Virus killer: Hong Kong's passenger fleets are equipped with High Efficiency Particulate Arrester filters to remove viruses, said Civil Aviation Department Director-General Albert Lam.

It is difficult for someone to catch severe acute respiratory syndrome in an airplane, says Albert Lam, the Director-General of Civil Aviation.

The entire passenger fleets of the two Hong Kong-based airlines are equipped with High Efficiency Particulate Arrester filters to remove viruses, Mr Lam told reporters at a press briefing today.

Mr Lam explained the operation of the inflight air circulation and filtration systems and said the HEPA filters, which are made of micro glass fibers, are 99.97% efficient in removing dust and airborne contaminants with a particle size of 0.3 micron and above, including viruses. Most coronavirus droplets are larger than five microns.

There are two to eight such filters installed in every aircraft. Hospital operating theatres use the same type of filter.

The World Health Organisation agrees that the filter systems in commercial aircraft have proven to be effective against infectious disease.

In flight, fresh air is introduced into the cabin continuously, and this fresh air is mixed with cabin air, which is passed through filters. The entire cabin air volume is exchanged every two to three minutes.

The average relative humidity in the cabin is 15% to 20% during flight. A low humidity environment has been shown to inhibit fungal and bacterial growth.

Meanwhile, air supplied to aircraft toilets and galleys is not re-circulated but is expelled from the aircraft, and air in the cabin is drawn down and extracted at floor level.

Mr Lam noted that there is no longitudinal flow of air through the cabin. The air supplied to one seat row leaves at approximately the same seat row, thus minimising airflow in the fore and aft directions. In other words, the air of one person passes another at floor level only.

Since mid-April, temperature checks for all arriving, departing and transit passengers at the Hong Kong International Airport have been implemented. In line with the WHO guidelines, Hong Kong carriers will not accept passengers with symptoms and signs compatible with SARS.

"Local air operators also carry out scheduled cabin cleaning with cleansing agents accepted for use on aircraft. Air travel is not a high-risk activity, neither is aircraft itself a high-risk environment for SARS transmission," Mr Lam reiterated.

Meanwhile, the International Civil Aviation Organisation  published guidelines in early May for its member states to help stop the international spread of SARS by air.