They definitely aren't designed to be washed. The seller (who is charging over the odds) has copped out by saying "some customers have told us they can be washed."
You'd think a supplier of medical supplies would know.
In fact there is a whole raft of research going on worldwide (now) to work...
But unless you used to putting one on and using it, you end up touching the area around your face more adjusting the mask. It also doesn't protect your eyes.
Lots of people wearing them in London, just fiddling with the things.
The only case for masks is they can help if the person wearing one...