OXYGEN PARTIAL PRESSURE

Oxygen forms 21% of the total air in the atmosphere. The partial pressure of oxygen in the atmosphere is therefore 21% of the total atmospheric pressure (Dalton's law). This gives an oxygen partial pressure of 160 mmHg at MSL. The total atmospheric pressure decreases with altitude and so does the partial pressure of oxygen.

In the human lungs, the partial pressure of oxygen decreases further (14%) due to the presence of the breathing waste products (carbon dioxide and water vapor). The required oxygen partial pressure for the human lungs to work properly is 55 mmHg. A reduction of the oxygen partial pressure below that value can lead to hypoxia. An oxygen partial pressure of 55 mmHg occurs at 10 000 ft. This means that above 10 000 ft oxygen percentage should be increased.

• Above 10 000 ft oxygen percentage should start increasing.
• At 33 700 ft the oxygen should become 100% of the cabin air in order to be equivalent to breathing air at MSL.
• At 40 000 ft breathing 100% oxygen is equivalent to breathing ambient air at 10 000 ft.
• Above 40 000 ft 100% oxygen under positive pressure should be provided.

Note: For a non-pressurized cabin breathing 100% oxygen at any altitude below 34 000 ft will provide an oxygen pressure higher than the oxygen pressure at MSL. However, when flying in a non-pressurized cabin the aircraft is usually not allowed to climb to altitudes higher than 10 000 ft.