Answer: Height Without Protective Equipment
If it has been a few years since your high school course, let's do a quick update of the basic concept required to estimate what the Armstrong limit is. 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. If you raise the atmospheric pressure, the temperature rises at which liquid is boiling. If you lower the atmospheric pressure, the temperature at which the liquid boils will decrease. For example, water boils at 21
When you're on that limit or higher, there are some exposed body fluids: the saliva in the mouth, the water that keeps the alveoli in your lungs wet, your tears and so on will boil. Not only is this completely confusing, but you swear out very quickly, as the entire deal inhibits your ability to breathe and absorb oxygen. If there is any silver lining on this, it is at least that we use the term "boils" here in purely scientific sense and not in everyday terms. The boil you would feel on your tongue would not blemish hot and hurt your tissue, it would be more related to having a mouth of popstones goddess where you could feel the water changes and the pox and move your tongue.
If all of this has been completely freaked out about the idea of being anywhere across the ocean, even less in an airplane, do not worry. The armstrong limit is only relevant to persons at altitudes reserved for military flights. The Armstrong boundary on the ground kickes at about 59,000 feet above sea level, which is about twice as high as commercial flights fly.
Finally, a little extra trivia to pack the entire treatment of the substance. The Armstrong Limit is not named after Neil Armstrong, no matter how natural the union may be, but General Harry George Armstrong from the United States Air Force. General Armstrong was the first person to recognize the phenomenon.
Image courtesy of USAF / Public Domain.