EMISSIONS from petrol vehicles are harmful to the human body and aggravate the greenhouse effect, experts from Hong Kong Polytechnic University said yesterday. The lecturers, who presented study results on petrol and diesel emissions, said although diesel created black smoke and harmful particulates, there was no proof petrol was a better option. 'Black smoke is the main disadvantage of a diesel car, but the fact that carbon monoxide from petrol is invisible doesn't mean it has no adverse effects,' said an assistant professor of civil and structural engineering, Hung Wing-tat. Professor Hung said the reliability of catalytic converters fitted to petrol engines to cut emissions, as required by law since 1992, had yet to be confirmed. Describing the two fuels as 'sour oranges and grapefruit', the experts said the Government's proposed switch from diesel to petrol, which uses 25 per cent more fuel, would not help cut pollution without stricter maintenance controls. 'The Government only checks black smoke. That's why it's biased against diesel,' said Professor Hung. The academics called for tighter maintenance controls through legislation and technicians' training. They also suggested a score system to punish drivers who failed to comply. A spokesman for the Taxi Association Federation, Ng Kwok-hung, agreed it would be better to improve diesel maintenance than switch to petrol, which would increase operating costs.