If you're like me, you wind up spending a lot of time rehearsing in basements and windowless rooms. I've started keeping dryer sheets (the fabric softener ones) in my kit, so that when I move into a rehearsal space that's musty or moldy I can tape a dryer sheet over the source of incoming air. It helps freshen the air in the whole room, and the fibers of the dryer sheet filter out some of the nasty airborne particles that can wreck up actors' voices.
