Aerostar 19 7/8 x 21 1/2 x 1 MERV 8 Pleated Air Filters (Actual Size: 19 7/8 x 21 1/2 x 3/4 ) - 4 Pack. Aerostar filters are a simple and cost effective step towards a healthier home environment free of airborne allergens and particles that can trigger various allergy and asthma symptoms. Made with electrostatically charged synthetic material these MERV 8 (comparable to MPR 600 and FPR 5) filters draw and capture allergens between 3 - 10 microns including lint dust mites spores and pollen. Aerostar air filters also improve the efficiency of your HVAC unit by removing dirt and reducing strain on the motor reducing utility and maintenance costs. For best results change the filter every 60 to 90 days and increase frequency to every 30 days during peak summer and winter months when your unit is working the hardest. Compatible with furnace and air conditioner units and easy to install. (Nominal Size: 19.88 x 21.5 x 1 ; Actual Size: 19.88 x 21.5 x 0.75 )