An ordinary facemask such as this one issued by a local hospital might seem good for your health.
But these facemasks are made of fine fibers, generally plastic. With a small amount of use, these fibers start coming loose and can be inhaled through your mouth or nose and into your lungs.
In this picture where we placed an led flashlight next to a lightly used hospital patient mask.
We’re not fiber or mask scientists, but we suspect these fibers don’t bio-degrade in your lungs. This certainly can’t be good for your lungs. Maybe they’ll be coughed up, maybe just lie there for years, or maybe cause some side effect such as asbestos fibers did. Or they might inflame your lung tissue and make it less effect in protecting you from infections such as a virus or bacteria.
Masks are hard to come by these days, but if you encounter these fibers when you use your mask, perhaps it might be time to switch. As the say “Be Safe!”.