Until what altitude can humans still breathe?

Humans can generally breathe comfortably up to about 8,000 feet (2,400 meters) above sea level without significant acclimatization. However, at altitudes above 10,000 feet (3,048 meters), some individuals may begin to experience symptoms of altitude sickness due to reduced oxygen levels. Above 18,000 feet (5,500 meters), the air becomes thin enough that supplemental oxygen is often required for prolonged exposure. Most people can survive at high altitudes, but breathing becomes increasingly difficult as elevation increases.