Air filters MERV rating
Before the CADR was developed, filters were rated by a standardized test—in the U.S., this test was known as ASHRAE Standard 52.1 (ASHRAE 1992). A test dust was introduced to the filter at a certain spot and the amount of dust intercepted was measured by the color of the spot (the Dust Spot rating, on a percentage scale from <20% to > 95%). A second test measured the fraction (by weight) of the test dust that came through the filter after a given amount of exposure time (the Arrestance rating, also on a percentage scale, from <65% to >98%).
However, these tests had major deficiencies. First, the test dust was not very similar to the type of dust that is found in modern-day outdoor air. Secondly, measuring by weight of the dust trapped gave high values for larger particles (which weigh more)—yet research indicates that the smaller particles, which can penetrate deep into the lung, may be more harmful. Thirdly, these tests did not provide an indication of how well the filter worked for different sizes of particles, since only a total amount was considered.
Fourthly, the tests did not have some of the practical problems of installing filters in homes, such as fitting the filter tightly into its assigned space to eliminate bypass. Fifthly, these short-term tests gave no indication of how the filter would perform over an extended period of time.
Therefore ASHRAE sponsored research to get a better idea of how filters actually worked with different sizes and compositions of particles (Hanley 1993a,b). In general terms, there are two processes at work: diffusion and interception/impaction. In diffusion, or Brownian motion, the particle moves in random directions as it is hit by molecules of air. The smaller the particle, the farther and faster it will move in these random directions, and the more likely it is to hit and be trapped by a fiber on the filter.
A large particle has more inertia, and as the airstream changes direction to get through the cross-hatchings of the filter, the large particle keeps right on going and impacts (or is intercepted by) the fiber. Therefore in very general terms, it must be that filters are most effective for the smallest and largest particles, and will reach a minimum at some intermediate size. This size turned out to be in the neighborhood of 0.1-0.5 µm .
With this knowledge, it was realized that a useful indicator of the efficiency of a filter would be its efficiency at this minimum level. Therefore ASHRAE Standard 52.2 was developed, and ranks filters according to their minimum efficiencies (MERV, for Minimum Efficiency Rated Value) (ASHRAE , 2007). There are 20 categories within the MERV appellation, with MERV-1 being the least and MERV-20 the most efficient. ASHRAE Standard 52.2 did not replace Standard 52.1, so that filter ratings may use either the newer MERV scale or the older ASHRAE Dust Spot Efficiency and/or Arrestance ratings.
The relationship between these three systems is shown in Table 7. The MERV 17-20 ratings apply to HEPA and ultra-low penetration air (ULPA) filters used in clean rooms, hospital surgery rooms, laboratories dealing with radioactive materials, etc. MERV 13 -16 rated filters are also used mainly in commercial locations requiring very low particle levels, such as hospital inpatient rooms, general surgery, smoking lounges, and high-end commercial buildings.
Air filters MERV rating
MERV ratings from ASHRAE Standard 52.2 compared to the ASHRAE Standard 52.1 Dust Spot Efficiency and Arrestance Tests