In the most polluted city on earth, the smog is so thick that it seems to consume its source. Iron foundries, smelting plants and cement factories loom out of the haze then disappear once more as you drive along Linfen's roads. The outlines of smoke stacks blur in the filthy mist. No sooner are the plumes of carbon and sulphur belched out than the chimneys are swallowed up again.

"We only see the sun for a few days each year," said Zhou Huocun, a doctor in the outlying village of Liucunzhen. "The colour of our village is black. It is so dirty that nobody airs their quilts outside any more so we are getting more parasites. I have seen a steady increase in respiratory diseases as the air quality gets worse and worse."