While I want to believe that clear sky means low pollution, sometimes I do find contradictory facts that suggest it might not always be the case.
I noticed that, for almost all my trips to Japan (tokyo/osaka), that usually within 2~3 hours after landing and heading downtown, my nose starts clogging up and I get the urge to dig those stuffs out. I don't have such an issue in Singapore or HK.
Don't ask me why... I don't have an answer.. maybe its because of cold weather ? the high number of smokers ?
I'm good with high humidity and industrial pollution though- my asthma pretty much vanished when I moved to Hong Kong.