Choking clouds of pollution have virtually shut down the northeastern Chinese city of Harbin, forcing schools to suspend classes, snarling traffic and closing the airport.
Weather officials said on Monday that concentrations of tiny airborne particles considered the most harmful to health were about 40 times higher than what is considered safe by the World Health Organization.
The official Xinhua news agency blamed the emergency on the first day of the heating being turned on in the city for winter.
Pollution from rapid development and heavy coal use plagues large areas of China, prompting pledges from the country's leadership to make improvements. The government has promised for years to tackle the pollution problem but has made little progress.
Officials said Monday the heavy smog is expected to continue for 24 hours.