Polyethylene polypropylene honeycomb air filtration media. Rolled, and folded in half. Then put into a plastic bag. This filter is magic at capturing dust and other particles. In fact it'll trap and eliminate 80% to 95% of particles 5 microns or larger. Perfect for use on return air filters. How does it work? As air flows over the woven honeycomb, the Polypropylene creates an electrostatic charge. That charge, which does not diminish over time, traps dirt and particles like a magnet. To clean, simply wash and air dry. The size you are purchasing is 26” x 48”. Cut to any size you wish with a scissor or utility knife. Do you have a boat or marine air conditioner? This filter is the same material used by the marine manufacturers although it is lighter. Marine air conditioner filters typically use a 12mil (.012") thread diameter, which makes for a stiffer filter. For use in vent hoods, air conditioners, condensers and heaters. Replaces that foam filter on your mobile home and RV furnace, blowers and vents. Replaces most all thin filters. Great for return air vents. 40x40 mesh aperature or 380 microns.