Emission from road transport is the primary source of CO. It contributed to 82% of total emission in 2007. The tightening of vehicle emission standards from Euro I in 1995 to Euro IV in 2006 have significantly reduced the CO emission. The various incentive schemes launched in 2007 to encourage owners to early phase-out their pre-Euro and Euro I commercial vehicles or to switch to new environmental friendly petrol private cars also helped reduce the emissions.