As winter continues to settle in, China is experiencing its 'worst' smog since autumn this year. And this time, it may last up to six days, affecting a vast area including Beijing, Tianjin, Hebei, and even Shandong and Henan.

So far 23 northern cities have issued a red alert -- the highest air pollution warning -- while 17 cities issued the lower-status orange alert. Experts say the smog blanketing northern China stems from a combination of local emissions, unfavourable weather and pollutants wafting in from elsewhere.