This is an issue for several reasons. I will only address the mechanical. The mask is a filter. When you wear the mask, what’s on the inside stays on the inside, and what’s on the outside stays on the outside. That’s the hope anyway.
Now, no matter which side we are talking about, when air is drawn into the mask, particulate matter sticks to the mask. When the air flow reverses, some particulates dislodge and enter the air again. On the inside of the mask, this is your crap. You breathe it out, then you breathe it back in. But, that’s okay. It’s your crap. It won’t hurt you and because it’s trapped inside, more importantly, it will not hurt others. So, if you’re sick WEAR A MASK!
Now, on the outside of the mask, the same thing occurs, only this time, it’s everybody else’s crap. If there is something harmful in the air on the outside of the mask, you draw it to the mask when you breathe in, and SPREAD it around every time you breathe out. So, if you come into contact with an infected person (old flattop) and his particulates stick to the outside of the mask, then every time you breathe out, you spread his disease back at him… and everyone else. Hello Typhoid Mary! This is why doctors change masks BETWEEN patients. Impractical for you.
What to do? Change masks often. If it’s disposable, throw it away. If it is cloth and reuseable, soak in a bleach solution before laundering. Most importantly, MARK THE INSIDE, AND NEVER EVER WEAR YOUR UNDERWEAR INSIDE OUT!!! Even if clean.
If you take it off DON’T put it back on!