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.

Sources

Dungeon Master’s Guide p. 110