China is experiencing its "worst" smog since autumn this year. This time, it may last up six days, and affect 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 this year stems from a combination of local emissions, unfavourable weather and pollutants wafting in from elsewhere.

So far 23 northern cities have issued a red alert -- the highest air pollution warning -- while 17 cities issued the lower-status orange alert.