Two students demonstrate wearing masks under a guidance of a teacher in an indoor physical education class as the outdoor sports activities for primary and middle schools were ordered to be halted due to heavy air pollution in Hupo Shanzhuang Elementary School in Hefei, capital of east China's Anhui Province, Jan. 14, 2013. Emergency response measures were adopted in many Chinese cities, where the air has held excessive levels of major pollutants in the past few days due to prolonged fog and smog. Heavy fog has caused highway closures and flight delays in several provinces. The elderly, children and those suffering from respiratory and cardiovascular diseases are advised to stay indoors to reduce exposure to polluted air. [Xinhua]