I have been using these for about 15 years now and love them. You would be amazed at how much stuff you can see trapped in one even after just a month. It will remove pollen, and even smoke and mold from the air. I would suggest getting a few for a season, especially if you dodn't have central air and are running the heat for the first time in the fall with a filter this fine. You will probably end up changing the first one after a month, then ever 3 after that unless there are constant pollutants in your house, like a lot of smoke or a mold problem.
Filter says 16x25x4 but it is actually 4.25 thick! Does not fit in my furnace which is exactly 4.0" This is an extremely frustrating problem and every store carries the new 4.25 filter and apparently nobody makes or carries the old 3.75's anymore. I could cram a 4" in there but not one thats 4.25. Trust me, I tried.