China's northern city of Harbin is on red alert for thick smog, which has forced schools and highways to shut down and flights being disrupted.
Visibility in several parts of Harbin City in Heilongjiang Province was reduced to below 50 meters, as the density of airborne particles was several times above World Health Organisation (WHO) recommended limits, the BBC reports.
According to the report, the reason behind the pollution was speculated to be the city's public heating system being switched on for winter.
PM2.5 levels, which are 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. It should not be more than 25 microgrammes per cubic metre according to WHO guidelines.
