Beyond MERV: a better way to evaluate filtration efficiency.
Air filters perform an important function in commercial and institutional facilities. Because indoor air typically is 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 defense for building occupants and HVAC equipment against indoor air pollutants. 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 the minimum efficiency reporting value, or MERV. MERV is assigned to filters based on their minimum fractional particle size efficiency, as determined under the ASHRAE 52.2 Standard.
Although MERV does provide some basic information for evaluating filter performance, there is a more complete way to compare the filtration efficiencies of air filters: by reviewing the efficiency values that are included in the ASHRAE 52.2. test report.
ASHRAE 52.2 and IAQ
The ASHRAE 52.2 test provides the efficiency of a 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 efficiencies represent the true measure of filter performance and give users a more complete picture of what the filter actually will do. 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. This enables users to evaluate all filters equally.
Many pleated filters have low E1 and E2 efficiencies. Manufacturers of these filters may try to divert attention from poor E1 and E2 performance by instead focusing on MERV and the non-standard "MERV-A" test. They also may work with filter distributors to get these poorer performers "spec’d in."
High E1 and E2 efficiencies are critical for providing for good indoor air quality (IAQ). Most of the respirable dust and particles people breathe into their lungs is three micrometers and smaller. Lung-damaging dust, for example, can be as small as 0.5 micrometers; some bacteria can be as small as 0.3 micrometers.
It’s 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 responsible for these illnesses, or at the very least, do little to help prevent them.
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; some of it also can be traced to reduced productivity because of "presenteeism" (when people go to work or school even when sick) and absenteeism. It’s estimated that U.S. adults miss 14.5 million work days every year because of poor IAQ, and presenteeism is thought to be up to 7.5 times more costly than illness-related absenteeism.
The MERV-A option
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 critics of ASHRAE 52.2. They were concerned that air filters featuring an electret charge performed at high-filtration efficiencies during initial testing, but 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 real-world performance of the filter.
The critics’ solution was to "level the playing field" by masking an electret filter’s electret charge. The MERV-A test that was proposed subjects filters to extreme particle loads—many times what a filter would be exposed to over its design lifetime. Further, only very fine particles are used in the testing. Although the stated goal of this was to represent real-world conditions, it does not. It represents a "worst-case" scenario. In addition, differences in environmental conditions and lab-to-lab variances also have been uncovered, leading to the conclusion that techniques that "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 step may be conducted at a users’ 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 affecting other filtration mechanisms 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, 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 mechanical-only filter against a mechano-electret filter.
Filters that provide a good balance of mechanical and electret efficiency almost always will outperform a filter that relies solely on mechanical efficiency.
The mechanical efficiency provides for sustained filtration efficiency, and the electret charge increases initial efficiency and is particularly useful in increasing capture efficiency for submicron particles. This is because, although submicron particles are much smaller than the void spaces present in most commercial electret media, the electrostatic forces within the media structure enable those particles to be removed with high efficiency.
Mechano-electret filters also typically deliver lower airflow resistance than mechanical-only filters, which translates directly to reduction in energy consumption and cost. The best mechano-electret filters have a depth-loading media with a gradient density structure. This combination can help to reduce airflow resistance, enhance dust loading and prevent face loading of the filter.
It’s important to remember that electret treatments are an enhancement of an underlying mechanical structure. The combination of different electret treatments and mechanical structures means that all electret filters are not created equally.
In addition to the performance factors measured under the ASHRAE 52.2 Test Standard, 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.
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 (130ºF).
Sustainability: 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, high-capacity pleated filters can extend filter life and reduce change-outs, which can reduce waste streams.
Cox, CAFS, is market manager for Kimberly-Clark Filtration, Roswell, Ga. He can be reached at email@example.com.