Lanzhou is the capital and largest city of Gansu Province in Northwest China. The prefecture-level city, located on the banks of the Yellow River, is a key regional transportation hub, connecting areas further west by rail to the eastern half of the country. Historically, it has been a major link on the Northern Silk Road. The city is also a center for heavy industry and petrochemical industry. Lanzhou ranks as one of the cities with the worst air quality in the world, due to industrial pollution and its situation in a narrow river valley. Lanzhou is home to 3,616,163 inhabitants at the 2010 census and 2,177,130 in the built-up area (urban) of 1,088 square kilometres (420 sq mi).