China’s smog has forced schools, highways and airport to shut in northern city of Harbin, Heilongjiang province.
The density of airborne particles was several times above World Health Organization (WHO) recommended limits.
Visibility was reduced to below 160ft in parts of the city, reports said.
Local media have linked the pollution to the city switching on its public heating system for winter.
PM2.5 levels, used to measure the amount of particulate matter smaller than 2.5 micrometres in the air, were above 500 micrograms per cubic metre on Monday morning, state-run news agency Xinhua reported.
WHO guidelines say average daily concentrations of PM2.5 should be no more than 25 microgrammes per cubic metre.
All of Heilongjiang province’s highways, and the Taiping International Airport in Harbin were forced to close, Xinhua reported.
A red alert for thick smog had been issued in the provinces of Heilongjiang, Jilin and Liaoning, the news agency added.
Earlier this year, air pollution in China’s capital, Beijing, also soared past hazardous levels.
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