The Flanders Flat Panel EZ flow is a standard grade furnace filter of the same type that has been protecting furnaces and central air conditioners around the world for over 50 years. The filtering medium is continuous filament spun glass. The frame is made with one continuous piece of chipboard to guard against corner separation and is strengthened by a metal retainer. For peak equipment protection and energy efficiency, these filters should be changed monthly in periods of high use. This filter is effective against the following contaminants: household dust, as well as debris and lint. Lasts up to 30 days. UL Class II rated. Protects heating and air conditioning equipment. Sturdy chipboard frame and metal retainer contributes to filter strength. Available in numerous sizes, and comes in a pack of 12.