In the United States, ASHRAE standard 52.2: 2007 gives us the requirements for filter efficiency. Efficiency is determined by types of particles, rather than an overall efficiency. This rating is known as MERV—Minimum Efficiency Reporting Value and is in contrast to percent efficiency.

MERV rates both how effective the filter is at stopping particles of a particular size (particle count efficiency) and a particular weight (weight efficiency). This differs from percent efficiency and the European standards (EN), of which there are several (EN 1822:2012 and EN 779:2012).

MERV ratings are on a scale from 1–20 and filter everything from ash and insects at the low end to smoke and metal oxides in the middle to aersoled micro particles at the high end. MERV differs from the EU standards in that they have a higher level of filtration, known as U15–U17, which MERV does not match. Comparisons between these classes of filters are approximate.