I have 2 coway units in a ~800sqft apartment and still have tons of dust but I open the windows a lot when its not smoky and I have a dog so who knows. The pre-

I have 2 coway units in a ~800sqft apartment and still have tons of dust but I open the windows a lot when its not smoky and I have a dog so who knows. The pre-filter does trap a lot of dust so there is obviously some (maybe minor) reduction.