oEPA
EPA 402-F-09-002 I July 2018 I EPA Indoor Environments Division I www.epa.gov/iaq
United States
Environmental Protection
Agency
RESIDENTIAL AIR CLEANERS
A Technical Summary
3rd Edition
Portable Air Cleaners
Furnace and HVAC Filters
Indoor Air Quality (IAQ)
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RESIDENTIAL AIR CLEANERS
FOREWORD
This document was developed by the U.S. Environmental Protection Agency (EPA), Office of Radiation and Indoor Air,
Indoor Environments Division. It focuses on air cleaners for residential use; it does not address air cleaners used in large
or commercial structures such as office buildings, schools, large apartment buildings, or public buildings. It should be
particularly useful to residential housing design professionals, public health officials, and indoor air quality professionals. It
may serve as a reference for anyone who designs, builds, operates, inspects, maintains, or otherwise works with buildings,
heating, ventilating and air conditioning (HVAC) equipment, and/or portable air cleaners/sanitizers. This includes home services
professionals, builders, remodelers, contractors, and architects.
In addition to providing general information about the types of pollutants affected by air cleaners, this document discusses
the types of air-cleaning devices and technologies available, metrics that can be used to compare air-cleaning devices, the
effectiveness of air-cleaning devices in removing indoor air pollutants, and information from intervention studies on the effects
that air cleaners can have on health and on health markers.
A briefer companion publication, designed for the general public, Guide to Air Cleaners in the Home, is also available on the
EPA website at www.epa.gov/indoor-air-quality-iaq/guide-air-cleaners-home.
ACKNOWLEDGMENTS
The EPA Office of Radiation and Indoor Air, Indoor Environments Division, thanks the many professionals who contributed to
the development of this document, including Terry Brennan of Camroden Associates, Lew Harriman of Mason-Grant Consulting,
Brent Stephens of Illinois Institute of Technology, and Vito llacqua of EPA's Office of Research and Development, National
Center for Environmental Research.
EPA would also like to thank the many reviewers of this document, including: William Bahnfleth, David Butler, Richard Corsi,
Hugo DestaiIlats, William Fisk, Howard Kipen, Jinhan Mo, Ju-Hyeong Park, Richard Shaughnessy, Tiina Reponen, and Charles
Weschler.
EPA Establishment Number: Federal pesticide law requires manufacturers of ozone generators to list an EPA establishment
number on the product's packaging. This number merely identifies the facility that manufactured the product. Its presence
does not imply that EPA endorses the product, nor does it imply that EPA has found the product to be safe or effective.
Ozone generators that are sold as air cleaners intentionally produce the gas ozone. No federal government agency has approved
these devices for use in occupied spaces. For more information regarding ozone generators that are sold as air cleaners, see
www.epa.gov/indoor-air-quality-iaq/ozone-generators-are-sold-air-cleaners.
ENERGY STAR® labels: Some portable air cleaners sold in the consumer market are ENERGY STAR® qualified. Please note
the following disclaimer on their packaging: "This product earned the ENERGY STAR® by meeting strict energy efficiency
guidelines set by the US EPA. EPA does not endorse any manufacturer claims of healthier indoor air from the use of this
product."
Disclaimer: EPA neither certifies nor recommends particular brands of air filters or home air-cleaning devices including portable
air cleaners or purifiers.
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RESIDENTIAL AIR CLEANERS
TABLE OF CONTENTS
SUMMARY 5
Research Overview 7
Air Cleaners and Indoor Air Quality 7
Air Cleaners and Health 8
Air Cleaners Must be Operated to be Effective 8
Portable Air Cleaners and Noise 8
Furnace Filters and Fine Particulate Matter 8
Furnace Filters and HVAC System Operation 9
Byproduct Emissions From Some Air Cleaner Technologies 9
Selecting and Using Portable and In-Duct Air Cleaners 10
INTRODUCTION 12
INDOOR AIR POLLUTANTS 12
THREE STRATEGIES TO REDUCE INDOOR AIR POLLUTANTS 14
TYPES OF AIR CLEANERS 14
UNDERSTANDING EFFICIENCY VERSUS EFFECTIVENESS 16
TYPES OF AIR-CLEANING TECHNOLOGIES 16
Air-Cleaning Technologies Used for Removing Particles 19
Fibrous Media Air Filters 19
Test Metrics for Fibrous Media Air Filters 20
High-Efficiency Particulate Air (HEPA) Filters 20
Types of Fibrous Media Air Filters 20
Practical Considerations for Using Fibrous Media Air Filters 22
Electrostatic Precipitators (ESPs) and Ionizers 25
Possible Negative Effects of Particle Charging 25
Cautions Concerning Ozone Production by ESPs and Ionizers 26
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Ultraviolet Germicidal Irradiation (UVGI) Air Cleaners 26
UVGI Technology 26
Types of UVGI Cleaners and Their Effectiveness 26
Disadvantages of UVGI Cleaners 28
Air-Cleaning Technologies Used for Removing Gases 28
Sorbent Media 29
Photocatalytic Oxidation (PCO) 30
Plasma 31
Intentional Ozone Generators 32
Practical Considerations for Using Air Cleaners for Removing Gases 32
Removal of Radon and Its Progeny 33
SELECTING AND USING A PORTABLE AIR CLEANER 33
Clean Air Delivery Rates (CADRs) for Portable Air Cleaners 34
Portable Air Cleaner Noise 36
Practical Considerations for Using Portable Air Cleaners 37
SELECTING AND USING A FURNACE FILTER OR OTHER IN-DUCT AIR CLEANER 38
Practical Considerations for Using In-Duct Air Cleaners 38
APPROXIMATIONS OF OPERATIONAL ELECTRICITY COSTS OF PORTABLE AND
IN-DUCT AIR CLEANERS 40
WILL AIR CLEANING REDUCE HEALTH EFFECTS FROM INDOOR AIR POLLUTANTS? 41
Evidence for the Impacts of Air Cleaners on Indoor Pollutant Concentrations 41
Evidence for the Impacts of Air Cleaners on Health Outcomes and/or Biomarkers of Health
Outcomes 42
Summary of the Impacts on Allergy and Asthma Health Outcomes 42
Summary of the Impacts on Cardiovascular Health Outcomes 43
Summary of Health Intervention Studies and Their Limitations 43
Detailed Descriptions of Health Intervention Studies 50
RESEARCH NEEDS 57
FURTHER RESOURCES 57
ACRONYMS AND ABBREVIATIONS 58
GLOSSARY 59
REFERENCES 62
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RESIDENTIAL AIR CLEANERS
This graphic depicts size comparisons for particulate matter (PM) in micrometers (pm).
Note that PM2.5 is not visible to the naked eye.
Fine beach sand
90 pm in diameter
Human hair
50-70 pm in diameter
PM10 PM2.5
<10 pm in diameter <2.5 pm in diameter
Figure 1. The image above depicts the size of fine (PM.,,.) and coarse (PM.,) particulate matter compared to a grain of sand
and human hair.
SUMMARY
Common indoor air pollutants include a wide
variety of particulate matter (PM) and gaseous
contaminants.
