How China is trying to battle smog in its cities
7 January 2014 Last updated at 00:05 GMT
Chinese authorities say they are confident they can tackle the smog that keeps engulfing many of the country's cities.
Shanghai is the latest to have to close its highways in a bid to clear the air and for many people pollution is now a major public concern.
The BBC's science editor David Shukman visited Beijing to look at the growing threat to public health - and the efforts to fix the problem.