UpStart Battery 8-Pack Replacement Holmes 3641 Humidifier Filter - Compatible Holmes HWF75, HWF72 Air Filter Replacement Holmes 3641 Humidifier Filter - Quantity, 8 Replaces Holmes HWF75, HWF72 Air Filter Traps Water Impurities & Dissolved Solids Before They Become Airborne For best results, change air filter at least once per season or more often depending on air quality and use. Essential for Allergy Sufferers. On Sale for a Limited Time!