Airborne PM ranges in size from a few nanometers
(nm) to tens of micrometers fjjm) and is
composed of both biological and non-biological
matter. Indoor particles are commonly categorized
into coarse particles (PM10) at 10 |jm to 2.5
[jm diameter, fine particles (PM.,,) at 2.5 [jm or
smaller, and ultrafine particles at 1 |jm (PM.)
or smaller. Types of indoor particles, ranked
generally from largest to smallest in size, include
pollen, fibers, fungal spores and fragments, dust,
pet dander, allergens, bacteria, vehicle exhaust
infiltrated from outdoors, viruses, and emissions
from smoking, cooking, and other combustion
sources. Fine particles (PM25) in outdoor air are
known to cause adverse human health effects.
Research on intervention studies summarized in
this document confirms that fine particles are also
a health concern for indoor exposures. To illustrate
their relative sizes, Figure 1 depicts fine and
coarse PM compared with human hair and sand.
Indoor biological particles include microorganisms,
bacterial and fungal spores, and fragments of
those spores. These particles can enter homes
through multiple routes. Bacteria enter homes
from outdoors and are emitted by human and pet
occupants. Fungal spores primarily enter homes
from outdoors and can grow on indoor surfaces
when moisture is present. Fungal spores can grow
inside heating, ventilating, and air-conditioning
(HVAC) systems in the presence of condensation
on cooling coils, drain pans, and internal thermal
insulation or on the surfaces of the air-handling
unit and ductwork.
Gaseous contaminants found indoors include
organic and inorganic compounds. Organic
compounds include a large number of volatile
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W
organic compounds (VOCs) emitted from building
materials, architectural coatings, and consumer
products; semivolatile organic compounds such
as pesticides and fire retardants; and aldehydes
such as formaldehyde from building materials
and other sources. Inorganic compounds include
carbon monoxide and nitrogen oxides emitted
from combustion sources, ozone that infiltrates
from outdoors, and radon that infiltrates from
the soil. Note that EPA does not recommend air
cleaning to reduce the health risks associated
with radon and radon progeny.
The most economical and effective way to
address indoor air pollution is usually to reduce
or eliminate avoidable sources of pollutants and
then to exhaust to the outdoors the unavoidable
particles, gases, and excessive water vapor that
come from normal indoor activities such as
cooking, cleaning, and showering.
Beyond minimizing sources and exhausting
indoor pollutants to outdoors, it is often possible
to dilute pollutant concentrations by ventilating
a home with cleaner outdoor air. However,
opportunities for dilution using outdoor air are
frequently limited by weather conditions or by
contaminants in the outdoor air.
When source reduction and dilution are insufficient,
air-cleaning devices can be useful. These fall into
two general categories: portable air cleaners and
HVAC or furnace filters and other duct-mounted air
cleaners installed in a home's central HVAC system.
Portable air cleaners are stand-alone units that
must be plugged in and turned on to operate.
Portable air cleaners are also commonly called air
purifiers or air sanitizers.
Furnace filters and other duct-mounted air
cleaners are installed either at the base of the air-
handling unit or upstream in return grilles. They
will filter the air whenever the HVAC system fan
is operating. They will not filter the air when the
HVAC fan is not on, even if the air-cleaning device
itself is on or activated.
Many portable and in-duct air cleaners combine
more than one air-cleaning technology to
accomplish their goals.
Two types of air-cleaning technologies are
commonly used in duct-mounted and portable
air cleaners to remove particles from the air:
fibrous media air filters and electronic air
cleaners (including electrostatic precipitators
[ESPs] and ionizers). Fibrous media air filters
remove particles by capturing them on fibrous
filter materials. ESPs and ionizers remove
particles by an active electrostatic charging
process that requires electricity to charge
particles that become attracted to and adhere
to oppositely charged plates or other indoor
surfaces. Another type of electronic air-cleaner
technology, ultraviolet germicidal irradiation
(UVGI), is designed to reduce the number of
viable airborne microorganisms by killing or
deactivating them.
A number of air-cleaning technologies are
designed to either remove gaseous air pollutants
or convert them to (ideally) harmless byproducts
using a combination of physical and chemical
processes. Gas-phase air-cleaning technologies
include adsorbent media air filters such as
activated carbon, chemisorbent media air filters,
photocatalytic oxidation (PCO), plasma, and
intentional ozone generators sold as air cleaners.
Compared to the control of PM, gas-phase
pollutant control is much more complex. Only
adsorbent and chemisorbent media air filters
have been shown to be effective gas-phase air
cleaners for some gaseous pollutants without
producing potentially harmful byproducts,
although not all gaseous air pollutants are
removed equally. Adsorbent media air filters have
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a finite capacity for adsorption and therefore
must contain sufficient sorbent media for the
application and must be replaced regularly.
To use portable air cleaners, furnace filters,
or other duct-mounted air cleaners to good
effect, it is crucial to understand the difference
between two parameters that influence the
performance of air-cleaning devices: efficiency
and effectiveness. The efficiency of an air-
cleaning device is a fractional measure of its
ability to reduce the concentration of pollutants
in the air that passes once through the device.
The fractional efficiency of a device is measured
in a laboratory, where all relevant variables are
controlled. The effectiveness 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.
The most helpful parameter for understanding the
effectiveness of portable air cleaners is the clean
air delivery rate (CADR), which is a measure of a
portable air cleaner's delivery of relatively clean
air, expressed in cubic feet per minute (cfm).
The CADR is a product of the fractional removal
efficiency for a particular pollutant and the airflow
rate through the air cleaner. A higher CADR
relative to the size of the room will increase the
effectiveness of a portable air cleaner. A CADR
can theoretically be generated for either gases
or particles; however, current test standards only
rate CADRs for the removal of particles.
The most helpful parameter for understanding
the efficiency of furnace filters and other in-duct
air cleaners is the fractional removal efficiency
for the polIutant(s) it is designed to remove. The
most widely used fibrous media air filter test
method for duct-mounted particle filters in the
United States is ASHRAE Standard 52.2, which
evaluates the removal efficiency for particles
0.3 to 10 |jm in diameter. Results are reported
as a Minimum Efficiency Reporting Value
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(MERV) ranging from MERV 1 to MERV 16
based on the average removal efficiency across
three particle size ranges: 0.3-1 |jm, 1-3 |jm,
and 3-10 |jm. Other commercially common
proprietary test metrics for in-duct air filters
include the Microparticle Performance Rating
(MPR) and Filter Performance Rating (FPR);
these are proprietary rating systems. In general,
the higher the filter rating, the higher a filter's
removal efficiency for at least one particle
size range. Although standards for testing the
removal efficiency of gas-phase in-duct air
cleaners also exist, they are not yet widely used
and reported.
Research Overview
A comprehensive review of current research (as of
early 2018) indicates the following:
Air Cleaners and Indoor Air Quality
• Intervention studies of air cleaners
operating in homes have consistently
found statistically significant reductions
in indoor exposures to indoor PM25, PM10,
and/or particle number counts with the
use of portable air cleaners, whereas levels
of allergens in dust were only sometimes
affected. Studies of air cleaners in homes
that address gas-phase pollutants are
extremely limited, and consistent reductions
have not been demonstrated.
