Fits Holmes, Sunbeam, and Bionaire Humidifiers requiring Filter "D" Improves efficiency of the humidifier Designed to inhibit the growth of bacteria, mold and mildew Enhanced with Arm & Hammer baking soda and antimicrobial treatment Holmes Filter Fits: Holmes: HM-3500, 3600, 3640, 3650, 3655-White Westinghouse: WWHM3300, WWHM3600 Why you Need a Humidifier Filter Evaporative cool mist humidifiers cannot run without the use of a wick filter. Replacing your Humidifier Filter The harder the water, the shorter the filter life. This is because minerals will get trapped in the filter over time and cause the filter to lose its ability to wick. We recommend you replace your filter every 4-6 weeks, depending on usage and hardness of water. In some evaporative cool mist humidifiers we have included a Filter Check indicator that will indicate when the filter needs changing. Removes Household Odors The Holmes Wick Humidifier Filter includes a unique feature, an exclusive layer of ARM & HAMMER Baking Soda which helps remove household odors. View larger Enhance your Home Environment A perfect solution to enhance your home environment, personal comfort and well-being. The Holmes HWF75CS (D) Wick Humidifier Filter transforms dry, damaging air with a balance of cleaner moisture and gentle restoration Helps Control Dust and Dirt This triple layer humidifier filter helps control dust and dirt build-up and helps prevent impurities from spreading by passing air through the filter.