Here is a photo of the inside of my evaporative humidifier (Vornado Evap40). Nice and clean, no algae or other ugly things growing in it! I use it non-stop throughout the winter as I heat my house with wood stoves which really causes low humidity issues. I "flipped" the square filters on my humidifier once last winter when one side became hard from mineral buildup and the like. and it hasn't evaporated either. However, I have never thrown away filters because of algae/bacterial problems. After reading other reviews, otherwise you have to throw away the filters every few weeks (really expensive). As I understand it, it's a good money saver and it certainly works.
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