Most studies have reported reductions in PM
exposures with the use of high-efficiency
particulate air (HEPA) or other high-efficiency
portable air cleaners on the order of approximately
50 percent or higher. Only a few studies
investigated the use of central in-duct particle
filtration, and reductions in PM exposures were
not as consistent, in part because of typically
low system runtimes. Only a few studies have
investigated the effects of gas-phase air cleaners
in homes.
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Air Cleaners and Health
• Most air cleaner intervention studies have
found statistically significant associations
between the introduction and use of
portable air cleaners in homes and at least
one measure or marker of improved health
outcome, although the improvements were
typically modest.
Specific health outcomes or markers of health
outcomes that have been correlated with
portable air cleaner use in homes include allergy
and asthma symptoms and several markers
of cardiovascular effects that are commonly
associated with exposure to PM of both indoor
and outdoor origin. However, most of the health
improvements were relatively small in magnitude
and, when multiple outcomes were measured,
typically only a fraction of health outcomes or
biomarkers of health outcomes were improved. To
date, no studies were found that systematically
investigated whether using sorbent media gas-
phase filtration, PCO, plasma, or ionizer air
cleaners in homes or other buildings has a
positive effect on the health of occupants.
Air Cleaners Must be Operated to be Effective
The amount of time that an air cleaner operates
influences its ability to reduce pollutant
concentrations and associated health risks. If
they are not operating, they will not be effective.
This limits the effectiveness of both categories
of air cleaners.
Typically, air cleaning is limited to less than
25 percent of the 8,760 hours in a year. In the
case of portable air cleaners, some intervention
studies show that after an initial period of
use and enthusiasm, the device is often not
maintained properly, operated less frequently,
turned off completely, or placed into storage,
often because of occupant annoyance related to
noise or other factors.
Portable Air Cleaners and Noise
• Operating noise can influence whether
occupants use portable air cleaners.
Portable air cleaner performance ratings
are determined at maximum airflow and
therefore typically maximum noise levels.
Quantified noise levels are seldom shown on
consumer product packaging.
Objectionable noise levels can reduce usage
and discourage the placement of air cleaners
in sleeping spaces where people spend a large
percentage of their time. Since noise is seldom
quantified or reported in a standardized manner
on consumer packaging, it can be challenging to
compare devices on the basis of noise rating. The
CADR label on product packaging is typically the
highest CADR achievable, which generally occurs
at the highest airflow setting. At lower airflow
settings an air cleaner may have lower noise
production, but it will also be less effective at
pollutant removal.
Furnace Filters and Fine Particulate Matter
• Furnace filters with a MERV 13 and above
rating require at least 50 percent removal
efficiency for 0.3-1 pm particles.
Particle filters that are tested following ASHRAE
Standard 52.2 test method—the most widely
used filter test standard in the United States—
are not required to report their fractional removal
efficiency for the small particles that contribute
most to indoor PM2 5 concentrations unless
they achieve a MERV 11 or above. MERV 11
filters must achieve at least 20 percent removal
efficiency for 0.3-1 |jm particles, while only MERV
13 and above require at least 50 percent removal
efficiency for 0.3-1 |jm particles. Because high
concentrations of fine particles are associated with
health risks—especially in sensitive populations
such as children, the elderly, and those with
existing respiratory health problems like asthma
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and allergies—EPA recommends that consumers
who are concerned about small particles choose
furnace filters with at least a MERV 13 rating
or as high a MERV rating as the system fan and
filter track can accommodate. However, selection
of any increased efficiency media furnace filter—
including MERV 13 to 16 or HEPA—must also
take into account the compatibility of the filter with
the existing ducted HVAC system to ensure that
airflow will not be impeded by the added resistance
of the filter. To accommodate a higher efficiency
furnace filter in an existing home, a trained
professional may need to modify the system.
Furnace Filters and HVAC System Operation
• The effectiveness of furnace filters and other
duct-mounted air cleaners is limited by the
operating hours of fan in the HVAC system
in which they are installed and whether they
are properly maintained.
In some locations, such as where air-conditioning
is not needed or where air-conditioning is
provided by window air conditioners, central HVAC
systems may not operate at all or not for many
months of the year. Low system runtimes can
greatly limit the effectiveness of a furnace filter
or other in-duct air cleaner simply by not passing
air through it long enough to yield substantial
reductions in indoor pollutant concentrations.
Because of these limitations in system operation,
experimental data and theoretical predictions
indicate that for particle removal, medium-high
efficiency furnace filters, such as some MERV 12
filters and most MERV 13 filters, are likely to be
almost as effective as HEPA filters in reducing the
concentrations of most sizes of indoor particles,
including those linked to health effects. However,
field studies have not yet confirmed that central
HVAC system fans operate long enough for high-
efficiency furnace filters and other duct-mounted
air cleaners to reduce concentrations of indoor
particles and gases sufficiently to demonstrably
improve health outcomes. Additionally, no
RESIDENTIAL AIR CLEANERS
filter or air cleaner, regardless of its rating, will
be effective if it is not properly maintained.
Manufacturers provide guidance on how often
filters must be replaced, cleaned, or otherwise
serviced to ensure that they perform as intended.
Byproduct Emissions From Some Air Cleaner
Technologies
Some air cleaning technologies may emit
potentially harmful byproducts during operation.
For example, PCO air cleaners have been
shown to generate formaldehyde, acetaldehyde,
nitrogen dioxide, and carbon monoxide. Plasma
air cleaners have been shown to form particles,
ozone, carbon monoxide, and formaldehyde
as byproducts. Additionally, many electronic
air cleaner devices—including portable and
duct-mounted ESPs, ionizers or ion generators,
uncoated UVGI lamps, and other products that
advertise the use of "plasma," "ions," and other
similar terms—can generate high amounts of
ozone. Ozone is a well-documented lung irritant.
Intentional ozone generators should not be used
in occupied spaces.
No federal agency has approved the use in
occupied spaces of air cleaners that intentionally
emit ozone. Ozone and ozone-generating devices
are also discussed in more detail in EPA's "Ozone
Generators that are Sold as Air Cleaners," which
can be found at www.epa.gov/indoor-air-quality-
iaq/ozone-generators-are-sold-air-cleaners.
The California Air Resources Board mandates
device testing for ozone production following
UL Standard 867, but currently no national
regulation or voluntary program exists that requires
independent measurement and certification
that the production of ozone from these devices
does not reach hazardous levels. Apart from
California Air Resources Board requirements, no
U.S. standard, regulation, or industry consensus
program requires measurement and disclosure of
ozone production by air cleaners.
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Selecting and Using Portable and
In-Duct Air Cleaners
Research suggests that when selecting and using
air-cleaning devices, consumers can make more
informed decisions by keeping in mind these
practical factors:
• Types of air cleaners with documented
improvements in indoor air quality and
health effects: Field-testing and simulation
studies show that high-efficiency furnace
filters (e.g., MERV 13 and above), duct-
mounted air cleaners, and portable air
cleaners with high CADRs can substantially
reduce levels of airborne particles and,
in some cases, gaseous pollutants in a
home. High-efficiency fibrous media filters
(including HEPA-rated filters for portable air
cleaners, and MERV 13 and above furnace
filters for central HVAC systems) and sorbent
media filters with adequate amounts of
media are generally most effective and
have the fewest limitations or adverse
consequences. Furthermore, studies have
shown that portable air cleaners with CADRs
that are adequate for the size of the space
can reduce some adverse health effects
and related biomarkers associated with PM
exposure in sensitive populations such as
children, people with asthma and allergies,
and the elderly, as well as in healthy
individuals.
