I bought a mask with an air filter in it that's designed for outside activity in cities like LA. It looks more like a surgeon's mask mixed with the aesthetics of a BMX helmet. But in the end, I never wear it.
1.) It's creepy for me and everyone around me.
2.) It doesn't seal well and therefore isn't even working.
I wonder also, though, about what exposure is like. I've heard it said that pollution levels in cars on the road is a certain number of times higher than the surrounding air. The lesson from that statistic is roll your windows down. But it doesn't stink from exhaust in cars so I don't know if that's a skewed statistic.