MERV Rating

The MERV (Minimum Efficiency Reporting Value) rating of a filter describes the size of the holes in the filter that allow air to pass through.  The higher the MERV rating, the smaller the holes in the filter, the higher the efficiency.”

This definition is incorrect on so many levels that it is difficult to know where to begin.  First, the MERV is not a rating.  The assumption that the higher the MERV, the better the filter, is wrong. MERV is intended to enable users to select filters based on their ability to remove target particles.

Secondly, the reference in the definition to the holes in the filter refers to the filtration mechanism called “straining.” This is where the particles are captured by the filter because they are too large to pass through the holes in the filter.  In fact, only about one percent of the particles captured by air filters are the result of straining.  The other mechanisms of filtration including impingement, interception, diffusion and, in some cases, electrostatic attraction account for the remaining 99%.  In other words, the definition not only is wrong about the meaning of MERV, but it also lacks a basic understanding of how air filters actually work.

So, let’s assume nothing and review the test method by which a MERV is determined.  For those of you who know this ASHRAE Standard, please bear with me.  For those of you who don’t, here is your opportunity to understand what these air filter MERV numbers really mean.

For more in depth information about air filter testing and all other aspects of air filtration the best resource is the NAFA Guide to Air Filtration – Fourth Edition published by and available from the National Air Filtration Association (www.nafahq.org).  This helpful and comprehensive book includes chapters on the principles of air flow, air pressure and air filtration, the types of air filters, air cleaners, filter testing, procedures for dealing with controlled environments, avoiding molecular contaminants, indoor air quality, calculating owning and operating costs of air filters, UV lights and photocatalytic oxidation (PCO). 

Air Filter Testing Standards

The testing standards for air filters have been developed by committees of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).  For many years tests for air filters were done according to ANSI/ASHRAE Standard 52.1-1992, Gravimetric and Dust Spot Procedures for Testing Air Cleaning Devices Used in General Ventilation for Removing Particulate Matter. This test produces an average atmospheric dust spot efficiency expressed in terms such as a 30%, 65%, 90%, etc., arrestance (the percentage of the test dust captured by the filter by weight), dust holding capacity and pressure drop measured in inches of water gauge (w.g.) or water column (w.c.).   The test dust used is a combination of cotton linters, carbon black and Arizona Road Dust.

The ANSI/ASHRAE 52.1 Standard is relatively easy to understand. However, it has some major deficiencies in that it does not provide a very good guide for selecting filters for particular contaminants. To do that effectively you need to know two things – one, the particle size of the contaminant you are trying to remove and two, the level of efficiency of the filter on that particle size. Thus, the ANSI/ASHRAE Standard 52.2 – 1999, Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size was developed to provide this data.

The ANSI/ASHRAE 52.2 Standard emphasizes the standardization and calibration of test components.  The testing apparatus must pass 13 qualification tests before filter testing begins.  Theoretically, this should ensure that a filter tested by one ASHRAE 52.2 laboratory should have the same results as it would in any other correctly operating laboratory.

Obtaining the Data

An air filter’s performance is determined by measuring the particle counts upstream and downstream of the filter being tested. Particle counts are taken over the range of particles six times. One begins with a clean filter, measures the particle size efficiency counts, loads or “conditions” the filter with ASHRAE Standard 52.1 synthetic test dust and measures the particle efficiency counts again.  This loading with test dust and measuring is done 4 more times.

The particle generator creates particles of a known size in the air stream. The objective is to create particles of sufficient numbers to obtain meaningful counts in all the measured particle ranges.  These particle size ranges are listed in Table 1.

This procedure results in efficiency numbers for the 12 particle size ranges for each of the 6 cycles. Efficiency is measured as the percentage of particles captured by the filter. The lowest of the 6 readings is then taken to determine the Composite Minimum Efficiency Curve.

The efficiency numbers in the twelve size ranges are then placed in three larger groups (E1, E2, E3) and the percentages in each group are averaged. This percentage is the Average Particle Size Efficiency, and these are used to determine the Minimum Efficiency Reporting Value (MERV).

Table 2 gives a breakdown of MERV parameters.  When using this table, you move up each Group until you arrive at the highest true statement. This will correspond with a number that represents the Minimum Efficiency Reporting Value (MERV) for the filter.

