Air in an airliner is far fresher than in an office building
May I lay to rest some widespread myths and untruths that have appeared in your newspaper. One is the myth that 'aircraft have highly recirculated air', as expressed by your medical columnist ('Your guide to Sars', South China Morning Post, April 3).
The other is the wholly inaccurate and misinformed report by Joseph Lo claiming that Cathay Pacific pilots are somehow able to increase the amount of fresh air that flows into the aircraft cabin and that this has been prohibited by the company because aircraft would use more fuel ('Pilots increase fresh air inside cabins', April 4).
The facts are as follows. When in flight, fresh air continuously flows into the aircraft cabin. The entire cabin air volume is exchanged every three to five minutes. During that cycle, half of the cabin air volume is drawn by fans through a set of special filters as it passes through the cabin.
By way of comparison, an office building's air-conditioning system will typically mix only 10 per cent to 15 per cent of fresh air in with recycled air. The air in an aircraft is clearly far fresher than in an office building.
Cathay Pacific aircraft are fitted with high efficiency particle air (HEPA) filters, with a 99.97 per cent efficiency rating, which effectively filter out bacteria and larger virus particles. Smaller viruses travel in larger respiratory droplets, which are generally captured by the filters as well.
Moreover, the dry air in aircraft cabins is inhospitable to germs, as most thrive in a moist environment. Droplets that may bear germs also quickly evaporate in the dry air.
Boeing and Airbus aircraft are designed to manage the air-conditioning systems automatically. Because aircraft cabins are pressurised, simply turning air-conditioning packs on to high makes virtually no difference to the volume of outside air drawn in during flight.
Furthermore, switching off circulation fans to achieve this aim, as suggested in Joseph Lo's report, would mean that no air would pass through the filtration system.
Whenever people congregate in close proximity there is a risk of disease transmission. However, the air quality in an aeroplane provides no more risk than other forms of public transport.
The airline industry's experience with other transmissible respiratory diseases supports the view that air travel poses minimal risk of infection.
DEREK CRIDLAND, Engineering Director, Cathay Pacific