Asia has more polluted cities than any other continent. That's what a comparison of 20 leading cities of the world shows. Rapid economic growth spurs the growth of big cities, which in turn bring in more vehicles. While the growth rate of vehicles in most developed countries has stopped or is very low, in nine Asian countries -- India, China, the Philippines, Thailand, Indonesia, Malaysia, South Korea, Taiwan and Singapore -- it is around 13 per cent. Unless governments invest in cleaner alternatives, air pollution will continue to choke millions.