Assuming you're running the filter on a fan and it's an enclosed space, unless you have a matching intake fan you'll be creating negative pressure in the space. So pretty much all the air flow will go out through the filter wherever it's placed as far as smell mitigation due to the vacume it creates. They're usually put at the top of the space above everything else since heat rises and even with good LED's or floursecents, there will still be heat. Putting the fan/filter near the top of the space will help cycle the most heat out of the space.