The drive for cleaner air is paying dividends. Concentrations of hydrocarbons released by vehicles in Melbourne's central business district of Australia are now 61 per cent less than they were 15 years ago - even though the number of motor vehicles has risen. Much of the air quality gains are a result of the use of catalytic converters, which have been mandatory in new cars since 1986, and unleaded petrol. The converters remove from car exhausts pollutants, including hydrocarbons, which are produced from fossil fuels.