• Noise from portable air cleaners: Noise is
an important issue with many portable air
cleaners, particularly when operating at
higher airflow rates, because users often
turn them off to avoid the noise. Noise can
be an important factor when selecting an air
cleaner.
• Sizing portable air cleaners: Portable air
cleaners should have a CADR that is large
enough for the size of the room in which
it is operated. For example, an air cleaner
that has a CADR of 250 for dust particles
can reduce dust particle levels to the same
concentration as would be achieved by
adding 250 cfm of clean air to the space
in question. Units tested according to
procedures established by the Association
of Home Appliance Manufacturers (AHAM)
carry an AHAM-Verified® label that suggests
the appropriate maximum room size for the
device. The size rating is intended to provide
an 80 percent reduction in particle levels
(at equilibrium conditions) as compared
to levels without the air cleaner operating.
Portable air cleaners often achieve a high
CADR by using a HEPA filter, although other
technologies can also achieve a high CADR.
• Placement of portable air cleaners: Place
portable air cleaners where the most
vulnerable occupants spend most of their
time. Infants, elders, and asthmatics are
more vulnerable than healthy adults. A
bedroom can be a good place to locate
and operate an air cleaner. Also, place any
portable air cleaners so that their clean air
reaches the breathing zone of occupants
as directly as possible, without obstruction
from furnishings or addition of fine particles
by common sources such as printers.
Otherwise, "short-circuiting" could occur,
in which the output flow does not reach the
intended area. Additionally, manufacturer
instructions may indicate that the air
cleaner be placed a certain distance from
any objects that might obstruct airflow.
• Consider the sequence of air-cleaning
technologies in an air cleaner: Many air
cleaners combine two or more air-cleaning
technologies to accomplish multiple goals.
The order in which individual technologies
are combined with respect to the direction
of airflow can be very important for
determining its effectiveness. For example,
an activated carbon filter installed upstream
of an air-cleaner technology that generates
gaseous byproducts would be less effective
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in limiting indoor concentrations of those
byproducts than an activated carbon filter
installed downstream.
• Installation of furnace filters and duct-
mounted air cleaners: Furnace filters and
duct-mounted air cleaners must have
easy access for regular filter replacement,
inspection, and any required maintenance.
Some furnace filters and duct-mounted air
cleaners may also require HVAC system
modifications for their installation, such as
a wider filter track or additional electrical
power. Any system modification and
installation should be done by a trained
professional.
• Monitoring and control: Some air cleaners
have monitoring and control features, such
as the ability to schedule operation, control
by a smartphone, or monitor filter status.
To the extent that these features result
in more operating hours when the spaces
are actually occupied, and more frequent
cleaning or replacement of filter media, they
should be able to improve the air-cleaning
effectiveness of the device.
• Pollutant sensors and indicators: Some
consumer-grade air cleaners now include
pollutant sensors or indicators of some
indoor pollutant concentrations, but to date
no studies were found that have investigated
their performance over time. Although
they may not be as accurate as more
expensive professional grade sensors, they
may provide useful indicators of relative
pollutant concentrations. These indicators
could provide occupants with immediate
visual feedback that their current activities
are either increasing or reducing pollutant
concentrations. These indicators could
also be used to automatically control the
operation of the device in response to real-
time pollutant concentrations.
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• Removal of moldy odors: Some air cleaners
can remove moldy odors and airborne mold
or bacterial spores and their fragments.
However, air cleaners will not prevent mold
growth, nor will they rid the house of mold.
To permanently remove the source of moldy
odors, it is necessary to remove mold growth
and eliminate the sources of moisture that
allow it to grow.
• Removal of chemical odors: Air cleaners
that are designed only to remove particles
cannot control gaseous pollutants, including
those that contribute to chemical odors.
For example, air cleaners designed only to
remove particles will not remove all of the
odorous compounds or the carcinogenic gas-
phase pollutants from tobacco smoke.
• Costs: Cost may also be a consideration in
using air cleaners. Major costs include the
initial purchase price, maintenance (such
as cleaning or replacing filters and parts),
and operation (such as electricity costs).
The cost of professional installation of an
upgraded media filter or an electronic air
filter in the HVAC system must also be
considered. The most effective air cleaners,
those with high airflow rates and efficient
pollutant capture systems, are generally the
most expensive. Maintenance and operating
costs vary depending on the device, and
these costs should be considered when
choosing a particular unit. Operating
cost is important because air cleaning
is an ongoing process, and units require
filter replacement or cleaning and other
maintenance to remain effective. Although
central HVAC systems can distribute
filtered air to more places throughout the
house, they commonly cost approximately
twice as much to operate as a typical
portable HEPA air cleaner operating for the
same amount of time.
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INTRODUCTION
The best way to address residential indoor air
pollution usually is to control or eliminate the
source of the pollutants and to ventilate the
home with clean outdoor air. But source control
is sometimes impractical as a remedial measure,
and ventilation may be limited by weather
conditions or the levels of contaminants in the
outdoor air.
If the usual methods of managing indoor air
pollutants are insufficient, air-cleaning devices
may be useful. Air filters and other air-cleaning
devices are designed to remove pollutants from
indoor air. They can be installed in the ductwork
of most home HVAC systems to clean the air in
the entire house, or the same technology can
be used in portable air cleaners that clean the
air in single rooms or specific areas. Most air-
cleaning devices are designed to remove particles
or gases, but some destroy, degrade, or transform
contaminants that pass through them.
This publication focuses on air cleaners for
residential use; it does not address air cleaners
used in large or commercial structures such
as office buildings, schools, large apartment
buildings, or public buildings. It should be
particularly useful to residential housing design
professionals, public health officials, and indoor
air quality professionals. In addition to providing
general information about the types of pollutants
affected by air cleaners, this document discusses:
• The types of air-cleaning devices and
technologies available
• Metrics that can be used to compare air-
cleaning devices
• The effectiveness of air-cleaning devices in
removing indoor air pollutants
• Information from intervention studies on the
impact that air cleaners can have on health
and on health markers
• Additional factors to consider when deciding
whether to use an air-cleaning device and, if
so, which type
INDOOR AIR POLLUTANTS
Two main categories of indoor air pollutants
can affect the quality of air in a home: PM and
gaseous pollutants.
PM can be composed of microscopic solids, liquid
droplets, or a mixture of solids and liquid droplets
suspended in air. Also known as particle pollution,
PM can be made up of a number of components,
including acids such as nitric and sulfuric acids,
organic chemicals, metals, soil or dust particles,
and biological contaminants. Among the particles
that can be found in a home are:
• Dust, as solid PM
• Fumes and smoke, which are mixtures of
solid and liquid particles
• Particles of outdoor origin, which are
complex mixtures of solid and liquid
particles
• Biological contaminants, including
viruses, bacteria, pollen, fungal spores and
fragments, dust mite and cockroach body
parts and droppings, and animal dander
Particles exist in a wide range of sizes. Small
particles can be ultrafine, fine, or coarse. Of
primary concern from a health standpoint are fine
particles that have a diameter of 2.5 |jm or less
(i.e., PM25). These fine particles can be inhaled,
and they penetrate deep into the lungs where
they may cause acute or chronic health effects.
