As long as you are super careful to clean every last bit of the respirator (like a CPAP) it should be good. I have a friend who found out the hard way when using a half-mask respirator. He got a bad bacterial pneumonia from a dirty unit. Warm humid breath condensate is a breeding ground for all kinds of nasties.

A freshly washed pieced of a cotton sheet and a couple of elastic bands may be just as effective, but way less cool.

Dan