Homedics Ultrasonic Humidifier Demineralization Cartridges help prevent calcium & lime build-up in the ultrasonic humidifiers. Reduces mineral build-up & "white-dust" caused by hard water when used in ultrasonic humidifiers; Works with all Homediccs ultrasonic humidifiers (1 gallon & larger); Simply drop one cartridge into water tank; Replace cartridge every 30-40 filings, depending on water hardness; Includes 4 cartridges; Countertop display merchandiser (holds 6 color boxes)