Ultrafine particles, smaller than 0.1 |jm (100 nm)
in diameter, penetrate far into the alveolar region
of the lungs and can translocate to the brain via
the olfactory nerve. Coarse particles, between 2.5
and 10 |jm in diameter (i.e., PM2510), usually do
not penetrate as far into the lungs; they tend to
12 www.epa.gov/iaq
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settle in the upper respiratory tract where they
can irritate the eyes, nose, and throat. Large
particles are greater than 10 |jm in diameter,
or roughly one-sixth the width of a human hair.
They can be trapped in the nose and throat and
expelled by coughing, sneezing, or swallowing.
Fine particles are directly emitted into indoor
air from a variety of sources including tobacco
smoke, chimneys and flues that are improperly
installed or maintained, unvented combustion
appliances such as gas stoves and kerosene
or gas space heaters, woodstoves, fireplaces,
electric stoves, printers, incense, candles, and
ozone reactions with emissions from indoor
sources of organic compounds. Fine particles
also include outdoor particles that infiltrate
indoors (such as traffic emissions or wildfire
smoke), viruses, and some bacteria.
Among the smaller biological particles found in
a home are some bacteria, mold and bacterial
fragments and spores, some plant allergens, a
significant fraction of cat and dog dander, and
a small portion of dust mite body parts and
droppings. Larger particles include dust, pollen,
some fungal fragments and spores, a smaller
fraction of cat and dog dander, a significant
fraction of dust mite body parts and cockroach
body parts and droppings, and human skin flakes.
Biological particles such as bacteria and fungal
spores and fragments enter a house by various
routes, including open windows, joints and cracks
in walls, and on clothing and shoes, food, or
pets. Fungi and some bacteria can be found in
either the vegetative or the spore stage of their
life cycle. Vegetative bacteria and fungi are in
the growth and reproductive stage; they are not
spores. Some bacteria form spores, an inactive
stage characterized by a thick protective coating,
to survive harsh environmental conditions. Most
fungi produce tiny spores to reproduce. Fungal
spores will enter the growth and reproductive
stage of their life cycle in locations where
RESIDENTIAL AIR CLEANERS
sufficient moisture and nutrients are available,
such as on basement walls, in refrigerators, on
HVAC coils, on air filters, and in drip pans.
Gaseous pollutants include inorganic gases such
as combustion gases (e.g., carbon monoxide and
nitrogen dioxide), ozone, and organic chemicals
that are not attached to particles. Hundreds of
different gaseous pollutants have been detected
in indoor air.
Sources of indoor combustion gases include
combustion appliances such as gas stoves,
tobacco smoke, and vehicles from which
exhaust infiltrates from attached garages or the
outdoors. Sources of ozone include infiltration
from outdoors and intentional or unintentional
generation by laser printers and some devices
sold as air cleaners.
Sources of airborne gaseous organic compounds
include tobacco smoke; building materials
and furnishings; and products such as paints,
adhesives, dyes, solvents, caulks, cleaners,
deodorizers, cleaning chemicals, waxes, hobby
and craft materials, and pesticides. Organic
compounds may also come from cooking;
human, plant, and animal metabolic processes;
and outdoor sources.
Radon is a colorless, odorless, radioactive gas that
can be found in indoor air. It comes from radium
in natural sources such as rock, soil, ground
water, natural gas, and mineral building materials
(e.g., granite countertops). As uranium breaks
down, it releases radon, which in turn produces
short-lived radioactive particles called "progeny,"
some of which attach to dust particles. Radon
progeny may deposit in the lungs and irradiate
respiratory tissues. Radon typically moves through
the ground and into a home through cracks and
holes in the foundation. Radon may also be
present in well water and can be released into
the air when that water is used for showering and
other household activities. In a small number
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RESIDENTIAL AIR CLEANERS
of homes, building materials also can give off
a significant amount of radon. EPA does not
recommend air cleaning to reduce the health risks
associated with radon and radon progeny.
THREE STRATEGIES TO REDUCE
INDOOR AIR POLLUTANTS
Three basic strategies to reduce pollutant
concentrations in indoor air are source control,
ventilation, and air cleaning.
Source control eliminates individual sources
of pollutants or reduces their emission. It is
usually the most effective strategy for reducing
pollutants. Many sources of pollutants in the
home can be avoided or removed (U.S. EPA
1995). For example, solid wood or alternative
materials can be used in place of pressed wood
products that are likely to be significant sources
of formaldehyde. Smokers can smoke outdoors.
Combustion appliances can be adjusted to
decrease their emissions. Any areas contaminated
by microbial growth should not only be cleaned
and dried, but the underlying moisture problem
should also be addressed.
Ventilation with outdoor air is also a strategy
for diluting indoor air pollutant concentrations,
provided that the outdoor air is relatively clean
and dry or that it can be made so through
mechanical means such as filtering. Outdoor air
enters buildings in three ways. Small amounts
of air are constantly entering by infiltration
through the building enclosure. Larger amounts
enter when windows and doors are left open for
extended periods and can also be brought in by
continuous supply or exhaust fans.
Most existing residential forced air heating
systems and air-conditioning systems in the
United States do not intentionally bring outdoor
air into the house. However, residential HVAC
design practice is changing. Current consensus
standards and some residential buildings codes
have recently changed to encourage or require
deliberate and continuous outdoor air ventilation.
To date, however, no national regulatory
requirement or standard exists that requires
removal of fine particles or gases from outdoor air
used for continuous ventilation.
Local exhausting of air from the kitchen when
cooking and from bathrooms when showering
provides occupants an effective way to achieve
reductions in the otherwise unavoidable high
concentrations of water vapor, particles, and
gases that result from daily household activity.
Note, however, that the act of exhausting air
from the bathrooms or kitchen pulls outdoor air
into that home. So to gain the greatest benefit
from exhaust, it is important any replacement
ventilation air be clean and dry.
Air cleaning has proven useful when used along
with source control and ventilation, although
it is not a substitute for either method. Air
cleaning alone cannot ensure adequate indoor
air quality where significant sources are present,
when exhaust and outdoor air ventilation are
insufficient, or when the operating hours of an
air-cleaning device are not sufficient to reduce
indoor pollutant concentrations. The remainder
of this document focuses on air cleaning.
For more information, see also the ASHRAE
Position Document on Filtration and Air Cleaning
(ASHRAE 2015a).
TYPES OF AIR CLEANERS
There are two basic categories of air cleaners:
portable air cleaners, and HVAC/furnace filters
and other duct-mounted air cleaners. Stand-alone
portable air cleaners are generally designed to
filter or clean the air in a single room or area.
Furnace filters and other duct-mounted air
cleaners are installed in a home's central HVAC
system and can provide filtered or cleaned air to
many parts of a home, but only when the HVAC
system fan is operating.
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RESIDENTIAL AIR CLEANERS
Furnace Filter
Figure 2. This graphic depicts the installation of an air filter in a
typical furnace.
