I've found that masks don't work well and scare little children and old people with your darth vader/bank robber look. Best options are to plan routes through side streets that avoid most traffic or, quite literaly, suck it up. It totally bothers me too, though. The worst is when you're behind a truck or car that is just spewing out exhaust and you can't go fast enough to get past them (always meeting up at lights and stop signs) and they won't go fast enough to leave you behind to enjoy clean air. Sometimes I just pull over and wait for a couple minutes to give those cars more distance.