Instruct: Classify Amazon reviews into positive or negative sentiment
Query: works fine, but mold build-up in warm homes

I liked the idea of invisible mist, and the humidifier worked just fine. However, since I live in an apartment building where I have no control over the heating of my apartment, and my apartment is quite warm, I found that with 7-10 days of use I would get mold build-up on the wicking filter. I could see the mold on the filter, and smelled mold when I operated the humidifier. After going through about three filters in the course of a month, and since those filters are quite expensive (about $8 each), I decided to just get a regular ultrasonic humidifier.