Most viruses and viral materials that are exhaled from a person, are exhaled inside of droplets of moisture that the masks often stop. However, stopping some of the viruses this way isn't a certain thing, because many droplets are small enough to go thru masks just like viruses do. In addition, these and other "bugs" that remain in the mask, are often inhaled back into the person, after growing in multitude, thereby affecting him adversely. In addition, the masks themselves often give off toxic substances from their material makeup. Take your pick. Masks aren't any kind of an ideal protection, either in or out.