The pressurised aircraft cabin provides protection against the hostile external environment encountered at cruising altitudes. The control system is automatic and is designed by the aircraft manufacturer to comply with the requirements of the regulatory authorities.
Although the partial pressure of oxygen is less than at sea level, it is more than adequate in a pressurised aircraft cabin for normal healthy individuals.
The total volume of air in the cabin is exchanged every 2 to 3 minutes, compared with an exchange rate of 5 to 10 minutes in an air-conditioned building.
Although up to half the air in a modern pressurised aircraft is recirculated, the amount of fresh air available to each occupant is greater than that available in air-conditioned buildings. Recirculating some of the air has the advantage of reducing localised cold draughts and increasing the humidity.
In modern aircraft, all the recirculated air is passed through high efficiency particulate filters which remove more than 99% of particles, including bacteria and viruses. These filters are identical to those used in hospital operating theatres.Research by the World Health Organisation (WHO) has found no evidence that air recirculation facilitates transmission of infectious disease aboard aircraft.
The cabin air, although dry, does not cause systemic dehydration or harm to health. Dry skin and eyes can lead to discomfort, which can be alleviated by the use of moisturising creams and eye drops.