As air moves through a building’s HVAC system, air filters trap and collect large and small particles such as dust, allergens and microorganisms. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), this filtration helps provide healthier indoor air quality. Several methods have been developed by trade groups or organizations to allow comparison of filters. These standards are designed to rate a filter’s particle removal efficiency. Currently, most filters are rated using the Minimum Efficiency Reporting Value (MERV) rating system that has been developed by (ASHRAE). A filter's MERV number indicates how it's rated to remove these particles. But what is MERV rating, exactly?

An air filter’s minimum efficiency reporting value (MERV) rating measures how effectively the filter stops dust and other contaminants from passing through the filter and into the air stream. The MERV rating system ranges from a low of 1 to a high of 20, with each rating indicating the percentage of particles removed within a specific particle size range (measured in microns, where a micron is one millionth of a meter).