Armstrong limit

[1] On Earth, the limit is around 18–19 km (11–12 mi; 59,000–62,000 ft) above sea level,[1][2] above which atmospheric air pressure drops below 0.0618 atm (6.3 kPa, 47 mmHg, or about 1 psi).

A test subject at the NASA Johnson Space Center accidentally exposed to near vacuum in 1965 "reported that ... his last conscious memory was of the saliva on his tongue beginning to boil.

[9] The Armstrong limit describes the altitude associated with an objective, precisely defined natural phenomenon: the vapor pressure of body-temperature water.

A pressure suit is normally required at around 15,000 m (49,000 ft) for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins.

If the user does not wear a pressure suit or a counter-pressure garment that restricts the movement of their chest, the high-pressure air can cause damage to the lungs.

For modern military aircraft such as the United States' F‑22 and F‑35, both of which have operational altitudes of 18,000 m (59,000 ft) or more, the pilot wears a "counter-pressure garment", which is a g‑suit with high-altitude capabilities.

The Armstrong limit is above most of Earth's atmosphere.
If the cockpit lost pressure while the aircraft was above the Armstrong limit, even a positive pressure oxygen mask (shown) could not protect the pilot.
A pressure suit developed for high altitude, 1937 (worn by Mario Pezzi )
Comparison of a graph of International Standard Atmosphere temperature and pressure with the Armstrong limit and approximate altitudes of various objects