A couple of tips for riding in urban/polluted areas:
1. Try to breathe through your nose rather than taking large breaths through your mouth. There are certain "filters" in your nose that will keep some of the particles out of your lungs.
2. If you're commuting and it's a ride you'll be doing regularly, look for a route that uses less trafficked streets.
3. If there is a particular vehicle (a bus, garbage truck etc) in front of you that is spewing out an unusually high amount of smog, slow down and let it get ahead of you. There's no shame in riding like a "granny" for a few hundred metres in order to protect your lungs.
4. Don't let it prevent you from riding. I've seen quite a few studies on this, and they tend to indicate that the level of pollution is much higher inside a car than outside a car. Basically if you commute by bike, you'll be getting a considerably higher quality of air, and that can't be a bad thing.
Re: the mucus issue. I think it probably has more to do with colder weather than with pollution. I tend to have more of it in "winter" than in summer, and our winters aren't even real winters. It should also be noted that our summers generally have heavier traffic as our population triples during the tourist season.
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I am clinically insane. I am proud of it.
That is all.