Officials in the Chinese capital Beijing have had to implement emergency measures for the first time to combat pollution which rose to the worst ever at the weekend.
Residents were warned to stay indoors, as the concentration of harmful particles in the air reached up to 45 times the recommended safety levels.
The thick blanket of smog descended on Friday and is being trapped by an area of low pressure.
One resident, Li Li said: "I have difficulty breathing. Today is better (than the weekend). We live on the east side of the city and the smog is worse there. When I opened the window this morning, I couldn't see clearly. The visibility was very low. I am worried about my family, especially those who drive."
Beijing's emergency plan involves cutting the use of official vehicles, and reducing production from more than 50 high emission industries.
The city sent out 14 teams of inspectors.
Pollution has been identified as one of the biggest challenges facing China, whose industries have catapulted the economy to being the second biggest in the world.