SEVERE smog has reduced the visibility of a northern Chinese city to less than half a football field.

Authorities in Harbin near the Russian border said on Monday primary and middle schools and some highways had been closed.

At least 40 flights from its international airport have also been cancelled or postponed, and the official Xinhua News Agency says some buses have stopped running.

The density of fine particulate matter used as an indicator of air quality has been found to be more than 24 times higher in the city than the level considered safe by the World Health Organisation.

Also on Monday, the manager of American jazz singer Patti Austin said she'd been forced to cancel a concert in the capital, Beijing, on Friday evening after suffering a severe asthma attack probably linked to air pollution.

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