Ask Captain Lim

In 1996, the New England Journal of Medicine stated that "during most commercial flights today, cabin air is remarkably clean". In January 2001, a major airline in UK admitted to a problem with its fleet of Boeing 777 after crew members reported a high incidence of nausea and fainting. It was suggested that poor air circulation led to still pockets of air forming at head height in part of the airplanes.

In the recent SARS (severe acute respiratory syndrome) epidemic, the subject of cabin air quality has further aroused interest in the minds of many passengers.

One air traveler has emailed me asking whether it is true that the Boeing 777 continuously uses the outside air during the cruise to ventilate the cabin. The answer is no. This is because outside air, say at 35,000 feet, is very dry. It would reduce the relative humidity inside the cabin further, causing more health symptoms such as dry, irritated eyes and dry or stuffy nose. This would make one even more susceptible to infection. So, on a Boeing 777, re-circulation fans are used to augment the airflow in the cabin. They provide a mixture of about 50 percent outside air and 50 percent re-circulated air such that a balance has been achieved to maintain a high level of cabin air quality.

The Boeing 777 cabin design provides a ventilation rate of 13 to 20 cubic feet per minute of air per passenger. The 50 % cabin air is highly filtered re-circulated air where viruses and bacteria are removed. The filtration systems used are High Efficiency Particulate Air Filters (HEPA-type), similar to that used in hospital operating rooms. So one can say that, in-flight cabin air is of an even better quality than air in the average office building or shopping complexes.

Personally, having flown the Boeing 777 on many long haul flights for the past 7 years, I have yet to receive any serious complaint of foul air in the cabin. It has also been suggested that air distribution in some of the aircraft cabin may contain microbes which have been re-circulated and are liable to cause illnesses. But studies have suggested that proximity and not air quality is the issue when it comes to contracting colds and other illnesses.

Although in flight cabin air is replaced 20 to 30 times an hour in most modern jets today, it was stated that passengers can still fall ill because filter do not trap all bacteria and viruses as cabin air is generally dry, making passengers' nasal membrane dry and more vulnerable to airborne infection.

Further, according to the US Federal Center for Disease Control, an infectious or contagious disease liked influenza can easily spread through 70 % of passengers during a flight. Wearing a filter mask will cut down the chances of being infected and if one cannot face using a mask, using nasal spray to keep nasal membranes moist would help to reduce the chances of infection.