Choking smog all but shut down one of northeastern China's largest cities on Monday, forcing schools to suspend classes, snarling traffic and closing the airport in the country's first major air pollution crisis of the winter. An index measuring PM2.5, or particulate matter with a diameter of 2.5 micrometers (PM2.5), reached a reading of 1,000 in some parts of Harbin, the gritty capital of northeastern Heilongjiang province and home to some 11 million people. A level above 300 is considered hazardous, while the World Health Organisation recommends a daily level of no more than 20. The smog not only forced all primary and middle schools to suspend classes, but shut the airport and some public bus routes, the official Xinhua news agency reported, blaming the emergency on the first day of the heating being turned on in the city for winter. Visibility was reportedly reduced to 10 metres. The smog is expected to continue for the next 24 hours. Air quality in Chinese cities is of increasing concern to China's stability-obsessed leadership because it plays into popular resentment over political privilege and rising inequality in the world's second-largest economy.