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.
The tougher Euro II standard was introduced in 1996.
At the end of last year, Kowloon Motor Bus, CityBus and sister company New World First Bus were operating 609 pre-Euro buses and 1,351 Euro I buses.
Of these, 311 were plying some of city's busiest roads, including Hennessy Road, Queensway - the worst affected - and Des Voeux Road Central on the island and Nathan Road in Tsim Sha Tsui.
Most of them were in the CityBus fleet.
But Mr Yau told lawmakers that forcing them off these roads would shift the air pollution problem elsewhere.
'The government needs to study carefully whether deploying all pre-Euro or Euro I buses outside low emission zones will shift the roadside air pollution problem to areas which are currently unaffected, or worsen their current situation,' he said.
Pre-Euro and Euro I buses accounted for about half the 100 tonnes of respirable suspended particles and nitrogen oxides emitted by franchised buses in 2006.
A Euro IV diesel vehicle emits 95 per cent less particulates and 60 per cent less nitrogen oxides than a pre-Euro vehicle, the Environmental Protection Department says.