Images show stark contrast between Hanoi's cityscape on clear days and its muted appearance during heavy air pollution. In the last three months, the Vietnamese capital city of Hanoi has regularly topped a list of the world's most polluted major cities, leaving its nine million residents struggling to breathe and even to see through a thick blanket of smog. "The situation is urgent," says Muthukumara Mani, World Bank lead environmental economist, based in Hanoi. IMAGES AND SOUNDBITES