No. 3 Linfen
A number of Chinese cities could've made this list for the same reason - pollution - but Linfen takes the cake because it has the dirtiest air in the entire world. A small city (pop. 4,000,000 - yes, that's a small city in China) located in southern Shanxi province, Linfen's air pollution problem is legendary, a result of being one of China's largest coal mining hubs. The air is filled with soot and pollutants and water is contaminated by arsenic. There is a heightened risk of bronchitis, pneumonia, and lung cancer for residents. The entire Shanxi province, a hard-scrabble region on China's western central plains, is plagued by air pollution. Shanxi has 270 billion tons of proven coal reserves, and coal is everywhere - piled in back alleys and sold in burnable cubes. Until China's government makes a more sincere effort at cleaning up the environment, the sun won't be shining in Linfen for sometime to come.