We don't have central air or heating. We do have a hepa based air purifier in the house. Still, a way was needed to filter the air entering the house. That way you take the load off the rather expensive hepa filter, plus you breathe better air even if it hasn't hit the hepa filter yet. I ended up designing an inexpensive 4-part filter system that attaches to a window and pushes filtered air throughout the house.The system consists of a replacement screening material that filters out dust, pollen, etc. (PollenTec¨ Clean Air Window Screen (21" x 10`)), this carbon filter, a furnace filter (15x25x4 Exact MERV 15 AC Furnace Filters Qty 2), and a standard 20" fan (Lasko #3720 20" Weather-Shield Performance Box Fan).The idea here is that the PollenTec screen is the first line of defense, trapping dirt, dust, pollen, etc., before these particles even enter the house and hit the main filter. The carbon filter is attached to the furnace filter such that air flows through the carbon filter first, and then through the furnace filter. The Nordic Pure furnace filter I used is the closest thing I could find to a hepa filter. These furnace filters are rated by the size of the particles they trap, known as the MERV (Minimum Efficiency Reporting Value). My research showed that a MERV rating of 13 would be quite effective, but I found a MERV 15 filter on Amazon and that captures even more bad guys (the higher you go in MERV however, the more the fan has to work to pull the air through). Furnace filters don't trap gases though -such as the fumes from a campfire- so that's where the carbon filter comes in. The fan is the final piece of the puzzle, which is set in place so that it draws in outside air through the three filters.To begin, I removed the existing screen from the window the rig would be placed in and installed the PollenTec screening in its place. If you have ever replaced the screen on a window or screen door before you know what this entails. It's not too hard a job, and having one of those little rollers (New York Wire 90611 2-Wheel Wood Handle Screen Tool) that push the spline into the groove that holds the screen is a big help. I didn't have have such a tool handy so I used a mini screwdriver. But that's just me. I recommend you get the tool because either tools will fly or tears will flow if you get the new screen 90% installed and then slip with the screwdriver and tear a whole in your brand new screen (I of course have never experienced either reaction, but I hear its possible).Next I attached the carbon filter to the furnace filter. I just cut the carbon filter to the size of the boxed furnace filter and used masking tape to fasten it in place. Worked like a charm.I then placed the 2-piece filter setup against the window opening and taped it in place. You can get really fancy here about how you mount the filter to the opening, but I found that again, masking tape was adequate for my situation (see photo). You want to choose a furnace filter that best fits the opening, and then be prepared to use cardboard, rubber, or whatever to seal any gaps.Now you have to attach the fan to the filter. The particulars of the window chosen of course dictate how much effort is needed to do this. One method would be to tape the 2-piece filter assembly right to the fan, then insert the fan in the window. What you need to achieve is a fairly tight seal between the fan and the filter so that when the fan is on it pulls air exclusively from outside the building. Because of the hi MERV rating the fan MUST make a good seal to the furnace filter in order to suck air through it.The entire job took about three hours and cost well under $200. Compare that to having a professional solution installed. Did it work? Yes! The only modification from here is to hook up an AC rheostat to reduce the fan speed further, and thus the noise. My theory is that as long as you are getting air flow you are producing positive pressure at this point in the house, and thus forcing out old air through other openings.