I though the pollution was worse a few years ago, and with the world recession output may well have reduced, as well as the opening up of other areas of manufacture, away from Shenzen, etc., but I'd like to see stats on that. I remember when it was clear, and the pollution was minimal (80s and much of the 90s) and changed in the space of a few years as Zuhai SEZ and Shenzen started to boom incredibly.
The pollution varies, in the winter though the climate is them comfortable, the prevailing wind changes,and brings more pollution, and during the summer, by contrast, it is hotter though the amount of pollution coming from the north is lower, however ozone is a problem due to the heating of exhaust fumes in the inner-city areas.