In reply to Bjartur i Sumarhus:
Never worn one cycling, but having done a lot of DIY with dust masks on, I'd hate to be doing something as aerobic as cycling with one on - as Gethin said, it's bad enough cycling with a Buff pulled up in the cold.
I agree with you about some, mainly diesel, cars, though - if I see such a car, I try and hold my breath for as long as possible until they've gone past.
That said, they're probably quite a good trainer for your lung muscles!