ASHRAE Journal - August 2020 - 31

TECHNICAL FEATURE

Efﬁciency (%)

Viral Filtration Efﬁciency (%)

E1 (0.3 µm to 1.0 µm), E2 (1.0 µm to 3.0 µm) and E3
FIGURE 3 Viral filtration efficiency with 95% confidence interval. N = 6.
(3.0 µm to 10 µm) efﬁciencies deﬁned according to
ASHRAE Standard 52.2-2017 are commonly used in resi98.5
97
100
97
dential and commercial HVAC applications. People are
95.5
87
90
89
familiar with E1, E2 and E3 efﬁciencies, while VFE is usu80
78
80
ally not reported for HVAC ﬁlters. It would be beneﬁcial
70
69
60
for ﬁlter users to know the relative relationship between
50
VFE and E1, E2 or E3.
43
40
Although MS2 virion is much smaller than potassium
32
30
chloride (KCl) particles, the challenge aerosol used in
20
21
ASHRAE Standard 52.2-2017, the MS2 bacteriophage
MERV 5
MERV 12
MERV 13
MERV 14
aerosol contained particles larger than MS2 virions and
Individual standard deviations were used to calculate the intervals.
particles collected by the impactor were greater than 0.65 µm.
TABLE 5 Descriptive statistics of pressure drop data.
Therefore, the comparison of VFE
FILTER N
MEAN, PA
MEAN, STD DEV, PA COEF VAR MEDIAN, PA MINIMUM, PA MAXIMUM, PA
IN. W.G.
versus E1, E2 or E3 should be relevant.
Figure 4 shows the comparison of the
MERV 5 6
43
0.17
2.3
5.2
44
39
45
viral ﬁltration efﬁciency versus the
MERV 12 6
38
0.15
3.5
9.2
36
34
43
E1, E2 and E3 efﬁciencies of clean ﬁlMERV 13 6
45
0.18
5.6
12.6
43
39
54
ters measured per ASHRAE Standard
MERV 14 6
60
0.24
3.1
5.1
60
54
64
52.2-2017. As shown in Figure 4, the
viral ﬁltration efﬁciency (VFE) is
FIGURE 4 Comparison of viral filtration efficiency versus E1, E2 and E3 (tested at 295 fpm).
always higher than E1, but lower than
E2 or E3 efﬁciencies. In other words, E1 efﬁ100
ciency can provide a conservative prediction for
90
the viral ﬁltration performance of a ﬁlter, also
80
70
indicating that viral particles collected on sam60
pling plates were likely to be in the size range
50
40
of 0.3 µm to 3 µm, i.e., the lower size limit of
30
20
E1 and upper size limit of E2. Note that the sta10
bility of the aerosol size distribution can have
0
signiﬁcant impact on the VFE measurement.
A (MERV 5)
B (MERV 12)
C (MERV 13)
D (MERV 14)
Filter Type
Therefore, it is important to ensure the stabilViral Filtration Efﬁciency
E1 - Initial
E2 - Initial
E3 - Initial
ity of the aerosol in the testing (see Figure 2).

Summary
An experimental study was carried out to investigate
effectiveness of residential HVAC ﬁlters at removing
airborne virus particles. MS2 bacteriophage organisms were grown on appropriate media, harvested,
resuspended in saline and then aerosolized into the
ASHRAE Standard 52.2-2017 test duct using a nebulizer.
Upstream and downstream air samples were taken
using impactors. The collection plates were incubated,
and the recovered plaque-forming units (PFU) were
enumerated to determine the ﬁltration efﬁciency of
ﬁlters against virus particles. The following conclusions

can be drawn from this study:
* High-efﬁciency residential HVAC ﬁlters were found
to be effective at capturing airborne virus particles.
* Filter viral ﬁltration efﬁciency (VFE) was found
to be generally correlated to its MERV rating, i.e., the
higher the MERV rating, the higher the viral ﬁltration
efﬁciency.
* In comparison to E1, E2 and E3 efﬁciencies measured
per ASHRAE Standard 52.2-2017, VFE was found to be
higher than initial E1 efﬁciency, but lower than initial E2
and E3 efﬁciencies.
AU G UST 2020

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ASHRAE Journal - August 2020

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