Bus and truck operators have barely begun replenishing their fleets with models meeting current European Union standards for pollution emissions, but the government is already considering bringing in tougher standards for new vehicles within two years. New vans and light buses and trucks may be required to meet the standards after a further two years, in 2011, said principal environmental protection officer Mok Wai-chuen. The Euro V standards, covering emissions of respirable suspended particulates, nitrogen oxides and hydrocarbons, will come into force in the European Union next year. The Euro V standard for nitrogen oxides is 42 per cent tighter than that under the current Euro IV. 'We have been following the timetable of the European Union closely in tightening emission standards,' Mr Mok said at a briefing organised by the Business Environment Council. Euro IV has applied to new trucks and buses registered in Hong Kong since October 1. New World First Bus and Citybus will add the first 28 Euro IV buses to their combined fleet of 1,600 by November. Kowloon Motor Bus, which runs more than 4,000 buses, bought its third Euro IV vehicle in February. KMB principal engineer Kane Shum Yuet-hung said the company had tentative plans to run Euro V buses in 2009. Vehicles are the second-largest source of air pollution in Hong Kong, contributing 25 per cent of respirable suspended particulates and nitrogen oxides. Diesel trucks are the biggest vehicle polluters, accounting for 90 per cent of particulate emissions and 80 per cent of nitrogen oxides. Alexis Lau Kai-hon, an atmospheric scientist at the University of Science and Technology, said improving city air would require better urban planning and traffic control. Meanwhile, Alfred Tam Yat-cheung, of the Council for Sustainable Development, said it would be difficult to develop guidelines for when schools should suspend outdoor activities because of high air pollution. International schools cancelled some sports events last week. The council released a consultation paper on cleaner air in June. It proposes adopting electronic road pricing and letting employees work from home when pollution is high. A kit to teach schoolchildren about climate change will be launched this week by the Observatory and the Education Bureau. Additional reporting by Loretta Fong