I try and replace the dust filter on my indoor air handler every two months. I think they recommend every month but I don't usually see enough dust on it when I check it every month. Every two months show some, not filthy, but that is when I change it. Sometimes I see small black spots that I assume are mold spores. I don't have a mold issue in this house and our region (Central VA) isn't known for mold issues in houses. Just curious if others see mold on their air filters. Maybe I went longer than 2 months this past time? It wasn't very dirty but I could see some dust on it. Humidity in my house has been low (55 - 60%) the last few months too.