Furnace filters and other duct-mounted air-
cleaning devices are typically installed in the return
ducts of HVAC systems, as shown in Figure 2, They
are installed either at the base of the air-handling
unit or upstream in return grilles. The typical
low efficiency furnace air filter is a simple air
cleaner that captures particles in the airstream to
protect fan motors, heat exchangers, and ducts
from soiling. Such filters are not designed to
improve indoor air quality, but the HVAC system
can be upgraded by using more efficient air filters
to remove additional particles. Other air-cleaning
devices such as ESPs, UVGI air cleaners, and a
number of gas-phase filters are sometimes used in
the ductwork of home HVAC systems. These air-
cleaning technologies are described in more detail
in subsequent sections.
Portable air cleaners are available as small
tabletop units and larger console units. They
are used to clean the air in a single room, as
shown in Figure 3, but not in an entire house.
The units can be moved to wherever continuous
and localized air cleaning is needed. Larger
fii'i* i mm I
ijy
sj J: si sjs.,7^ ¦
Portable Air Cleaner
Figure 3. This image shows an example of a typical portable air cleaner
installation.
console units may be useful in houses that are
not equipped with forced air heating and/or air-
conditioning systems. Portable air cleaners are
also commonly called air purifiers or air sanitizers.
The basic components of a portable air cleaner
include a filter or other air cleaning technology
and a fan that propels air through that filter/air
cleaner. Portable air cleaners may also have a
panel filter with bonded fine granules of activated
carbon, an activated carbon filter encased in
a frame, or other sorbent mixtures to remove
gases and odorous compounds. Beyond simple
filtration and sorption of odorous compounds
using carbon, some portable air cleaners add
further technologies to increase pollutant removal,
inact.ivat.ion, or conversion. Technologies can
include electrostatic precipitation, ion generation,
or ultraviolet (UV) lamps in combination with
catalysts for photocatalytic conversion of gaseous
contaminants. Some units marketed as having
the quietest operation may have no fan; however,
units that do not have a fan typically are much
less effective than units that have a fan.
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RESIDENTIAL AIR CLEANERS
UNDERSTANDING EFFICIENCY
VERSUS EFFECTIVENESS
To use portable air cleaners, furnace filters, or
other duct-mounted air cleaners to good effect, it
is crucial to understand the difference between
two parameters that influence the performance of
air-cleaning devices: efficiency and effectiveness.
Efficiency: The efficiency of an air-cleaning device
is a fractional measure of its ability to reduce
the concentration of pollutants in the air that
passes once through the device. The efficiency
of a device is measured in a laboratory, where
all relevant variables are controlled. Efficiency
ratings allow comparison between different
devices when they are tested under the same
conditions (e.g., the same flow rate, air speed,
pollutant concentrations).
Effectiveness: The effectiveness of an air-cleaning
device or system is a measure of its ability to
remove pollutants from the spaces in which it is
operated.
The effectiveness of the device or system is
a function of its use in real-world situations.
While this can be simulated under controlled
conditions in a laboratory test space, the in-use
effectiveness of any device depends on many
factors including its location, installation, airflow
rate, and operating hours. In fact, these factors
may have stronger influences on its effectiveness
than does its laboratory-tested efficiency. As an
example, while a given device can have a high
laboratory-measured and certified efficiency,
its effectiveness (i.e., its effect on pollutant
concentrations in the occupied space that it
serves) will be zero if no air passes through that
device because it is turned off, or if very little air
passes through the device because its airflow rate
is too low or if its operation is intermittent, or if
its filter media is so clogged that little or no air
can pass through it.
In addition, the air cleaner removal rate must
also be competitive with other removal processes
that occur within the space to be effective
(Batterman et al. 2005; Shaughnessy and Sextro
2006). Other removal mechanisms within the
space include surface deposition (for particles)
or adsorption (for gases), indoor air reactions
(typically for gases), and ventilation (outdoor air
exchange). For example, an air cleaner operating
in a space with multiple open windows may be
less effective than when operating in a space with
closed windows because ventilation through the
open windows is likely to be a more dominant
removal mechanism (assuming outdoor air is
cleaner than indoor air).
TYPES OF AIR-CLEANING
TECHNOLOGIES
Within each category of air cleaner, one or
more air-cleaning technologies may be used
to accomplish its goals, and some air-cleaning
technologies have clear advantages over others.
The available technologies vary in the type
of pollutant that they can remove or reduce
(e.g., different PM sizes, different kinds of
gases, airborne microbes), their mechanism of
action (e.g., pollutant collection, conversion,
inactivation, destruction), and the potential side
effects of their use (e.g., primary energy use
requirements, secondary impacts on equipment
performance, direct emissions of pollutants,
secondary pollutant formation) (ASHRAE
2008; NAFA 2007). Table 1 summarizes the
most commonly used air-cleaning technologies
available in products currently on the market
and the pollutants they are designed to control.
Each technology is explained in more detail in the
following sections. The list does not include other
potential air-cleaning strategies such as material
coatings that are designed to passively remove
gaseous pollutants or biofiltration strategies such
as ornamental potted plants or active bio-walls.
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CD
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ID
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Table 1. Summary of Air-Cleaning Technologies
Targeted
indoor air
pollutant(s)
Air-cleaning
technology
Mechanism(s) of action
Test standards (and rating metrics)
Advantages
Disadvantages
Fibrous filter
media
Particles
Collection: Filter fibers capture
particles
* Mechanical filtration media rely on
mechanical forces alone
* Electrostatically-charged (i.e.,
"electret") media use mechanical
fibers with an electrostatic charge
applied to collect oppositely charged
particles, enhancing removal
efficiency
* If rated efficiency is high, they can have
excellent removal capabilities for many
particle sizes
* Mechanical media filters see improved
efficiency with loading
* Regular replacement is required
* Used particle filters can be a source of
sensory pollution/odors
* High pressure drops on some fibrous media
filters can negatively impact HVAC systems
* Electret media filters see reduced efficiency
with loading
* Confusing number of test standards and
rating metrics
Filters:
• ANSI/ASHRAE Standard 52.2 (MERV)
• ISO 16890 (ePM)
• ISO 29463 (HEPA)
• Proprietary test standards (FPR, MPR)
Portable air cleaners:
• AHAM AC-1 (CADR)
Electrostatic
precipitation
(ESP)
Particles
Collection: Corona discharge wire
charges incoming particles, which
collect on oppositely charged plates
* Can have high removal efficiency for a
wide range of particle sizes
* Low pressure drop and minimal impacts
on HVAC systems
* Low maintenance requirements
* Sometimes ESPs have high ozone and
nitrogen oxide generation rates
* Efficiency typically decreases with loading
and plates require cleaning
* High electric power draw requirements
ANSI/UL Standard 867 for electrical
safety and ozone emissions (similar to
IEC 60335-2-65) (pass/fail; no rating
metric)
Ionizers
(i.e., ion
generators)
Particles
Collection: Similar to ESP, ionizers
use a high-voltage wire or carbon
fiber brush to electrically charge air
molecules, which produces negative
ions that attach to airborne particles;
the charged particles are then collected
either on oppositely charged plates in
the air cleaner or become attracted
to other surfaces in the room and
deposited elsewhere
