Smog has been engulfing northern China ever since the region began enjoying central heating in winter. Some experts blame cloudy, windless weather for the smog, but research shows the burning of coal to provide central heating is one of the main reasons for the increase of particulate matter in the atmosphere which causes smog.

Fossil fuels account for as high as about 90 percent of all energy sources in China, which is the highest in the world. Worse, coal accounts for 65 percent of China's total energy consumption while the share of natural gas is just 6 percent. In contrast, the percentage of coal and natural gas is about 32 and 24 in the US. Consider this: to produce the same amount of heat, coal lets off double the volume of emission than natural gas.

To further worsen matters, coal is burned in rather less efficient ways in China. For example, to provide central heating, many cities use small and medium-sized boilers to burn coal, which emit more particulate matter compared with large boilers.