The heavy-duty spun glass is a continuous filament spun glass. The frame is made with 1 continuous piece of chipboard to guard against corner separation. A tough metal media retainer is adhered to the frame on the upstream side for extra filter strength (one-sided metal reinforcement, on air exit side). Maximum air flow. Alleviates strain on equipment. Filters pollen, pet dander, dust/lint, dust mites. For peak equipment protection and energy efficiency, these filters should be changed monthly in periods of high use. UL Class II.