Green groups blame the city's high roadside air pollution on the inadequacy of steps to curb bus and truck emissions and the failure to implement other long-discussed ideas.

Despite a series of attempts to tackle the problem over the past decade, high pollution levels have persisted.

The groups also pointed to reluctance among operators to join a voluntary subsidy scheme to get rid of old, polluting trucks and buses, and say it should be made compulsory.

Friends of the Earth environmental affairs manager Hahn Chu Hon-keung said: "The government is never short of proposals to improve air quality; lots of ideas have been thrown at it in the past decade. It's just a question of determination."

Efforts to tackle traffic pollution include the 2000 introduction of taxis burning liquefied petroleum gas instead of dirtier diesel.

More recent measures include tax concessions for those buying environment-friendly petrol cars in 2007, and the introduction of electric cars this year. By May, about 7,500 tax-reduction applications had been approved, about 11 per cent of all newly registered motor vehicles.

But Green Power chief executive Man Chi-sum said old diesel trucks, buses and minibuses were the types of vehicle that contributed most to dirty roadside air, and the most urgent task was to deal with them.

About a third of the franchised bus fleet on Hong Kong's roads still does not meet Euro IV emission standards, the second-highest of five sets of standards that have been introduced progressively in Europe since 1992 and adopted internationally.

The last of the 1,800 pre-Euro and Euro I buses - which are more than twice as polluting as the newer models - will only be taken off the roads in 2015. Bus companies have said that replacing them immediately would be costly and require a fare rise.

"Whenever these buses start their engines, black smoke pours out," Dr Man said. "It is time the government took bold action [such as] setting up a fund to subsidise bus companies in speeding up the process."

A HK$3.2 billion subsidy scheme introduced in 2007 to replace the pre-Euro and Euro I diesel vehicles is regarded as a failure, with owners of fewer than a quarter of them applying for the subsidy by the end of last year. Dr Man and Greenpeace campaign officer Prentice Koo Wai-muk said the government should increase grants, make the scheme mandatory and set a deadline for replacement.

In November, the government proposed raising licence fees for more than 30,000 older commercial vehicles, but nothing further was done after truck drivers' groups expressed opposition.

New measures such as electronic road pricing, further bus-route rationalisation and pilot low-emission zones have been proposed by concern groups for many years. They are now included in a consultation paper on the air-quality-objective review to be released this summer. But no timetable has been laid out for their introduction