MERV rating or, Minimum Efficiency Reporting Value, is a measurement scale used to report the effectiveness of an air filter at capturing particles of a certain size. The higher the MERV number, the more efficient the air filter is at filtering particles of a known size.
A MERV 13 filter, for instance, is capable of filtering out particles as small as 0.3 microns in diameter. In comparison to a MERV 8 rated filter which is only capable of filtering out particles as small as 3.0 microns in diameter.
Before purchasing and installing a high-efficiency filter it is first important to check the maximum MERV rating of your HVAC system. Higher MERV rated filters reduce air flow to the system which may cause damage or force the HVAC system to shut down.