The government has pledged to study the feasibility of setting up low-emission zones from which heavily polluting buses would be banned.
The study will consider their pros and cons, including the extent to which they would improve roadside air quality and their impact on passengers and road traffic.
The pledges were made by Secretary for the Environment Edward Yau Tang-wah in the Legislative Council in a written response to a question from Hong Kong Island lawmaker Martin Lee Chu-ming.
Low emission zones (LEZ) are increasingly popular in Europe. Heavily polluting vehicles are banned from them or charged for entering them.
London has recently decreed an LEZ in the city centre and is progressively introducing daily charges of GBP100 (HK$1,540) to GBP200 for vehicles which flout its emission caps. The charge applies initially to heavy trucks; in July it will be extended to trucks over 3.5 tonnes and to buses and, in 2010, to vans and minibuses.
Mr Yau said a third of the vehicles run by Hong Kong's three big bus franchises were pre-Euro or Euro I. The total of 1,960 had dropped by only 80 since September 2006.
Pre-Euro vehicles were built before 1992, when the European Union introduced its first emission standards for heavy vehicles, known as Euro I.
