Electrostatic air filters are better at capturing airborne particulates than other types of filters. As air passes over the the polypropylene fibers in electrostatic filters, it causes a static electrical charge around those fibers that attracts and hold particulates until washed off. These 7/16″ and 1/2″ thick electrostatic filters have 2 inner layers of fine mesh polypropylene netting between 2 outer layers of expanded aluminum face grid and are enclosed in a one piece aluminum frame. Electrostatic filters are higher efficiency than metal mesh filters and are washable and durable. GOOD FILTER COMPANY custom size filters come in sizes up to 2000 square inches.