I use very often the air washer "Venta LW 15 7015501 Air Purifier (White / Grey)" and this is perfect since it is in large size, which avoids to buy all 4 mornings. I personally do not put much it is advisable to put it.
By cons, impossible to say whether this is really useful or not. I can just say that I am much less sick since I use it and I do not know allergies I had before!