth, the water that keeps the alveoli in your lungs wet, your tears and so on, will boil away. Not only is this absolutely disturbing, but you will faint very quickly as the whole thing inhibits your ability to breathe and absorb oxygen. However, if this is a silver lining, we will at least use the term "cooking" in a purely scientific sense and not in the colloquial sense. The cooking that you would feel on your tongue would not be hot and harmful to your tissues. It would be more like having a sip of pop-rock candy in which you can feel the water's transformation and let the steam rise and get away from your tongue
Well, if you've done it all completely crazy, somewhere over there to be at sea level, let alone in an airplane, do not worry. The Armstrong border is only relevant to people who are at altitudes reserved for military flights. The Armstrong boundary for Earth begins at approximately 59,000 feet above sea level, which is approximately double the altitude at which commercial flights fly.
Finally a few details to complete the whole treatment of the topic. The Armstrong limit is not named after Neil Armstrong, no matter how natural the association may be, but General Harry George Armstrong of the United States Air Force. General Armstrong was the first to recognize the phenomenon.
Courtesy of the USAF / Public Domain.