I pack N95 masks in my briefcase PSK and in my BOB, cars and family survival box. From a practical viewpoint for me, anything larger than a N95 mask becomes difficult to pack particularly in a EDC. As one poster pointed out, one of the most important functions is to keep your hands away from your nose and mouth. The airborne particles are important too but I believe the N95 will go a long way to block those as well.

I was overseas during the 2009 swine flu scare and spent about a half day searching the small French town I was in for N95 masks. I swore I wouldn't travel without one again. Spending time in an airport during an influenza scare is not very comfortable. That would likely be ground zero for contagion entry to a country.