For lower efficiency filters i.e., those with MERV of 4 or less, the above table has been amended to add a test for Average Arrestance.  Arrestance is the % of particles BY WEIGHT captured by the filter.  Since the large particles represent most of the weight captured by a filter, arrestance numbers can be quite high – even for relatively inefficient filters.  This information is covered in Addendum b to the Standard.  Addendum b also specifies that filters with an average arrestance of at least 65% are a MERV 1.  If the Average Arrestance is tested between 65% and 70%, they are a MERV 2.  If the Average Arrestance is between 70% and 75%, they are a MERV 3.  If the Average Arrestance is greater than 75%, they are a MERV 4.  As a practical matter, most of the filters tested in this category are MERV 4.

Another important consideration is the speed of the airflow during the test.  Table 3 provides the seven acceptable airflow speeds.

The Minimum Efficiency Reporting Value (MERV) must be stated with the speed of the airflow at which the filter was tested.  Most commercial filters are tested at 492 fpm (2.5 m/s).  Certain types of filters perform differently at different airflow velocities.

Let’s go over a few examples just to make sure you understand how a MERV is calculated. (All test results were obtained at 492 fpm.)

Example 1: Filter tests on a pleated filter give us the following Average Particle Size Efficiencies- E1=16.7, E2=44.7, E3=66.1 

What is the MERV Number?  Answer: MERV 7

Example 2: Filter tests on another pleated filter give us the following Average Particle Size efficiencies – E1=28.4, E2= 66.9, E3=85.9 

What is the MERV Number? Answer: MERV 11

Example 3: Filter tests on a bag filter give us the following Average Particle Size efficiencies – E1=61.0, E2=90.8, E3=99.1 

What is the MERV Number?  Answer: MERV13

One must admit that this Standard provides an ingenious way of evaluating the particle efficiency of an air filter.  The single number is an easy way to express the results of some complex testing procedures.  But from its adoption in 1999 to the present, ANSI/ASHRAE 52.2 has been the center of discussion and controversy in the air filter industry.

First, the ANSI/ASHRAE 52.2 Standard did not totally replace the old 52.1 Standard.  Data from the 52.1 tests were still needed to obtain arrestance percentages for filters with a MERV of 4 or less.  In addition, the 52.2 Standard did not provide for a measurement of “dust holding capacity” which many still believe is a measure of the service life of a filter.

Secondly, there has been concern that the test results do not correspond to how the filters perform in real world situations.  Many filters use an electrostatically charged media (electret) which can occur naturally or be the result of charging the media during manufacturing.  This enhances the ability of these filters to capture particles. Laboratory and field studies have shown that as some electret media filters are exposed to particles (load) their “charge” is reduced and they drop in efficiency. (See the paper entitled, “The Long-Term Performance of Electrically Charged Filters in a Ventilation System” by Raynor and Chae in a 2004 edition of the Journal of Occupational and Environmental Hygiene.)

However, not all electret media filters drop in efficiency as they load.  Likewise non-electret media filters have also been shown to drop in efficiency both in testing and in actual field use situations. (See the paper entitled “Real Time Evaluation of Ventilation Filter-Bank Systems” by Moyer, et al. of the US Dept. of Health and Human Services in the January 2007 edition of the Journal of Occupational and Environmental Hygiene.)

In January of this year the ASHRAE committee responsible for the 52.2 Standard approved two changes:

Addendum B provides for testing of arrestance to determine MERVs for filters of MERV 4 and under and dust holding capacity for filters with MERVs 5 to 16.  The intent is to eliminate the need for the ASHRAE 52.1 Standard.  (The last Public Review comment period to eliminate Standard 52.1 has just concluded with all comments supporting the removal of the old Standard and the inclusion of Addendum B in Standard 52.2)

Appendix J is an optional test including a different conditioning step using nanometer sized potassium chloride (KCL) particles sprayed on the filter.  The tests run with this conditioning step would produce a separate test result called a MERV-A.  Those manufacturers that choose to conduct this test would state the MERV Number and the Suffix A of their filters (i.e., MERV 8-A).

The forward to Appendix J makes clear that it is “not part of the standard.  It is merely informative and does not contain requirements necessary for conformance to the standard.  It has not been processed according to the ANSI requirements for a standard and may contain materials that have not been subjected to public review or the consensus process.”

Some air filter manufacturers have stated that they will be using the MERV-A procedures in their testing and will show both a MERV and a MERV-A on their filters.  Other filter manufacturers have concerns that the Appendix J procedures dramatically increase the cost of filter testing without sufficient proof that the new methods are reliable, reproducible, and truly represent performance in real life situations.

