Hitup Home Depot, all the local ones out here got 3m respirators for 30 bills. Comes with a set of cartridges for the mask, and a set of extra filters. Good deal. Respirators serve two purposes, one the obvious, two it serves as a good way to mask your face from the eye in the sky etc. Go out for the gusto and keep it raw!
I say you use one when you can, if you are wearing one, don't wear one around people... if you are out in the middle of east bumfuck, rock it, especially around cameras. A lot of the cheepos and five fingers will be up in your grille in a low tunnel or highway area... I know out here it don't cirulate too well out on the highways, especially if you got two of your crew next to you and it's windy that night. Just use your brain.