For a non pressurised aircraft, the supplemental oxygen is required to operate at pressure altitudes above 10 000 ft. From sea level to 10 000 ft, pilots breathe outside air only. A mixture of 100% oxygen and outside air will be required from 10 000 ft to 34 000 ft. From 34 000 ft to 40 000 ft, pilots must breathe 100% oxygen. Without pressurization, sufficient oxygen can be delivered up to an altitude of about 40 000 ft (12 000 m). At 40 000 ft, the ambient air pressure falls to about 0.2 bar, at which maintaining a minimum partial pressure of oxygen of 0.2 bar requires breathing 100% oxygen under positive pressure.