But after a half hour or so, water vapor trapped in the mask makes it hard to inhale, since a flexible mask pulls itself up against your mouth and allows air through only a small portion of the mask. There's no problem exhaling because the mask puffs out and air passes out through its whole area.
A foam mesh sleeve, as used to protect fruit such as Asian pears, placed inside the mask holds it away from your mouth and ensures that most of the mask area is available all the time. The foam used for fruit seems to be closed cell polyethylene, it doesn't absorb water and can be washed with the mask. It tends to stay in place if the mask is pulled down when not needed, but could be stitched to the mask.
