21 Oct 2013, Harbin, Heilongjiang Province, China — Pedestrians wearing face masks walk on a road in heavy smog Harbin, northeast Chinas Heilongjiang province, 21 October 2013. Choking smog all but shut down one of northeastern Chinas largest cities on Monday (21 October 2013), forcing schools to suspended classes, snarling traffic and closing the airport, in the countrys 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 Xinhu — Image by © Imaginechina/Corbis