Minimum Efficiency Reporting Value, commonly known as MERV, is a measurement scale designed in 1987 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to report the effectiveness of air filters in more detail than other ratings.
For example, often a high-efficiency particulate air (HEPA) filter is impractical in residential central heating, ventilation, and air conditioning (HVAC) systems due to the large pressure drop the dense filter material causes.
[1] The MERV scale is designed to represent the worst-case performance of a filter when dealing with particles in the range of 0.3 to 10 micrometers.
[2] The 3M scale is known as microparticle performance rating (MPR) and measures the ability to filter particles 0.3 to 1 microns.
[3] Although the standards measure slightly different attributes and thus, are not strictly convertible, one retailer reported the following typical equivalents:[3]