"The bacterial colony numbers were greater on the face-side than the outer-side; the fungal colony numbers were fewer on the face-side than the outer-side. A longer mask usage significantly increased the fungal colony numbers but not the bacterial colony numbers. Although most identified microbes were non-pathogenic in humans;
...
We propose that immunocompromised people should avoid repeated use of masks to prevent microbial infection."
Sprich: Risikopatienten sollten die Masken öfter wechseln.