In a recent paper given at the National Air Filtration Association Technical Conference in 2008 and published in Air Media magazine (Fall 2008) Monroe Britt presented results of ASHRAE 52.2 tests in different laboratories.  The filters used in these tests were all pleated filters produced in the same factory, using the same materials, by the same people with the same specifications (a size of 24”X24”X2” and a total of 19 pleats).  The tests were conducted for initial resistance and for initial efficiency only. The media used in the filters was not electrostatically charged (electret).     A total of 8 laboratories were used – six of the laboratories were commercially operated independent third-party labs and two were “in-house.”  The filters were then prepared using the new optional ASHRAE Conditioning step (Appendix J).  Laboratory C used a modified conditioning step that involved spraying the filter with an isopropyl alcohol (IPA) spray much like is used in the EN779 European test method.

Since one of the claims is that the ASHRAE test dust does not perform like actual dust in real life situations, there have been studies to determine the composition of dust in indoor and outdoor environments.  It appears that the most frequently occurring dust is in the 4 nanometer (.04 micrometers) to 8 nanometer (.08 micrometers) range.  The Conditioning Step is done by bombarding the filter with millions of particles of these sizes produced by particle generators using potassium chloride (KCL).  Parameters are established for determining when a filter has been sufficiently “conditioned.”

Table 4 provides a summary of the test results.

This data from eight different laboratories gives us some very interesting information on the variations in test results and the reliability of the Appendix J Conditioning Step.  One would think that with the controls over equipment and test procedures in the ASHRAE 52.2 Standard that you would receive the same initial MERV number for the same filter.  As it turns out 6 of the 8 laboratories found that the filter was a MERV 8 while two of the laboratories found that it was a MERV 7.  A more careful look at the test results gives us some indication of why that is the case and illustrates why there are some limitations to the MERV system.  Look at the results for Lab H.  The E3 efficiency is 67% or a MERV 7.  Just 3% more and it becomes a MERV 8.  In looking at the variation in the other test results, it would not be surprising that another test of the same filter in the same laboratory would produce a MERV 8.  It could be within the margin of error in the testing methods.

If you were the filter manufacturer producing the filters used in these tests, which test would you use?  Would you say your filter is a MERV 7 or a MERV 8?  Possibly this explains some of the variation in claims between manufacturers for what appears to be basically the same filter made from the same materials.

In every case, except one, the efficiency of the filter decreased after conditioning.  In half of the cases the drop was sufficient to reduce the filter by a full MERV number.  Generally, it is believed that the new conditioning step in Appendix J would drop the MERV on filters made with electret media.  It is interesting that we see a consistent drop in efficiency for the mechanical filters used in these tests.

Although the ANSI/ASHRAE 52.2 Standard is a useful tool in helping to select air filters, it is not an exact science.  Despite the best efforts to standardize and certify test procedures, there are going to be some variation in test results.  Given this variation, some in the filter industry have suggested that customers look at a MERV number in terms of (+) or (-) 1.  Filter purchasers should keep this in mind.  Be wary of purchasing a better test result – rather than a better filter.

Filters need to be changed regularly to ensure they perform with minimal pressure drop and consistent filtration efficiency.  Lengthening filter change intervals to “save money” is a false economy.

Some manufacturers have their own in-house ANSI/ASHRAE 52.2 testing facilities.  These have their place in the development of products.  However, with the possibility of differing test results, it would seem that a commercially operated third-party laboratory should be used to maintain impartiality.   The National Air Filtration Association (NAFA) Product Certification program could also play a significant role in this regard as well.

Proper filter installation is an important component in filter efficiency.  Small gaps between filters or around the perimeter of the filter housing can dramatically reduce the effectiveness of filtration.  Especially in the case of high efficiency filters, proper sizing, and the use of gasketing and spacers are essential.  For particularly critical applications such as in certain areas of health care facilities, there should be on-site (in situ) testing.  This is the only way to ensure the particle capture efficiency of the entire system.  In fact, ASHRAE has recognized this with the development of a new guideline – ASHRAE 26-2008 “Guideline for Field Testing General Ventilation Devices and Systems for Removal Efficiency In-Situ by Particle Size and Resistance to Airflow.”

The MERV numbers derived from the tests in the ANSI/ASHRAE 52.2-2007 Standard have become a common part of our vocabulary in both the residential and commercial markets.  It is important to understand how they are derived and what they mean.  Likewise, it is also critical to be aware of their limitations.  It is tempting to use MERV numbers to “rate” filters, but the system was not designed for this – nor is it particularly meaningful when used in this way.   This is especially true for filters with MERV numbers that are within (+1) or (-1) of each other.

The ANSI/ASHRAE 52.2 Standard is a useful tool in helping to assess the efficiency of air filters.   However, at the end of the day we must keep in mind that the Standard is an attempt to simulate real life conditions in a laboratory.  As such, it will continue to have its drawbacks since it cannot possibly reproduce all the factors determining air filter performance in all applications.