Acute altitude sickness arises after at least four hours spent at an altitude above 2,000 m. Ascending to heights greater than 2,500 m can trigger a range of symptoms including headache and vomiting. Men are at greater risk of altitude sickness than women, for reasons unknown.

Altitude sickness, specifically Acute Mountain Sickness (AMS), typically begins to affect unacclimatized individuals at elevations above 8,000 feet (2,438 meters). While some highly sensitive people may feel mild symptoms as low as 5,000 feet, the majority of clinical cases occur once you cross the 8,000-foot threshold. At this height, the barometric pressure drops significantly, resulting in fewer oxygen molecules per breath. Symptoms often include headache, nausea, and fatigue, usually appearing 6 to 12 hours after arrival. In 2026, medical experts categorize altitudes into "High" (5,000–11,500 ft), "Very High" (11,500–18,000 ft), and "Extreme" (above 18,000 ft). To prevent sickness, the standard recommendation is to avoid ascending more than 1,000 feet of "sleeping altitude" per day once you are above 10,000 feet, allowing your body time to undergo the biological process of acclimatization.