Beijing skyline shrouded by smog
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Anger as Chinese cities blanketed by thick smog

6 December 2011 Last updated at 12:50 GMT

Sales of face masks are reported to have surged, as dense smog affects parts of northern China for a third day

More than four million comments on pollution were posted on one of China's main social networking sites, many bemoaning transport delays and the risks to health.

But the smog has raised health concerns. The BBC's Damian Grammaticas has more from Beijing.