Air filtration: Evaluating filtration efficiency

Indoor air is typically two to five times more polluted that outdoor air.
Indoor air is typically two to five times more polluted that outdoor air.
Filter media from Kimberly-Clark Professional Filtration is used in a wide variety of air filters.
Filter media from Kimberly-Clark Professional Filtration is used in a wide variety of air filters.
Air filters being installed.
Air filters being installed.

Air filters perform an important function in commercial and institutional facilities. Because indoor air is typically two to five times more polluted that outdoor air, air filters are needed to remove respirable particles such as microorganisms, dust and allergens from the breathing air. In fact, air filters provide the primary defence for building occupants and HVAC equipment against air pollutants. ilters are increasingly being more widely used in public buildings to remove air pollutants and understanding their effectiveness is crucial. Tony Fedel of Kimberly-Clark Professional Filtration suggests looking beyond the Minimum Efficiency Reporting Value for a better way to evaluate filtration efficiency.The extent to which filters remove airborne particles is referred to as filtration efficiency. When it comes to evaluating filtration efficiency, many people turn to MERV. The Minimum Efficiency Reporting Value is assigned to filters based on their minimum fractional particle size efficiency, as determined under the ASHRAE 52.2 Standard. While MERV does provide a simple basis for choosing a filter, in order to better understand its in-use performance, you also need to understand some basics about the type of media used in the filter. Generally, there are two types of air filter media. One type relies solely on its mechanical structure to filter the air. The other type of media utilises both its mechanical structure and the added benefit of an electret charge. The mechanical efficiency provides for sustained filtration efficiency and the electret charge increases initial efficiency and is particularly useful in increasing capture efficiency for E1 submicron particles. This is because, while submicron particles are much smaller than the void spaces present in most commercial electret media, the electrostatic forces within the media structure allow those particles to be removed with high efficiency. Therefore, in addition to choosing a MERV rating level, it is important to consider the media type. For example, two similar filters of the same MERV rating (one using mechanical only media and the other using electro-mechanical media) can have different filtration efficiencies and initial pressure drops. To conduct a more thorough filter selection review, it is important to request and review each filter’s ASHRAE 52.2 test report.

ASHRAE 52.2 and indoor air quality

The ANSI-certified ASHRAE 52.2 test is conducted in a controlled environment, with the range of particle sizes the filter will experience in the field, allowing users to evaluate all filters equally. The ASHRAE 52.2 test provides the efficiency of the filter over three particle size ranges: E1 (very fine particles in the 0.3 to 1.0 micrometer range), E2 (fine particles in the 1.0 to 3.0 micrometer range), and E3 (coarse particles in the 3.0 to 10.0 micrometer range). The E1, E2 and E3 ASHRAE 52.2 test values for a given filter provide a more complete picture of a filter’s filtration performance over the three particle size ranges. The 52.2 test also provides additional information such as initial pressure drop, which can impact energy usage during the filter’s life. Many pleated filters today (especially at commonly used MERV 8) have very low E1 and E2 efficiencies. In fact, under the ASHRAE 52.2 Standard, there is no minimum requirement threshold for E1 particulate capture below a MERV 14 rating and no minimum requirement threshold for E2 particulate capture below a MERV 9 rating. Therefore, when specifying filters, be sure to ask the filter supplier for the ASHRAE 52.2 test report data. Manufacturers that only offer filters utilising mechanical media may try to divert attention from their poor E1 and E2 performance by instead focusing on MERV only or the non-standard ‘MERV-A’ test. They may also work with filter distributors to get these poorer performers ‘spec’d in’. On the other hand, filters with electret-charged media often provide better E1 and E2 performance than is required by MERV. High E1 and E2 efficiencies support healthier indoor air quality (IAQ). Most of the respirable dust and particles people breathe into their lungs is three micrometres and smaller. Lung-damaging dust, for example, can be as small as 0.5 micrometres while some bacteria can be as small as 0.3 micrometres. In the U.S. alone, it has been estimated that 50 percent of all illnesses and 27 percent of all respiratory illnesses are attributable to poor IAQ. One could deduce, therefore, that filters with poor E1 and E2 efficiencies may be partially responsible for these illnesses, or at the very least, do little to help prevent them. The impact of poor IAQ is staggering. Some estimates place the cost of poor IAQ to the U.S. economy at $168 billion. Part of that cost relates to direct medical care, while some of it can be traced to reduced productivity due to presenteeism (when people go to work or school even when sick) and absenteeism. It is estimated that in the U.S. alone, adults miss 14.5 million work days every year because of poor IAQ, while presenteeism is thought to be up to 7.5 times more costly than illness-related absenteeism.