* Typically low power draw requirements
* Quiet
* Low maintenance
* Generates ozone
* Typically low effectiveness because of very
low airflow rates and clean air delivery rates
(CADRs)
None specific to ionizers, although
AHAM AC-1 can be used to measure
CADR
Ultraviolet
germicidal
irradiation
(UVGI)
Microbes
Destruction: UV light kills/inactivates
airborne microbes
* Can be effective at high intensity with
sufficient contact time
* Can be used to inactive microbes on
cooling coils and other surfaces
* Uncoated lamps can generate ozone
* Potential for eye injury
* Effectiveness increases with lamp intensity,
which is typically low in residential UVGI air
cleaners
* High electrical power draw requirements
* Inactivates but does not remove microbes
Air irradiation:
• ANSI/ASHRAE Standard 185.1
Surface irradiation:
• ANSI/ASHRAE Standard 185.2
Adsorbent
media
Gases
Collection: Gases physically adsorb
onto high-surface-area media (typically
activated carbon)
* Potential for high removal efficiency for
many gaseous pollutants in air cleaners
with a sufficient amount of media for
the application
* No byproduct formation
* Regular replacement is required because
its adsorption capacity is exhausted and
physical adsorption is a reversible process,
meaning pollutants may not be permanently
captured
* Effectiveness of many consumer-grade
systems with small amounts of activated
carbon is unknown
* High pressure drops on some sorbent media
filters can negatively impact HVAC systems
* Different removal efficiency for different
gases at different concentrations
* Standard test methods are not widely used
Media:
• ANSI/ASHRAE Standard 145.1 (no
rating metric)
In-duct air cleaners:
• ANSI/ASHRAE Standard 145.2 (no
rating metric)
No effectiveness standards
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Table 1 (continued). Summary of Air-Cleaning Technologies
Air-cleaning
technology
Targeted
indoor air
pollutant(s)
Mechanism(s) of action
Advantages
Disadvantages
Test standards (and rating metrics)
Chemisorbent
media
Gases
Collection: Gases chemically adsorb
onto media coated or impregnated with
reactive compounds
* Potential for high removal efficiency for
many gaseous pollutants
* Chemisorption is an irreversible process,
meaning pollutants are permanently
captured
* Regular replacement is required because its
chemisorption capacity is exhausted
* Effectiveness of many consumer-grade
systems is unknown
* High pressure drops on some sorbent media
filters can negatively impact HVAC systems
* Different removal efficiency for different
gases at different concentrations
Media:
• ANSI/ASHRAE Standard 145.1
(no rating metric)
In-duct air cleaners:
• ANSI/ASHRAE Standard 145.2
(no rating metric)
No effectiveness standards
Catalytic
oxidation
Gases
Conversion: Most utilize photocatalytic
oxidation (PCO) in which a high-
surface-area medium is coated
with titanium dioxide as a catalyst;
incoming gases adsorb onto the media
and UV lamps irradiate and activate the
titanium dioxide, which reacts with the
adsorbed gases to chemically transform
them
* Can degrade a wide array of gaseous
pollutants (e.g., aldehydes, aromatics,
alkanes, olefins, halogenated
hydrocarbons)
* Can be combined with adsorbent media
to improve effectiveness
* Can generate harmful byproduct such as
formaldehyde, and acetaldehyde, and ozone
* No standard test methods
* Often relatively low removal efficiency for
many indoor gases, but high variability in
removal for different gases
* Lack of field studies to validate
performance
* Catalyst often has a finite lifespan
None specific to PCO
Plasma
Gases
Conversion: Electric current is applied
to create an electric arc; incoming
gases are ionized and bonds are broken
to chemically transform the gaseous
pollutants
* Can have high removal efficiency
* Can be combined with other air-
cleaning technologies (e.g., PCO) to
improve performance and minimize
byproduct formation
* Wide variety of plasma generation types
yields confusion on how a product actually
works
* Byproducts are formed from many
plasma technologies, including particles,
ozone, formaldehyde, carbon monoxide,
chloroform, nitrogen oxides, and a large
number of other organic gases
* Most studies have investigated gaseous
removal while fewer have evaluated particle
removal
None specific to plasma
Intentional
ozone
generation
Gases
Conversion: Intentional generation
of ozone using corona discharge, UV,
or other method to oxidize odorous
compounds and other gases
* Reacts with many indoor gases
* Can be combined with other less-
harmful technologies such as adsorbent
media
* High ozone generation rates
* High amounts of byproduct formation
* Can cause degradation to indoor materials
None specific to ozone generators
Note that "Gases" are inorganic gases (e.g., carbon monoxide, nitrogen dioxide, ozone) and organic gases (e.g., volatile organic compounds, aldehydes).
-------
Passive material coatings (Darling et al. 2016)
and active bio-walls (Soreanu et al. 2013; Waring
2016) have shown some promise, but they are
not widely commercially available, and published
research on their effectiveness remains limited.
Potted plants have been shown to be ineffective
and impractical for pollutant removal because
there is no active airflow and the number of plants
required to effectively clean air is not feasible in
most environments (Cruz et al. 2014; Girman et
al. 2009; Waring 2016).
Air-Cleaning Technologies Used for
Removing Particles
Two types of air-cleaning technologies are
commonly used in duct-mounted and portable air
cleaners to remove particles from the air: fibrous
media air filters and electronic air cleaners.
Air-cleaning devices designed only to remove
particles are incapable of controlling gases and
some odors. For example, they will not remove
the odor and many of the carcinogenic gas-
phase pollutants from tobacco smoke and the
musty/moldy odor from microbial contamination.
Particles of liquid tobacco smoke trapped by
an air filter may give off odorous organic gases
(Offermann et al. 1992).
Particle size and mass affects the performance of
both types of particulate air-cleaning technologies
because particles must first be suspended in the
air to be removed. Whether installed in the ducts
of HVAC systems or used in portable air cleaners,
most air filters have a good efficiency rating
for removing coarse particles. These particles
include dust, pollen, some mold spores, animal
dander, and particles that contain dust mite and
cockroach body parts and droppings. However,
because these larger particles settle out of the air
and onto surfaces rather rapidly, air filters are not
likely to remove them effectively from the home
(Institute of Medicine 2000; Shaughnessy and
Sextro 2006; Wood 2002). Therefore, since many
RESIDENTIAL AIR CLEANERS
indoor allergens are large particles, effective
allergen control requires routine cleaning and
dust control. For more on allergen control, visit
www. e p a. go v/ast h m a.
Although human activities such as walking,
sweeping, and vacuuming can resuspend
particles, most of the larger particles will resettle
before they enter the HVAC system or portable
air cleaner to be removed by a particle air filter.
It should also be noted that a significant fraction
of cat and dog allergens and a small portion of
dust mite allergens associated with mite feces are
carried on small particles. Consequently, they are
more easily dispersed throughout a house, remain
airborne longer, and are more likely to be removed
by air cleaners (Custovic 1998; Luczynska 1988).
Fibrous Media Air Filters
Fibrous media air filters remove particles by
capturing them onto fibrous filter materials.
Fibrous media filters vary widely in their ability
to remove particles. Particle removal efficiency
depends on a number of parameters including
particle size, face velocity, filter thickness, filter
porosity, filter fiber dimensions, dust loading
conditions, and whether or not the media are
modified by the manufacturer to initially have an
electrostatic charge on the fibers (e.g., electret
vs. non-electret media). In general, fibrous media
filters without an electrostatic charge tend to
increase in efficiency with dust loading over time,
and fibrous media filters with an electrostatic
charge initially tend to decrease in efficiency with
dust loading as the charge is diminished over time.
