This really doesn't involve masks, but it is along the same lines. When traveling in airports I refuse to grab the railing on the esculators. If a terrorist wanted to spread chemical/biological agents all they would need to do is apply some to the esculators' rails. I don't know how many people travel through the Atlanta Airport everyday, but millions to be contaminated in no time.

More realistic is the fact the person ahead of me doesn't wash his/her hands after using the restroom.