As part of the same study, they compared very specific areas over time, as well as following levels of pollution over time. A cloud of polluted air, after all, can move around depending which direction the wind blows. This takes pollution to different parts of the city, at random, to both richer and poorer areas. “We just followed this cloud on a daily level and see what happened to crime in areas when the cloud arrives… We found that wherever it goes crime rate increases,” he explains.