Nearly 90 per cent of air pollution is caused by smoke-belching diesel engines, a report has revealed. And the main source of pollutants is exhaust fumes from 70,000 vehicles made before 1992. A paper to be delivered at an environmental conference tomorrow will urge the Government and private sector to offer an incentive package allowing owners of pre-1992 models to change their cars. Newer models could cut pollution by up to two-thirds, environmentalists said. The paper's author, Steve Choi, of pressure group Better Environment Hong Kong, said: The biggest challenge not only for the Government but also the private sector is to speed up the programme.