There are some limitations to the study, considering the participants only went up to 11k.
True, but the official minimum altitude at which altitude illness begins is 8,000 ft. Plenty of people get sick and a few even die at "only" 11,000 ft.
Some, including the Boeing medical department, think it should be lower, ie, 7,000 ft. And, for persons with certain medical conditions, it may be only 5,000 ft. The FAA recommends the use of oxygen above 5,000 ft in general aviation (non-pressurized) aircraft flying at night. The rule is routinely ignored as the main manifestation is partial loss of color vision and pilots figure they can still distinguish a blue mountain from a green one.
