Motor vehicles largely contribute to local street-level pollution which typically impacts the largest groups of people. Diesel vehicles such as trucks and buses are the biggest culprits as they are the main source of vehicular emissions. An increase in vehicles on the road over the past two decades has also been attributed to a sharp increase in air pollution in Hong Kong. It is considered by many to be difficult to clean up local street-level pollution because of the pollutants trapping themselves beside tall skyscrapers that surround streets.