Intermediate altitude: 1500-2500meters (m) (4921-8202feet (ft)): Decreased performance, no impairment of oxygen transport. No risk to low risk of severe HAI. There have been a few case series describing susceptible individuals getting AMS and HAPE at intermediate altitude; however, this is generally considered a rare occurrence. SpO2 expected to remain >90%. 1-3

High altitude: 2500-4500m (8202-14,763ft): Decreased arterial oxygen saturations, hypoxemia during sleep/exercise. Moderate risk of severe HAI, including HACE and HAPE. SpO2 can drop below 80%. 3

Very high altitude: 4500-5500m (14,763-18,044ft): Requires period of acclimatization. High risk of severe HAI. Prevalence of AMS is described as high as 70% in individuals ascending Mount Rainier (14,411ft). SpO2 at 17,500ft has been described as ranging from 65-81%. 3-4

Extreme high altitude: >5500m (18,044ft): Severe hypoxemia/hypocapnia. Incompatible with prolonged human habitation. SpO2 on the peak of Everest Summit (29,000ft) is described as 54-62%. 3

Rapid ascent for military operations: Ascent from sea level to high altitude 2500-4500m (8202-14,763ft) in less than 48 hours without proper pre-acclimatization or an ascent rate faster than 500m per day when above 3000m (9,842ft) altitude without proper pre-acclimatization.