People traveling to high altitude are at risk of developing acute altitude illnesses, including AMS, HACE, and HAPE. Incidence and risk are assessed by history of acute altitude illness, sleeping elevation above sea level on day 1 of travel, and rate of ascent. AMS incidence can range from approximately 3% in persons traveling to 2000 m (6561 ft) to 25% to 40% at altitudes above 4300 m (14鈥�107 ft),1,2 with symptoms varying from mild to marked.3 While rare, HACE and HAPE are potentially life-threatening if untreated.