ASHRAE Journal - December 2020 - 20

TECHNICAL FEATURE

FIGURE 1 Change in filter efficiency in real life. Both filters were white when new. a) MERV
and test methods specified by organizations
11 charged-media filter used for full recommended lifetime (three months). Efficiency
such as Institute of Environmental Sciences
for 0.3 μm to 0.4 μm particles dropped from above 40% to below 10% during this time.
This filter's efficiency also dropped 25 percentage points in the 7 μm to 10 μm size
and Technology (IEST), the International
range. b) MERV 15 uncharged filter used for 10 months. During use, the efficiency of this
Organization for Standardization (ISO) or the
filter increased by 7 percentage points in the 0.3 μm to 0.4 μm size range. It was 100% effiEuropean Committee for Standardization (CEN)
cient in the 7 μm to 10 μm size range before and after use.
normally must be used for their performance
B.
A.
evaluation (IEST-RP-CC001),6 ISO 29463-5:2011,7
EN 1822:20098). Paraphrasing the suggestions in
Table 12-1 of ASHRAE Standard 52-1999, MERV
17 to MERV 20 would be equivalent to HEPA classifications per IEST Testing of Type A (99.97%
efficient at 0.3 µm) to Type F (99.999% on 0.1 to
0.2 µm).
can have much lower efficiency than when clean or have
The Standard 52.2 committee is currently taking steps
somewhat higher efficiency. Examples from RP-1360 of the
to remove the last reference to theoretical MERV 17 to
impact of use on charged and uncharged filters are shown
20 filters and the relationship of MERV to HEPA from
in Figure 1 and described below.13
the standard. We recommend that specifiers calling for
To account for the charge drop-off, Standard 52.2 added
MERV 17 to MERV 20 change their recommendations to
the appropriate HEPA classification for correctness, clar- a conditioning step using 40 nm to 50 nm (tiny) particles
to imitate the exposure a filter may see in real life when
ity and ability to be implemented.
big particles are not present. This step is specified in
Why Some Filters Increase Efficiency With Time While Others Decrease
Appendix J. However, Appendix J is optional. If used, the
Another issue of particular concern, especially with
test gives a MERV-A instead of a MERV. Using MERV-A can
the current worry over whether COVID-19 transmisgive a better idea of how a charged-media filter, especially
sion by HVAC systems is likely, is how filters' efficiencies one that is prefiltered, will behave. This drop-off can take
change over time. This depends to some extent on the
weeks or months to occur; it is not usually an effect that
dust characteristics in the specific building, but whether changes the performance immediately after installation.
the efficiency increases or decreases with dust-loading
Owen et al. 2013,13 and Li and Siegel 202014 found that
Appendix J reasonably bracketed the performance of
depends largely on the type of filter installed.
charged filters that were used in real buildings.
Media filters that do not rely on charge often increase
in efficiency with loading, mostly due to the buildup of
Get on Board With Mass-Based Filter Efficiency
bigger particles on and inside the filter, which develops
Most recently, Standard 52.2 added a particulate matadditional filtration capability. Charged-media filters,
ter (PM) efficiency calculation. PM, especially PM2.5, is
however, often lose efficiency over time. For the small
considered a major health issue.15,16 The term is also used
particle sizes, this effect can be up to 50% of the original
for describing air pollution. EPA,17 California's South
value.9,10,11 This is due to deposition of many tiny particles that mask the effect of the charge.
Coast Air Quality Management District18 and CDC19 use
PM as an outdoor air pollutant level marker so that it is
Filters, including charged filters, may also experience
now often reported in the news. Several ASHRAE stanan increase in efficiency as they are dust-loaded due to
dards committees have gotten onboard and requested a
the buildup of larger particles as mentioned above and
described in Raynor and Chae 2003.12 They also noted that PM-based efficiency to use in their standards for specifylarge efficiency increases such as those seen with ASHRAE
ing filters. To meet this need, Standard 52.2 added the
dust* in ASHRAE Standard 52.2 were not seen for their inPM52.2 efficiency values. Using the minimum efficiency
situ filters. Results from Research Project RP-1360 showed values from the already available Standard 52.2 test data,
that filters that look very dirty and caught substantial dust
and assuming a flat particle size distribution in the air,

* ASHRAE Standard 52.2 consists of a clean filter efficiency test followed by five separate dust loading steps using ASHRAE dust with
efficiency tests after the dust loads. " ASHRAE dust " is a defined dust mixture including ISO Fine dust, cotton linters and carbon black.
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ASHRAE Journal - December 2020

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