Ann Intern Med 145 (7): 497 - 506, 2006. doi: 10.7326/0003 - 4819 - 145 - 7 - 200610030 - 00007 Key Points About 20% of people who ascend to 2500 m (8000 ft) and 40% of those who ascend to 3000 m (10, 000 ft) in one day develop some form of altitude illness, the most common form of which is acute mountain sickness (AMS).