Beijing traffic officer in smog
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China smog crisis: 'Air turns toxic grey'

7 January 2014 Last updated at 17:38 GMT

Officials in China say they are confident green technology will help overcome the country's notoriously polluted air.

Apocalyptic scenes of dense smog have recently forced major cities including Shanghai and Harbin to virtually shut down.

The BBC's science editor David Shukman reports.