Just bought (4) units on behalf of our daughter-in-law, who has allergies, for our home with three cats. We noticed a difference right away, and haven't been sick since we fired them up. Wow....we literally didn't know what we were missing. Quality of units far surpasses what you could find in the Big-Box stores, and the filters themselves are truly well engineered & effective. We should have done this years ago. Thank you, Alen!