The cities that are cleaning up their act

We asked a panel of experts how communities from Mexico City to Belfast are becoming healthier and more sustainable

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There is no better example of a city turning things around - and quickly, said Seth Schultz, director of research, measurement and planning at C40 Cities. "It was not long ago that Mexico City held the dubious distinction of [being] the city with the world's dirtiest air. That is no longer the case."