ASHRAE Journal - April 2022 - 29

TECHNICAL FEATURE
Note that while Figure 4 allows for scaling test chamber
results to a CADR based on volume, factors exist that are
not incorporated into Figure 4 that may impact the realized
CADR in the environment where the air cleaner is
used. For example, in the case of electronic air cleaners,
the concentration of added reactive constituents should
scale with realized loss rates for target air pollutant(s).
The concentration of added constituents can differ
between testing and actual use due to device settings,
types of surfaces present, surface area to volume ratios
and the background air matrix (i.e., constituents in air
other than the target pollutant or pollutants), among
other reasons. For any air cleaner, imperfect mixing in
the space (e.g., short-circuiting) may also reduce the
realized effectiveness of an air cleaner.
As an example of how to use this fi gure, consider a
manufacturer test report of " 99.9% removal of a pathogen
in 60 minutes. " By identifying the appropriate curve
(gray line) and time (60 minutes), one can determine
the volume-normalized CADR (i.e., CADR/V on the vertical
axis of Figure 4) for these conditions to be ~0.1 cfm/ft3
or (6 m3/h)/m3. Thus, if the test were conducted in a
100 ft3 (2.8 m3)chamber, the resulting CADR from that
test would be approximately 10 cfm (17 m3/h). If the test
were conducted in a 10 ft3 (0.28 m3) chamber, the resulting
CADR from that test would be only ~1 cfm (~1.7 m3/h).
For comparison, AHAM recommends that portable air
cleaners have a CADR of ~⅔ of the fl oor area of the space
being served. By this rule of thumb, an air cleaner with
~10 cfm (~17 m3/h) would be suitable for a 15 ft2 (1.4 m2)
room, or perhaps a small closet. As with prior examples,
this shows that an impressive sounding performance
statement must be carefully scrutinized in terms of test
conditions and expected impact in a real indoor environment.
Use of Figure 4 also demonstrates that the volume
of the test chamber scales proportionally with the
CADR. In other words, a high value of percent removal
does not inherently mean the device has a high CADR
even if achieved in a short duration test.
The examples shown in Figures 2 and 3 demonstrate
that loss rates from chamber tests can be determined
by plotting the natural logarithm of the reduction in
the target compound achieved as a function of time. For
these more complex analyses involving regression across
values of removal measured over time, we developed a
spreadsheet application to make calculations, visualize
data and perform regressions to determine loss rates,
FIGURE 4 The test chamber volume-normalized clean air delivery rate (CADR) as
a function of removal achieved in a specified time. For a given removal and time
to achieve that removal, the resulting CADR of the air cleaner is determined by
multiplying the associated value on the vertical axis. Left axis: cfm/ft3, right axis:
(m3/h)/m3 by the test chamber volume (ft3 or m3).
10
Net Log
Removal
Net
Removal
1.0
1 (90%)
2 (99%)
3 (99.9%)
4 (99.99%)
5 (99.999%)
6 (99.9999%)
600
60
0.1
6
0.01
0.6
15
30
45
60
Time in Chamber (Minutes)
CADRs and equivalent clean air changes per hour provided
to a space. This spreadsheet application provides
instructions, including rationale for and comparisons to
generally accepted air cleaning thresholds. This spreadsheet
tool can be accessed at: https://www.pdx.edu/
healthy-buildings/ace-it.
Charting a Path Forward for Research Needs and
Improving Test Methods and Standards
Given these persistent issues and others, we provide
the following suggestions for future research needs and
improving test methods and standards for evaluating
the effi cacy and impacts on indoor air of current and
emerging air cleaning technologies:
1. Fundamental studies that elucidate underlying
mechanisms of action of air cleaning technologies.
Statements from manufacturers and distributors often
confuse and confl ate terms, such as interchanging
" ions " and " radicals, " which limits the ability to
evaluate the expected primary (i.e., air cleaning)
and secondary (i.e., by-product formation or other
unintended consequences) effects of the device
operation.
A P R I L 2 0 2 2 ashrae.org ASHRAE JOURNAL
29
75
90
CADR Per Test Chamber Volume
(cfm/ft3)
(m3/h)/m3
https://www.pdx.edu/healthy-buildings/ace-it https://www.pdx.edu/healthy-buildings/ace-it http://www.ashrae.org
ASHRAE Journal - April 2022

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