Gansu

This is Lanzhou, the old Silk Road outpost that has matured into a metropolis of almost 4 million as the capital of Gansu province in China’s still-poor northwest. And as your lungs can attest, it is one of the most polluted cities in the country.

A towering wall of sand rushed over factories and apartment blocks in northwestern China's Gansu province as seasonal sandstorms barrelled across the country, causing air pollution and traffic accidents. Aerial images as it struck showed an apocalyptic scene as a billowing cloud of yellow dust smothered Gansu's Linze county on Sunday.