Product review (5 stars): Try putting them into the dishwasher!
I live in NJ and the cold winters require a lot of heating which in turn can create very dry air. Since I started to use a humidifier during these times I am less prone to colds and itchy / dry skin (dry sinuses can make you more susceptible germs). Most frequent reason for me to change filters was that they started to develop spots of mold and a fusty smell. Last week, after nearly 1.5 months of use another filter reached that stage again. Not having a replacement around, I tried and put the filter upright into the dishwasher together with my regular dishes. Result: Funky smell and mold spots were gone, filter back in the humidifier and everything runs as before - money saved!