This has bugged me for years. The classroom I teach in has a photo lab and a kiln room, each with a motorized exhaust fan. I have asked custodians, administration, and the health&safety specialists we have a contract with. No one can tell me how to measure efficiency or even what the efficiency should be for these rooms and they don’t seem concerned. I’m guessing the room’s air should be replaced (x) times per hour and that rate might vary depending on use. Any insight appreciated.