Build a do-it-yourself air purifier for about $25 [video]

One data point: my father made a similar filter and was running it constantly. One night the fan inexplicably caught on fire, burned down the living room and almost burned down the house. So either get good quality fan, or spend the money on ceritifed purifier that can run for long hours at a time.

I just wanted to add that you can basically put the filter behind the fan and have it suck air through it instead of taping it to the front of the fan. I bought the Heavy Duty Lasko Box fans (they pull more air) and run 3 of them 24/7/365 along with HEPA Filtration on the HVAC system. What's amazing is how these filters turn black over a month or so from just VOC's floating around from cooking to off gassing of furniture, hard wood, carpets, etc.

Even during allergy season the inside of our house is like a clean room. Every one sleeps better and there is no smell or anything.