Filtration is rated in increasing efficiency

Minimum Efficiency Reporting Value, commonly know as MERV rating is measurement scale designed in 1987 by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) to rate the effectiveness of air filters. For Example, a HEPA filter is often impractical in central HVAC systems due to the large pressure drop the dense filter materials causes. Experiments indicate that less obstructive medium-efficiency filters of MERV 7 to 13 are almost as effective as true HEPA filters at removing allergens, with much lower associated system and operating costs.

The 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. The MERV rating is from 1 to 16. Higher MERV ratings correspond to a greater percentage of particles captured on each pass, with a MERV 16 filter capturing more than 95% of particles over the full range.