How high in the atmosphere is it dangerous for humans to stay?
Most people will need supplemental oxygen to avoid hypoxia at an altitude above 4500m (15000ft). At altitudes above 11900m (39000ft), the oxygen must be delivered through a sealed mask with increased pressure to avoid hypoxia, and the user also needs a pressure suit or a counter-pressure garment to avoid possible lung damage caused by the extra pressure. At around 15000 m (49000 ft), a well-conditioned and experienced pilot also needs a pressure suit to safely operate the aircraft. At the Armstrong limit, which is around 18000-19000m (59000–62000ft), water will boil at the normal temperature of the human body. Without a full-body pressure suit or pressurized cockpit/cabin, any exposed body fluids will boil away, including the liquids wetting the alveoli in the lungs. This results in a rapid loss of consciousness and eventually death if pressure can't be restored within 60–90 seconds.