I'm sure, and moisture is a big no-no (I throw them away if there's significant moisture on them, like if I went out and it rains), but all in all a mask made to be worn 8 hours will still work fine if you use it in small increments of time. Also found those recommendations by the CDC that suggest rotating as I described above - and that's for hospital use, apparently. Though they do suggest a 72 hours downtime instead of my 48 hours suggestion.