4" ducting, 120 CFM fan vented to the outside with a standard 4" dryer vent, cycles fresh air into the boxes all the time. The boxes get cleaned weekly but you never get the "OMG Cat, wtf did you eat!?!?!?!!" moments when they take a dump. It also makes cleaning the boxes more plesent. The clumping cat litter works much better this way. Cats are healthy, happy and don't mind the fan noise..