In The Other Side of Everest, the British mountaineer and filmmaker Matt Dickinson records how Howard Somervell, on a 1924 British expedition up Everest, "found himself choking to death after a piece of infected flesh came loose and blocked his windpipe." With a supreme effort Somervell managed to cough up the obstruction. It turned out to be "the entire mucus lining of his larynx."
Bodily distress is notorious above 25,000 feet—the area known to climbers as the Death Zone—but many people become severely debilitated, even dangerously ill, at heights of no more than 15,000 feet or so. Susceptibility has little to do with fitness. Grannies sometimes caper about in lofty situations while their fitter offspring are reduced to helpless, groaning heaps until conveyed to lower altitudes.
The absolute limit of human tolerance for continuous living appears to be about 5,500 meters, or 18,000 feet, but even people conditioned to living at altitude could not tolerate such heights for long. Frances Ashcroft, in Life at the Extremes, notes that there are Andean sulfur mines at 5,800 meters, but that the miners prefer to descend 460 meters each evening and climb back up the following day, rather than live continuously at that elevation.