Hamilton Beach True Air Replacement Blue Filter for 05520 and 05521 humidifiers (05920) Replacement filter for Hamilton Beach True Air 05520 and 05521 humidifiers Filter replaces quickly with drop in installation; accommodates a 4-gallon steam output Best used in rooms up to 1200 square feet in size; change filters every three months and clean humidifier weekly to prevent mold and germs Filter measures 11.1 by 8.8 by 3.1 inches