The barometric altitude of a passenger aircraft cabin varies between sea level and 2 100 metres depending on the type of aircraft and its actual flight altitude. The oxygen content of the cabin air is always 21%, but when the barometric altitude of the cabin increases, the alveolar oxygen partial pressure decreases. The arterial oxygen partial pressure (pO2) of a healthy individual at sea level is approximately 13 kPa, and at 2 100 m altitude is still about 8 kPa. Hypoxic symptoms appear only above 3000 m when the arterial oxygen partial pressure falls below 7 kPa. The respective haemoglobin oxygen saturation values are 98% at sea level, 92% at 2100 m and 87% at 3000 m. The cabin air is re-circulated so that it is changed 6-12 times in an hour. The cabin air is very dry with relative humidity varying between 10% and 20% during the flight. Cabin pressure changes cause pressure changes in closed body cavities unless the pressures can be equalised. The most common symptoms are caused by infective or allergic conditions of the middle air or sinus cavity.