A woman covers her mouth with a scarf in the smog in Harbin. AFP/Getty Images

Vehicles in Harbin are engulfed by smog Sunday. The air quality was so bad that the city closed schools on Monday. ChinaFotoPress/Getty Images

Choking smog all but shut down one of northeastern China's largest cities Monday, forcing schools to suspend classes, snarling traffic and closing the airport in the country's first major air-pollution crisis of this fall and winter.

Smog — a mixture of atmospheric pollutants, including car exhaust and factory emissions — is measured on an air-quality index measuring PM2.5, or particulate matter less than 2.5 micrometers in diameter. A level above 300 is considered hazardous, and the World Health Organization recommends a daily level of no more than 20.

In some parts of Harbin, the gritty capital of northeastern Heilongjiang province and home to some 11 million people, readings reached 1,000.



The smog not only forced all primary and middle schools to suspend classes but also shut the airport and some public bus routes, the official Xinhua news agency reported, blaming the emergency on the first day of the season that heating was turned on in the city. Visibility was reportedly reduced to 10 meters.

The smog is expected to continue for the next 24 hours.