Product review (3 stars): It Works
This cartridge is a necessary evil for your Air-O-Swiss humidifier.  I wish I'd bought a different humidifier that didn't need so many replacement parts, but this particular replacement part is definitely a must-have.  It has little crystals in it that absorb that "mildew-y" smell that humidifiers can get after alot of use.  This just screws on to the large twist on lid that comes with the humidifier....it goes inside the water tank.  Pretty simple, and again, it's a necessary evil so get out your wallet and buy one when the original starts to smell funky.