ASHRAE Journal - August 2021 - 22

TECHNICAL FEATURE
1 μm the filter efficiency takes a dip down to 99.97% (due
to the effects of diffusion tapering off before inertial
impaction and interception begin to dominate), with the
most penetrating particle size (MPPS) occurring between
0.2 μm and 0.3 μm.17
Particle penetration through the filter is greatly
affected by the flow velocity. A general rule of thumb
is by reducing the filter velocity by half, the pressure
drop is reduced by half, and the particle penetration is
reduced by almost an order of magnitude.17
HEPA filter test methods are typically specified by the
Institute of Environmental Sciences and Technology's
(IEST) IEST-RP-CC001 and IEST-RP-CC007, the
International Organization for Standardization's (ISO)
ISO 29463-5:2011, or the European Committee for
Standardization's (CEN) EN 1822:2009. A true HEPA filter
will have verified performance according to a reputable
standard. However, many filters are sold as " HEPA "
filters that are not tested according to one of these standards
and may fall short on performance.
Unlike HEPA filters, other commercial filters are rated
according to ASHRAE Standard 52.2 and assigned a minimum
efficiency reporting value (MERV) between 1 and
16. ASHRAE Standard 52.2-2017 provides the particle
size efficiency for three different size ranges (Table 3).20
Studies have shown that the majority of virus particles
expelled during cough events (80% to 90%) are smaller
than 1 μm to 2 μm in diameter, but recent studies
have also looked at these particles after they have been
expelled from the body and the size-fractions where
viruses are present.14
Research by Azimi, et al., reviewed several studies where
influenza virus samples were captured in various indoor
environments and found a mean distribution that was
then applied to the ASHRAE ranges in ASHRAE Standard
52.2.14 The mean viral distribution from the studies
resulted in 20% in the 0.3 μm to 1 μm range, 29% in the
1 μm to 3 μm range and 51% in the 3 μm to 10 μm range.14
Research by Zhang, et al., studied the viral performance
of high-efficiency electrically charged residential
type filters.21 His tests showed that the MERV 12 filters
that were tested had a mean viral filtration efficiency of
78%, MERV 13 of 89%, and MERV 14 of 97%. MERV 5 was
used for comparison and had a mean efficiency of 32%.21
These efficiencies are dependent on viral load and airflow
conditions, but generally show an increase in performance
the higher the MERV rating.
22
ASHRAE JOURNAL ashrae.org A U G UST 2021
TABLE 3 Minimum efficiency reporting value (MERV) performance.20
MERV
Range 2
8
9
10
11
12
13
14
15
16
(0.3 μm to 1.0 μm)
N/A
N/A
N/A
20 ≤ E1
35 ≤ E1
50 ≤ E1
75 ≤ E1
85 ≤ E1
95 ≤ E1
(1.0 μm to 3.0 μm)
20 ≤ E2
35 ≤ E2
50 ≤ E2
65 ≤ E2
80 ≤ E2
85 ≤ E2
90 ≤ E2
90 ≤ E2
95 ≤ E2
COMPOSITE AVERAGE PARTICLE SIZE EFFICIENCY, % IN SIZE RANGE
Range 1
Range 3
(3.0 μm to 10.0 μm)
70 ≤ E3
75 ≤ E3
80 ≤ E3
85 ≤ E3
90 ≤ E3
90 ≤ E3
95 ≤ E3
95 ≤ E3
95 ≤ E3
Note: Data taken from ASHRAE Standard 52.2-2017 Table 12-1.
The filter efficiencies listed above are single-pass filter
efficiencies. These filter efficiencies can be added
to a contaminant generation model or an infection risk
assessment model to make some general observations. A
study by Mousavi, et al., looked at filter efficiencies and
compared contaminant decay time to various outdoor
air ratios (10% to 100%).22 The study showed that for
low filter efficiencies (MERV 12 and under) introducing
additional outdoor air into the space significantly
lowered the contaminant decay time. For MERV 13, 14,
15 the decay time was only slightly affected by additional
outdoor air; and for HEPA filters, additional outdoor air
didn't affect the decay time.22
A study by Bohanon looked at several different scenarios
involving an infected person and the risk of infection
for others in the same residence.23 The study found that
by improving the filtration in the system (up to MERV
14), the risk of infection to others in the house could be
lowered by two-thirds. MERV 11 filters also performed
well and were able to provide protection within a few
percentage points of the MERV 14s.23 A study by Azimi, et
al., modeled a hypothetical office building with 25 occupants
and a single infector.14 The relative risk of infection
was compared to the annual cost of filtration. MERV
13 and MERV 14 filters achieved the optimal combination
of low risk at reduced costs. For the office building,
HEPA filtration only offered incremental advantage over
MERV 13 through MERV 16 at 1.6 to 2.3 times the cost of
operation.14
As filters start to accumulate buildup, their efficiency
can change. Uncharged filter media often increases in
efficiency with loading, while charged-media filters
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ASHRAE Journal - August 2021

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