Blacksmith Institute mentioned Ürümqi in 2007 World's Worst Polluted Places caused by air pollution along with Linfen and Lanzhou, all in China. In 2008, Toronto Star listed Ürümqi as one of the Top Ten worst places to live in the world due to sulphurous pollution. Heavy haze is extremely common in winter, which frequently affects air traffic. Officials believed that severe winter air pollution in Ürümqi is mainly caused by energy-heavy industries and the outdated coal-firing winter heating system. According to a report by Department of Environmental Science and Engineering of Fudan University, the average PM2.5 and TSP concentrations in the winter of 2007 were 12 times higher than USA standard for PM2.5 and 3 times the National Ambient Air Quality Standard of China for TSP. The sulfur dioxide from industrial emissions mixed with the local anthropogenic aerosol with the transported soil dust from outside the city were the main sources of the high concentration of sulfate, one of the main factors causing the heavy air pollution over Ürümqi.