The government should cut the number of diesel-powered vehicles on the streets, and make the remainder less polluting, to protect pedestrians now the World Health Organisation has classified diesel fumes as a cause of cancer, lobby groups say.
WHO experts said that the more diesel exhaust fumes people breathed in, the more they risked getting lung and bladder cancer.
At the end of March, about 130,000 diesel-powered vehicles were registered in Hong Kong.
The Clean Air Network and think tank Civic Exchange claim the government has been too slow in toughening environmental standards for diesel engines.
Clean Air said the WHO's decision reinforced the need for the government to set out policies to encourage the use of cleaner fuel.
According to the group, more than 80 per cent of diesel vehicles have engines that meet the emissions standards in force in the EU before the year 2000. These pre-Euro-IV- standard vehicles account for most of the city's emissions of two key pollutants, being responsible for 88 per cent of pollutant particles in the air and 76 per cent of nitrogen oxide.
The European Union has introduced progressively tighter emission standards for vehicles since 1992. Its Euro V standard for buses and trucks was introduced in 2008; the Euro VI standard comes in next year.
Clean Air general manager Helen Choy Shuk-yi said bus companies had been reluctant to upgrade their vehicles on cost grounds.
Mike Kilburn, of Civic Exchange, said it was crucial officials speed up the phasing out of old diesel vehicles.
He also said the government may not be fully informed on the emission levels of construction machinery and other equipment, such as diggers and cranes. 'There is no standard for [their] maintenance,' he said.
The government must respond to the WHO findings since people could now easily understand the harm diesel exhaust fumes cause, Kilburn said.
An Environment Bureau spokesman said it had noted the WHO decision and would consider measures to tighten controls. He also said the government was planning new laws to control emissions from machinery and equipment.
The WHO said that while the risk of getting cancer from diesel fumes was small, since so many people breathed in the fumes in some way, raising the status of diesel exhaust to carcinogen from 'probable carcinogen' was an important shift.
The WHO's updated stance was based partly on the findings of a study conducted by the US National Cancer Institute/National Institute for Occupational Safety and Health, which found that a wide exposure to diesel-related emissions by 12,300 underground miners increased their risk of death from lung cancer.
Choy said she expected the WHO to revise its pollution benchmarks along with the latest decision.
'Hong Kong will then fall far short of international standards in terms of its air quality benchmarks,' she said.
The government says it will toughen air-quality targets in 2014.
The 80pc of diesel vehicles that don't comply with the latest standards account for this percentage of the city's particulate matter