MERV stands for Minimum Efficiency Reporting Value. MERV ratings are an air filter industry standard as dictated by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) under standard 52.2. The higher the MERV rating, the more particles are captured as well as the ability to filter smaller and smaller particles. Most residential furnace filters range from MERV 4 to 13, but can go up as high as MERV 16 for some specialty filters.
While higher MERV ratings are better at filtering, air flow can be reduced slightly. MERV ratings deal with efficiency and particle sizes, and do not indicate the air flow rate of the filter. Most modern HVAC equipment can easily deal with higher MERV ratings because a good quality air filter will still allow for adequate air flow for the equipment. For the most part, filters with an initial resistance of 0.25" W.G. and a final resistance of 1.0" W.G. (this is referred to as static pressure) is perfectly acceptable.
Particle removal efficiency can be broken down in to 3 different ranges. Range 3: Large particle sizes of 3.0-10.0 microns and is the qualifier for MERV 4 to MERV 7. Range 2: Particle sizes of 1.0 to 3.0 is used as a qualifier for MERV 8 to MERV 12. Range 1: Particle sizes 0.3 to 1.0 and is used as a qualifier for MERV 13 to MERV 16. Filters rated MERV 4 to MERV 7 only have to meet the minimum requirements for Range 3 particles. Filters rated MERV 8 to MERV 12 must meet the minimum requirements for Range 2 and Range 3. Filters rated MERV 13 to MERV 16 must meet the minimum requirements for all 3 ranges.