High-altitude sickness, or acute mountain sickness occurs when people ascend rapidly to elevations above about 8,000 feet (2,500 meters), where the air has significantly less oxygen. The body reacts to this hypoxia with symptoms such as headache, dizziness, fatigue, loss of appetite, and disturbed sleep. It results from decreased oxygen pressure causing fluid shifts and mild swelling in the brain and lungs. Risk increases with faster ascent, higher altitude, dehydration, or exertion before acclimatization. Severe forms—high-altitude cerebral or pulmonary edema—reflect dangerous fluid accumulation in the brain or lungs due to the body’s inability to adapt to low oxygen levels.