Smog is an ongoing problem as China increasingly urbanizes and the motor-vehicle population grows and grow.

Harbin is the capital of Heilongjiang province, in north-east China, and has a population of around 11 million people. On Monday schools were closed, as was the airport, due to air pollution. Visibility was at 10 metres.

Xinhua news agency said the extra pollution was due to the first day of the heating being turned on in the city for winter.

Growth in China continues rapidly, such externalities (like smog) are often a part of growing economies.

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