The Flanders NaturalAire Floor Register Air Filter helps prevent dirt from entering your home's ductwork and reduces airborne particles. The MERV (Minimum Efficiency Reporting Value) 4 rating helps reduce dust, pollen, and lint from your home or office air space. Each air filter has a suggested lifespan of up to 90 days. NaturalAire Floor Register Air Filters can easily be cut to fit almost any register measuring up to 4 inches by 12 inches.