These air screens are excellent. I live in a densely populated city so dirt is mixed with pollen and other particulates. I've had a screen in place for more than a month and as another reviewer commented, the interior windowsill remains clear -- great evidence of the filtering that the screen is providing. I'm highly allergic to specific trees but this year I can sleep with my bedroom open because of this wonderful filter that fits snuggly under my window. I had not found another manufacturer that produced an 8 inch high filter (most are much higher), but this lower one is perfect for allowing fresh air at night as well. Additionally, the ease with which the filters can be washed off under running water is another plus. I can wholeheartedly recommend purchasing this fine product if you want to breathe easier!