Aerostar 21 1/2x23 5/16x1 MERV 8 Pleated Air Filters (Actual Size: 21 1/2" x 23 5/16" x 3/4") - 6 Pack. MERV 8 filters are comparable to MPR 600 and FPR 5. Designed for superior filtration efficiency, these filters are made with electrostatically charged synthetic material that draws and captures allergens between 3 - 10 microns, including lint, dust mites, spores, and pollen. 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. They also improve the efficiency of your HVAC unit and therefore reduce maintenance and utility costs, by removing dirt and reducing strain on the motor. For best results, change the filter every 60 to 90 days but increase frequency to 30 days during peak summer and winter months when your unit is working the hardest. Compatible with furnaces and AC units.