What altitude is classified as extreme hypoxia?

Extreme hypoxia is typically classified as occurring at altitudes where the partial pressure of oxygen significantly decreases, leading to a marked reduction in arterial oxygen saturation. While individual tolerance can vary, extreme hypoxia is generally recognized to begin around 20,000 feet above sea level. At this altitude, the oxygen levels are insufficient for normal physiological function, and individuals may experience severe cognitive and physical impairments without supplemental oxygen.

Climbing to higher altitudes, such as 22,000 feet or 24,000 feet, only exacerbates the hypoxic condition, but the threshold of extreme hypoxia is often set at 20,000 feet for aviation and medical purposes. This designation is crucial for pilots, mountaineers, and others who may operate in low-oxygen environments, as it indicates the point at which significant physiological effects start to manifest.