The question is so obvious that you could easily forget to ask it: why do cities so often have a poor east side? To be clear, the mystery is not why every city has its leafy and its grubby sections – it costs money to live in nice places and to avoid nasty ones, which tends to group people into them by wealth. The mystery is why the poor groups always end up in the east.Of course, the true picture is never neat nor simple, but by common consent a British-biased list of cities with poor eastern districts would include: London, Paris, New York, Toronto, Bristol, Manchester, Brighton and Hove, Oxford, Glasgow, Helsinki and Casablanca. No doubt there are some cities where poverty clusters in the west, but they seem harder to find; perhaps Delhi and Sydney?The story seems easier to explain case by case. In London, for example, the docks are downstream in the east, and docks are rarely very salubrious in cities. For much of its history, the Thames also took the city’s waste, and smell, eastward. No wonder the poor wound up living there, you might say.But then consider Paris, where the Seine flows westward, as does the Avon in Bristol. Or New York, which effectively has two rivers, and is a completely different shape. Or even Brighton, which has no rivers at all. Despite their differences, the story never varies: for poverty, look east.One theory is that it’s all about air pollution. In the middle latitudes where most of the world’s cities can be found, the prevailing winds are westerlies, which means they blow to the east. Crudely, it has long been thought that they might take smoke and odours with them, and now a new study by Stephan Heblich, Alex Trew and Yanos Zylberberg for the Spatial Economics Research Centre suggests this theory might be right.