If you have your mask hanging from your rear-view mirror, you may want to find a different spot to put it. Hanging anything from your rear-view mirror is a safety hazard, and against the law in the state of Pennsylvania.
Anything that you hang from your rear view mirror no matter the size can potentially block your view of something major further down the road such as another driver, a pedestrian, an animal, or a biker. Mounting things to the windshield is also a safety hazard that can obstruct your view.
Penalties such as fines and court costs can come from violations of this law.