MERV ratings are 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.
MERV ratings range from 1 - 16 and measurements are in microns. Some of the common particles related to MERV ratings are pet dander, insecticide dust, smog, dust, viruses, wood, tobacco smoke, spores, bacteria and pollen.
Some of the most common filters found in residential use only have a merv rating of 1 to 4. These are typically disposable panel type filters and do not do a good job of filtering the air because they will not stop particles smaller than 10 microns.
MERV 5 to 8 rated filters are a better choice and are more commonly found in commercial applications. These filters will collect particles as small as 3 microns and are a good choice for home owners also.
MERV 8 is more efficient when compared to standard pleated filters and has a high dust holding capacity that does not sacrifice air flow.