ASHRAE Journal - April 2022 - 25

TECHNICAL FEATURE
a net 4 log reduction in pathogen surrogate after 30
minutes.
* Air Cleaner Technology B reduces viable
SARS-CoV-2 by over 99% in 60 minutes.
They often sound impressive. For example: " Pathogen
killing with over 99% effi cacy in less than an hour, " or
" Multiple orders of magnitude of pathogen reduction
in 30 minutes, " or " Reactive species that kill COVID-19
in the indoor environment. " Such statements are frequently
made based on results from test reports from
commercial laboratories.
To understand these test reports and translate their
results to real-world situations, several important factors
must be considered. These may include but are not
limited to:
* Lack of standardized performance metrics, which
limits the ability to compare devices and translate to
operation in real occupied spaces;
* Testing in small sealed chambers that can overestimate
performance in actual buildings;
* Testing that does not account for control conditions
(such as natural decay rates);
* Testing conducted at elevated (or sometimes unreported)
concentrations of additive/reactive constituents
that might not refl ect real-world use;
* Lack of chemical by-product testing or demonstration
of complete oxidation;
* Omission of test parameters, such as chamber volume
or mixing conditions;
* Variation in parameters between control and test
conditions.
Here we illustrate some of these factors using generic
examples of performance data from test reports, again
focusing primarily on effi cacy given the nascent stage of
by-product testing that exists.
Hypothetical Example 1:
Concentration Decrease in a Medium-Size Chamber
Manufacturer A has provided a third-party test report
in which their air cleaning technology was tested for its
ability to inactivate aerosolized E. coli in a medium-sized
500 ft3 (14.2 m3) chamber. The report highlights 98%
reduction in E. coli viability after 60 minutes. The test
report provides the following E. coli concentration data
(in CFU/m3) for both control (air cleaner off) and test
(air cleaner on) conditions in the fi rst three columns of
Table 1.
TABLE 1 Hypothetical air cleaner test results (on a concentration basis) and
subsequent loss rate calculations for an E. coli inactivation test conducted in a
medium-sized 500 ft3 (14.2 m3) chamber.
TEST REPORT DATA
TIME (MIN)
15
30
45
60
CONCENTRATION (CFU/m3)
CONTROL
5,000
4,200
3,300
2,700
2,000
TEST
5,000
2,700
1,200
400
100
LOSS RATE CALCULATIONS
ln(C/C0)
CONTROL
-0.174
-0.416
-0.616
-0.916
TEST
-0.616
-1.427
-2.526
-3.912
After 60 minutes of testing with this air cleaner operating,
the concentration of E. coli is only 100 CFU/m3 compared
to an initial concentration of 5,000 CFU/m3-a
reduction of 98% from the initial concentration, which is
about 95% lower than the control condition concentration
at that same point in time. We can use these data to
calculate fi rst-order loss rate constants during both the
control and test conditions, as shown in the last two columns
of Table 1. To do so, we take the natural logarithm
of the concentration at each time step (C ) divided by the
concentration at the initial time step (C0), which is again
a simple fi rst-order exponential decay model assuming
Cbg from Equation 4 is set to 0 and that the test chamber
is reasonably well-mixed. We fi t a straight-line regression
through these calculated data points versus time
to estimate fi rst-order loss rate constants, as shown in
Figure 2. If the chamber is not well-mixed, then the effective
volume of treated air could be even smaller than the
chamber volume, and the effective loss rates and resulting
CADR could be lower than shown here.
The resulting loss rate constants are approximately
0.0145 per minute during the control condition (which
represents the natural decay rate in the chamber) and
approximately 0.0593 per minute with the air cleaner
operating. The difference in loss rate constants in this
setup is thus approximately 0.045 per minute. Using
Equation 3, we can calculate an equivalent CADR by multiplying
this loss rate difference by the volume of the
chamber (500 ft3 or 14.2 m3), which yields an estimated
CADR of approximately 22 cfm (37 m3/h) (Equation 5).
3
CADR=× = 22
min
500 0.045
ft
3
1
ft
min
(5)
In this case, an equivalent CADR of 22 cfm (37 m3/h) is
not a particularly effective air cleaner compared to the
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