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..
I built this for my cats. There is a 120mm computer fan and a motion detector set to run the fan for 5-minutes when the cat enters the box. Kitties shouldn't have to take a #2 without fresh air, should they?