[snarf21]

Mask effectiveness is to help prevent you spreading things. When you exhale, water droplets are still quite large and some will get caught on the fibers in even a t-shirt mask. After that has dissipated in the air, it will be smaller and will be breathed in through all but the best masks. There were lots of videos online when this became an issue where people showed how when it is really cold, you can "see" your breath (this is solely caused my warm moist air condensing into a "cloud" in the cold air). Put on even a crappy mask and you can't "see" it anymore. Sure it isn't 100% blocking but clearly is blocking moisture. Also, if you wear a heavy mask for a while, you will notice that it feels damp inside.