Updated at 6.35pm:
The government has proposed tightening emission standards for newly registered heavy vehicles and motorcycles, an Environmental Protection Department spokesman said on Friday.
He said the under the proposal newly registered heavy vehicles - those weighing more than 3.5 tonnes - would have to conform to Euro IV emission standards.
Euro emission standards are requirements defining acceptable limits for exhaust emissions of new vehicles sold in European Union countries.
A heavy vehicle meeting the Euro IV standards emits about 30 per cent less nitrogen oxides and 80 per cent less particulates than an existing Euro III model.
The spokesman said light vehicles, weighing up to 3.5 tonnes, had had to comply with Euro IV emission standards from January.
The proposal is expected to come into effect on October 1.
'It also requires all newly registered motorcycles to comply with Euro III emission standards from January 1, 2007,' he said.
A Euro III motorcycle emits about 90 per cent less hydrocarbons and 50 cent less nitrogen oxides than an existing model.
Japanese and United States standards would be accepted if they had the same effect in reducing emissions, the spokesman added.
The proposal, or the Air Pollution Control (Vehicle Design Standards) (Emission) (Amendment) Regulation 2006, was gazetted on Friday.
It will be tabled at the Legislative Council for negative vetting on May 24.
The air quality in Hong Kong has been deteriorating in recent years. Environmental concern groups have called on the government to implement tougher measures to help reduce pollutants. They have also urged officials to discuss the problem with authorities in the neighbouring Guangdong province, where vehicle numbers are growing exponentially.