Linfen is a prefecture-level city in the southwest of Shanxi province, People's Republic of China, bordering Shaanxi province to the west. It is situated along the banks of the Fen River. It has an area of 20,275 square kilometres (7,828 sq mi) and according to the 2010 Census, a population of 4,316,612 inhabitants of which 944,050 live in the built-up area made up of Yaodu urban district. The GDP of Linfen ranked second in Shanxi Province. It was known as Pingyang (平陽) during the Spring and Autumn period. In 2006, the American Blacksmith Institute listed Linfen as one of the ten most polluted cities in the world.