Linfen, China

Number of people potentially affected: 3,000,000
Type of pollutant: Coal and particulates
Source of pollution: Automobile and industrial emissions

This soot-blackened city in China's inland Shanxi province makes Dickensian London look as pristine as a nature park. Shanxi is the heart of China's coal belt, and the hills around Linfen are dotted with mines, legal and illegal, and the air is filled with burning coal. Don't bother hanging your laundry — it'll turn black before it dries. China's State Environmental Protection Agency says that Linfen has the worst air in the country, which is saying something, considering that the World Bank has reported that 16 of the 20 most polluted cities in the world are Chinese. One Linfen native summed up the city's plight to a TIME reporter last year: "This place of ours is no good."

— by Bryan Walsh