The maximum altitude where a person can survive without supplemental oxygen depends on various factors, including the individual's health, physical condition, and acclimatization to high altitudes. However, in general, the "death zone" is considered to start at an altitude of around 8,000 meters (26,000 feet) above sea level.
At this altitude and higher, the atmospheric pressure becomes extremely low, resulting in a lack of sufficient oxygen for the human body to function properly. Without supplemental oxygen or specialized equipment, the human body cannot sustain itself for an extended period at these altitudes.
It's important to note that prolonged exposure to high altitudes without acclimatization and proper preparation can be extremely dangerous and potentially fatal. It is always recommended to consult with medical professionals and undergo proper training and acclimatization processes when venturing into high-altitude environments.