At intermediate altitude (1,500 to 2,500 metres above sea level) altitude illness is unlikely, though possible. 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.

For most healthy individuals, the effects of altitude typically start to become noticeable at around 8,000 feet (approx. 2,400 meters) above sea level. This is the threshold where the concentration of oxygen in the blood begins to drop significantly as the atmospheric pressure decreases. In a commercial airplane, the cabin is usually pressurized to simulate an altitude of 6,000 to 8,000 feet, which is why some passengers experience mild headaches or fatigue during flight. Acute Mountain Sickness (AMS) symptoms—such as dizziness, nausea, and shortness of breath—usually manifest when you ascend quickly to altitudes above 8,000 feet without acclimatizing. For "extreme" elevations above 11,500 feet, nearly everyone will feel the effects of reduced oxygen. It is important to remember that physical fitness is not a predictor of altitude tolerance; the best way to prevent issues is to ascend slowly (no more than 1,000 feet of sleeping elevation per day once above 8,000 feet) and stay well-hydrated while avoiding alcohol.