High Altitude

Environmental Hazard

Traveling at altitudes of 10,000 feet or higher above sea level is taxing for a creature that needs to breathe, because of the reduced amount of oxygen in the air. Each hour such a creature spends traveling at high altitude counts as 2 hours for the purpose of determining how long that creature can travel.

Breathing creatures can become acclimated to a high altitude by spending 30 days or more at this elevation. Breathing creatures can’t become acclimated to elevations above 20,000 feet unless they are native to such environments.

Source: Dungeon Master’s Guide p. 110