MERV, or Minimum Efficiency Reporting Value, is a number from 1 to 16 that is relative to an air filter’s efficiency. The higher the MERV, the more efficient the air filter is at removing particles. At the lower end of the efficiency spectrum a fiberglass panel filter may have a MERV of 4 or 5. At the higher end, a MERV 14 filter is typically the filter of choice for critical areas of a hospital (to prevent transfer of bacteria and infectious diseases). Higher MERV filters are also capable of removing higher quantities of extremely small contaminant (particles as small as 1/300 the diameter of a human hair). A higher MERV creates more resistance to airflow because the filter media becomes denser as efficiency increases. For the cleanest air, a user should select the highest MERV filter that their unit is capable of forcing air through based on the limit of the unit’s fan power.
Most filters become more efficient as the filter is used in the system. Care should be taken when considering filters that incorporate an electrostatic charge. Although offering a reasonable MERV value these filters will actually drop in efficiency as the filter loads with contaminant. A number of electrostatic filters are presently being offered for residential as well as the commercial/industrial applications. Refer to the filter packaging or literature to determine if the filter you are purchasing relies on an electrostatic charge to boost its MERV. If it does, request that the manufacturer provide you with the “discharged MERV” for the filter to determine its actual in-use performance.