I can always tell when it is time to change out the furnace/AC filter in my house. The house starts to smell dusty and we sneeze more in the spring with a dirty filter. I know that these filters work- they are the only type that I buy for my home.I don't worrry about pet dander, dust, or pollen when I have a Filtrete filter in place. I know that when any of these filters are in place, my home is a much sweeter place to live.