3M 8000 is pretty cheap and meets the standard: .3 micron (histo ~2 micron). If you aren't sporting a beard it would work fine.
http://www.allergybegone.com/3m-8000-respirator-mask.html
Avoiding the dust (breathing any concentrated amounts of particulates) is definitely the most useful purpose of face masks.
If decreasing the chances of disease transmission is the primary goal, and you are thinking like an actuary, then I'd suggest wearing your masks to Wal Mart, in the airport, and to school (much higher exposure to disease vectors than elsewhere).
http://www.allergybegone.com/3m-8000-respirator-mask.html
Avoiding the dust (breathing any concentrated amounts of particulates) is definitely the most useful purpose of face masks.
If decreasing the chances of disease transmission is the primary goal, and you are thinking like an actuary, then I'd suggest wearing your masks to Wal Mart, in the airport, and to school (much higher exposure to disease vectors than elsewhere).