Air Quality Improvement: Filtration removes up to 99.97% of airborne particles as small as 0.3 microns in size including dust, smoke, pollen, pet hair and dander, and more. Maintenance: Recommended to change this filter once every twelve (12) months under normal conditions, and more often under harsh air conditions. Approximate Measurements: 5 x 10 7/10 x 1 4/5 inches. Replacement Quantity: Each package contains one (1) replacement filter. Filter-Monster branded replacement filter compatible with Whirlpool 1183900 filter. Engineered to fit Whirlpool Tall Tower Air Purifier models APT40010R, APMT2001M. Recommended to replace filter every twelve (12) months for maximum efficiency, or more often in harsh environments. Contains one (1) replacement filter. Approximate Dimension: 5 x 10 7/10 x 1 4/5 inches.