A bout of choking smog that brought life to a standstill in a far northeast Chinese city for three days has dissipated, with flights resuming and students returning to school Wednesday. The heavy pollution, which drew national and international headlines, was largely due to coal-fired heating and the burning of large amounts of straw as winter approaches, local environmental officials in the city of Harbin have said. Figures from monitoring stations on Wednesday morning showed that levels of the most harmful airborne particles, known as PM2.5, had dropped to an average of 123 micrograms per cubic metre in Harbin. That was down sharply from Monday\'s figure of 1,000 micrograms per cubic metre. The World Health Organisation\'s recommended standard is 25. The overall air quality index in the city, which has a population of more than 10 million and which hosts a famed annual ice festival, was 162, or \"moderately polluted\". At the height of the smog on Monday, that figure had exceeded 500, the highest level on the Chinese scale. Visibility had been so bad -- less than 50 metres in some areas -- that two city buses got lost for hours while on their regular routes, according to local media. Officials had warned residents to stay indoors, and many of those who did venture outside reported wearing multiple face masks in order to block out the harmful air particles.