ASHRAE Journal - April 2022 - 22

TECHNICAL FEATURE
Moreover, to date, these test reports do not adequately
characterize emissions of primary or secondary products
formed during operation of the device and/or chemistry
initiated in the indoor space. The peer-reviewed literature
on the potential for chemical by-product formation
from electronic air cleaners (other than ozone generation)
remains limited in scope, and publicly available
test methods do not yet adequately address by-product
formation.9,10 The most widely used test standards for
by-product formation focus only on ozone: UL 867 and
UL 2998. UL 2998 is the more stringent " zero ozone
emissions " standard (allowing up to 5 ppb in a standard
27 m3 to 31 m3 [954 ft3 to 1,095 ft3] test chamber), while
UL 867 is the less stringent standard (allowing up to
50 ppb in a standard test chamber).
However, no standard test methods exist for evaluating
the potential for formation of other types of by-products
such as nitrogen oxides (NOx), volatile organic compounds
(VOCs), including aldehydes or other chemical
compounds due to air cleaner operation. Thus, while
this article focuses primarily on characterizations of efficacy
rather than by-product formation, readers should
understand that concerns for by-products resulting
from additive oxidizing air cleaning technologies should
not be limited to ozone.
Three Commonly Used Performance Metrics
* The single-pass efficiency of an air cleaning device
is a fractional measure of its ability to reduce the concentration
of pollutants in the air that passes through
the device once (Equation 1). The fractional efficiency
(dimensionless) of a device is typically measured in a
laboratory test duct but can also be extracted from welldesigned
chamber or even field tests.11.12
Efficiency=−1
C
C
downstream
upstream
(1)
where Cupstream and Cdownstream are the contaminant
concentrations immediately upstream and downstream
of a filter or air cleaner, respectively (#/m3 or equivalent).
Efficiency is a straightforward metric for subtractive
technologies but perhaps less useful for additive
or mixed technologies that rely on the addition of constituents
to a space. Efficiency is usually expressed as
a percentage, in which case the above is multiplied by
100. Efficiency can also be calculated for multiple pass
units by comparing loss rates between air cleaner on and
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off conditions and accounting for the volume of the test
space and the flow rate through the device.
* The effectiveness (dimensionless) of an air cleaning
device or system is a measure of its ability to remove
pollutants from the spaces it serves in real-world situations
(Equation 2).13 It is simply a comparison of pollutant
concentrations in a space with and without an air cleaning
technology operating, assuming other parameters
like source rates, ventilation rates and other loss rate
mechanisms are held constant. Effectiveness can also
be assessed by comparing pollutant loss rates in a space
measured during air cleaner on and off conditions. This
performance metric can be applied to any air cleaning
intervention, regardless of underlying mechanism of
action.
Effectiveness=−1
C
C
on
off
(2)
where Con and Coff are the contaminant concentrations
in a space with the air cleaner on and off, respectively
(#/m3 or equivalent).
* The clean air delivery rate (CADR)-sometimes
also referred to as the effective cleaning rate (ECR)-of
an air cleaner is an important measure of the amount
of contaminant-free air delivered by an air cleaner.14
The terms CADR and ECR are used interchangeably in
the peer-reviewed literature, although the Association
of Home Appliance Manufacturers (AHAM) specifically
uses CADR to characterize its standard metric for an air
cleaner's efficacy. Our use of the term CADR herein does
not imply endorsement of AHAM or its CADR standard,
but rather reflects the term as used generally in the
scientific literature and in practice.
A higher CADR relative to the size of the room will
increase the effectiveness of an air cleaner. The CADR
is pollutant-specific and can be measured/calculated
for specific gases, specific particle sizes and/or specific
microorganisms, pathogens or surrogate pathogens.
Current test standards such as ANSI/AHAM AC-1 only
rate CADRs for the removal of particles.15
The CADR of any air cleaning device-whether it is
installed as an in-duct device or a stand-alone/portable
unit-is readily measured in a controlled chamber14-18
or even a well-controlled field environment.19,20 The
general test procedure to measure CADR relies on elevating
concentrations of a contaminant of interest and
measuring its subsequent first-order loss rate constant
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ASHRAE Journal - April 2022

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