China smog: "The air is so polluted it's darkened the sky"
For the fourth day in a row, large sections of northeast China remain covered by a thick layer of smog considered hazardous to human health.
Millions of people have been told to stay indoors, and most factories and schools have been shut.
It comes as President Xi Jinping attends the opening of the UN Global Conference on Climate Change in Paris.
Celia Hatton reports.