Self-retaining spun glass media. Heavy one-piece chipboard frame. Low initial pressure drop for energy efficiency. Record installation date on the designated space. MERV 1 Lasts up to 30 days. UL Classified. 24 filters per carton. This is the same type of filter that has been protecting furnaces and central air conditioners around the world for over 50 years. The low pressure drop fiberglass media both saves on energy cost and it has higher air flow.