MERV-A exposed

When ASHRAE published its updated Standard 52.2 in 2007, it included an Appendix not found in the previous standard: Appendix J: Optional Method of Conditioning a Filter Using fine KCl Particles. The Appendix was created to address the interest of critics of ASHRAE 52.2, who were concerned that air filters featuring an electret charge performed at high filtration efficiencies during initial testing while their filtration efficiencies could decline during actual use. Thus, they argued, the resulting MERV of the filter (as indicated by that initial test) would not represent the true minimum filtration efficiency of the filter. The critics’ solution was to ‘level the playing field’ by masking an electret filter’s charge. The MERV-A test that was proposed subjects the filters to extreme loads of very fine KCl (potassium chloride) particles – many times what the filter would be exposed to over its real-world, installed, useful life. While the stated goal of this was to represent real-world conditions, it does not, in fact, represent real-world conditions at all. In fact, the test was designed only to drive down the efficiency of electret filters. It represents a ‘worst-case’ scenario that is likely to never happen. In addition, differences in environmental conditions and lab-to-lab variances have also been uncovered, leading to the conclusion that techniques which ‘condition’ the filters are not repeatable. These are some of the reasons the electret masking step was not added to the 52.2 Standard as a mandatory part of the test but was included as an option only. This optional test method may be conducted at the filter manufacturer’s discretion, but only if the standard, ANSI-certified 52.2 test method is conducted to determine the filter’s true performance. In fact, it is impossible to isolate the structural and physical properties of an electret-charged filter media from the charge distribution without impacting other filtration mechanisms and/or other filtration properties. Moreover, these same conditioning techniques have been shown to decrease the filtration efficiency of certain mechanical-only filters as well. Be careful not to confuse the results of testing under the optional Appendix J, which should be reported as MERV A. MERV A results should not be confused with the filter’s E1, E2 and E3 Fine Particle Efficiency as a way to measure filter performance. Ask to see ASHRAE 52.2 test reports and request an energy cost analysis of the filter with mechanical-electret filter media versus one of the same MERV rating that utilises mechanical-only filter media.

EN 779 exposed

This European Standard and test procedure also classifies filter performance. To compare results for different filters, the test requires filters to be tested against two synthetic aerosols, a fine aerosol for measurement of filtration efficiency as a function of particle size within a particle size range 0.2 microns to 3.0 microns, and a coarse one for obtaining information about dust holding capacity and, in the case of coarse filters, filtration efficiency with respect to coarse loading dust (arrestance). This European Standard applies to air filters having an initial efficiency of less than 98% with 0.4 micron particles. Filters should be tested at an air flow rate between 0.24 m3/s (850 m3/h) and 1.5 m3/s (5400 m3/h). The performance results obtained cannot by themselves be quantitatively applied to predict performance in service with regard to efficiency and lifetime. Other factors influencing performance to be taken into account are described in Annex A (Normative) and Annex B (Informative).

Like with the MERV-A option, EN 779 requires electrostatically charged filters to be ‘discharged’ through immersion in a solvent. Thus, the procedure has a similar problem in that it does not represent real-world conditions. Also, it is impossible to isolate the structural and physical properties of an electret-charged filter media from the charge distribution without impacting other filtration mechanisms and/or other filtration properties.

Additional performance factors

In addition to the performance factors measured under the ASHRAE 52.2 Test Standard or EN779, consider these variables when selecting a filter.

Energy efficiency

How the filter’s resistance to airflow affects the energy consumed by the HVAC system. Energy expenditures can account for about 81 percent of the annual operating costs of an air filtration system. Filters with mechano-electret media typically deliver lower airflow resistance than filters with mechanical-only media, which translates directly to reduction in energy consumption and cost.

Moisture resistance and temperature limitations

How high humidity/moisture and temperature affect the filter. For example, studies have shown that filtration efficiency of electret-treated media is unaffected by relative humidity and by long-term warehousing at high temperatures (54.4°C/130°F).


Consider the entire product lifecycle when selecting filters – from raw material sourcing to manufacturing, from packaging to transport, and from design and usage to final disposal. For example, some filters provide superior performance while using less media than other filters. In addition, choosing high-capacity pleated filters can extend filter life and reduce change-outs, which can reduce waste streams.


MERV 12 and below rated filters with poor E1 and E2 filtration efficiencies may do an adequate job of keeping larger airborne particles from fouling HVAC equipment, but they are not likely to remove the very fine respirable particles that may lead to health and productivity problems. That is why it is so important to look beyond a filter’s MERV at the Fine Particle Efficiency rating as a true measure of a filter’s filtration efficiency. Filters with media that provides a good balance of mechanical and electret efficiency will almost always outperform a filter with media that relies solely on mechanical-only media efficiency. At the end of the day, it is important to remember that electret treatments are an enhancement of an underlying mechanical structure of the media. The combination of different electret treatments and mechanical structures means that all filters with electret media are not created equally.