This rigidly constructed air filter maintains its integrity where others fall short. Always the highest quality, Glasfloss disposable panel filters have metal grids on both the air entry and the air leaving sides. The standard size filter element shall be constructed of a single piece of heavy-duty, one piece craftboard frame. The metal grid shall be thermally bonded internally to the frame. The metal grid shall be 28 gauge and provide 82% open area. All corners shall be closed and sealed. The filter media shall be constructed in a progressively dense fashion from air-entering to air-leaving side. The PolyStrand® filter media shall be made from fibers completely coated with a polyester coating. Glasfloss disposable panel filters shall be rated to withstand temperatures up to 180 degrees Fahrenheit. All filters shall be Classified under U.L. std. 900.