What does MERV rating stand for? Minimum Efficiency Reporting Value. MERV ratings are a method of determining the efficiency of an air filter used with heating and air conditioning equipment. The efficiency is determined by adding particles of varying sizes (1-12) into a controlled testing environment. The particles are added upstream of the test filter and a laser particle counter samples the air before it enters the filter and after it leaves the filter. The two particle counts are compared to calculate the Particle Size Efficiency of the tested filter. Once this is determined, a MERV Parameters chart is used to determine the MERV rating. Read More »