Air filters perform an important function in commercial and institutional facilities. Because indoor air typically is two to five times more polluted than outdoor air, air filters are needed to remove respirable particles such as microorganisms, dust and allergens from the breathing air. In fact, air filters provide the primary defense for building occupants and HVAC equipment against indoor air pollutants. The extent to which filters remove airborne particles is referred to as filtration efficiency. When it comes to evaluating filtration efficiency, many people turn to the minimum efficiency reporting value, or MERV. MERV is assigned to filters based on their minimum fractional particle size efficiency, as determined under the ASHRAE 52.2 Standard. Although MERV does provide some basic information for evaluating filter performance, there is a more complete way to compare the filtration efficiencies of air filters: by reviewing the efficiency values that are included in the ASHRAE 52.2. test report. In addition to the performance factors measured under the ASHRAE 52.2 Test Standard, one may consider variables such as energy efficiency, moisture resistance and temperature limitations, and sustainability when selecting a filter.