Use of coal may be falling in China, but it remains a major energy source and pollutant.

Residential heating comprises just 2.6 percent of China's coal use, according to the National Bureau of Statistics.

Every winter, major cities in northern China are heated through coal-fired power stations, but in certain climatic conditions smog can be trapped. For example, in Shenyang, Liaoning province, PM2.5 levels rocketed to well over 1,400 on Nov 8.