1For decades, the city was famous for the thick grey smog that hung over its streets.
2Children were kept indoors on bad days, and hospitals filled with patients struggling to breathe.
3The turning point came when the mayor introduced a low emission zone in the centre.
4Older diesel cars and delivery lorries now had to pay a daily charge or stay outside the boundary.
5All the money raised from the daily charge was used to buy a fleet of clean electric buses.
6Within just three years, the level of dangerous particles in the air had fallen sharply by a third.
7Thousands of trees were planted along the main roads, creating green corridors through the city.
8Cyclists now enjoy wide new lanes along roads that were once reserved exclusively for cars and taxis.
9Not everyone is happy, however, because some shop owners say that fewer customers drive into town.
10Yet asthma cases among young children have dropped, and local doctors are clearly noticing the difference.
11Other polluted cities around the world are now sending teams to study how the change was achieved.