However, filters that become excessively loaded
will tend to decrease the effectiveness of a furnace
filter or portable air cleaner because of reduced
airflow through the filter and/or increased bypass
airflow around a clogged filter, so it is important to
follow manufacturer recommendations for regular
filter replacement.
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RESIDENTIAL AIR CLEANERS
Test Metrics for Fibrous Media Air Filters
Manufacturers use a number of test standards
to evaluate the particle removal efficiency of
fibrous media air filters. Removal efficiency is
typically characterized for different particle sizes
(for furnace air filters) or for different particle
sources and sizes (for portable air cleaners). The
most widely used fibrous media air filter test
method for duct-mounted particle filters in the
United States is ASHRAE Standard 52.2, which
is a national consensus standard that evaluates
the removal efficiency for particles 0.3 to 10
|jm in diameter. Results are reported as a MERV
ranging from MERV 1 to MERV 16 based on the
average removal efficiency across three particle
size ranges, including 0.3-1 |jm, 1-3 |jm, and
3-10 |jm. Other test metrics for in-duct air filters
include the proprietary MPR (Micro-particle
Performance Rating) and FPR (Filter Performance
Rating). None of these test standards measures
the removal efficiency of particles smaller than
0.3 |jm, although it is technically possible to
measure below 0.3 |jm, as frequently is done by
research laboratories.
In general, the higher the MERV rating, the
higher a filter's removal efficiency for at least one
particle size range tested in the ASHRAE 52.2
test standard. However, only MERV 11 filters
and above are explicitly tested for their ability
to remove the smaller 0.3-1 |jm particles that
are of greatest health concern because they
make up a significant fraction of PM25 mass
concentrations. MERV 11 filters must achieve
at least 20 percent removal efficiency for 0.3 to
1 |jm particles, while only MERV 13 and above
require at least 50 percent removal efficiency for
0.3 to 1 |jm particles. It should also be noted
that a recent filter test standard published by the
International Organization for Standardization
(ISO 16890- 1:2016: Air Filters for General
Ventilation-Part 1: Technical Specifications,
Requirements and Classification System Based
Upon Particulate Matter Efficiency [ePM]) was
developed to explicitly address particle removal
on the basis of PM10, PM25, and PM2 mass
concentrations, but it is not yet widely used in
the United States (ISO 2016; Stephens 2018;
Tronville and Rivers 2016).
High-Efficiency Particulate Air (HEPA) Filters
In residential air cleaners, filters described as
being HEPA filters are generally equivalent to
MERV 16 and offer the highest available particle
removal efficiency of fibrous media air filters for a
wide range of particle sizes.
Note that, in health care and industrial settings,
the HEPA designation has more explicit and
narrowly defined performance characteristics, and
more rigorous test standards are applied to its
use. While the HEPA-designated home air filters
usually perform at high levels comparable to a
MERV 16, there is no widely accepted definition
of HEPA performance in consumer products.
Thus, they are unlikely to be equivalent in
performance to HEPA-designated filter systems
used in health care buildings and industrial
processes, but still have very high removal
efficiency (i.e., usually 99% or higher) for the
reported particle sizes tested.
Types of Fibrous Media Air Filters
Flat or panel filters are relatively inexpensive
filters generally consisting of coarse glass fibers,
coated animal hair, vegetable fibers, synthetic
fibers (such as polyester or nylon), synthetic
foams, metallic wools, or expanded metals and
foils. The filter media may be pre-treated by the
manufacturer with a viscous substance, such as
oil, that causes particles to stick to the fibers.
Flat or panel air filters typically have a MERV of
1 to 4 and thus have very low removal efficiency
for most particle sizes, albeit with slightly higher
efficiency for large particles (Macintosh et al.
2008; Stephens and Siegel 2012, 2013). These
filters are usually about 1-2 inches thick. They
are commonly used in residential furnaces and
air-conditioning systems, and they are also often
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RESIDENTIAL AIR CLEANERS
used as pre-filters for higher efficiency filters. For
the most part, such filters in in-duct applications
are used only to protect the HVAC equipment
from the buildup of unwanted materials on fan
motors, heat exchangers, and other surfaces,
rather than to protect occupants from exposure to
airborne fine particles.
Pleated, extended surface, and unpleated pad
filters typically have a MERV of 5 to 13 or
higher and generally have higher particle removal
efficiency for most particle sizes compared to
pane! filters. However, their removal efficiency
for smaller particles varies substantially by MERV
and can even vary within different makes and
models of filters with the same MERV rating (U.S.
EPA 2008). Pleating the filter medium increases
surface area, reduces air velocity through the filter
media, and allows the use of smaller fibers and
increased packing density of the filter without a
large drop in airflow rate. Additionally, these filters
often have an extended lifespan because of their
increased surface area. A wire frame in the form
of a pocket or V-shaped cardboard separators may
be used to maintain the pleat spacing. The media
used in pleated filters can be fiber mats, bonded
glass fibers, synthetic fibers, cellulose fibers, wool
felt, and other cotton-polyester material blends.
The airflow resistance of these filters generally, but
not necessarily, increases as the MERV increases
for a given thickness. The reason that airflow
resistance does not necessarily increase with MERV
is that higher MERV-rated filters often use more
filter media by increasing the pleating and the filter
thickness. However, filters with electrostatically
charged media can have higher MERV ratings
without an increased airflow resistance. Three main
types of electrostatically charged media are used:
resin wool, a plastic film, or a fiber called electret
(an electrostatically sprayed polymer). Their
electrostatic charge attracts and captures particles.
The fibers of electret filters are somewhat larger
than the fibers of other flat filters, resulting in
relatively low pressure drop and greater efficiency
in filtering smaller particles.
Higher efficiency filters with a MERV of 14 to 16
will typically have a higher average resistance to
airflow than medium-efficiency filters of the same
thickness, although most manufacturers now rely
on extended depth filters and extensive pleating
to achieve these high MERV ratings with low
resistance to airflow.
H EPA filters are another type of pleated filter.
They also have very deep pleats with a much
larger surface area than conventional pleated
filters. Consequently, they remove fine and
ultrafine particles with higher efficiency than
lower rated fibrous media air filters.
Figure 4 shows an example of several
commercially available residential fibrous media
air filter products for central in-duct applications,
ranging from 1 inch fiberglass MERV 4 filters to
5-inch deep pleated MERV 16 filters.
1-irich depth
5-inch depth
1 r
I
5s
ii
c.. 3
1 ;
1
^5-
MERV 4
MERV 6
MERV 11
MERV 10
MERV 13
Scir
[ J;. J
MERV 16
Figure 4. Example of several commercially available residential fibrous media air filter products for central in-duct applications.
Image credit: Brent Stephens.
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RESIDENTIAL AIR CLEANERS
Practical Considerations for Using Fibrous Media
Air Filters
The performance of fibrous media air filters in
residences depends not only on the removal
efficiency of the media, but also on factors such
as the:
• Indoor particle size and size-specific mass
concentrations
• Amount of dust loaded on the air filter
• Airflow rate, velocity, and resistance to
airflow through the filter media
• Bypass airflow that flows around the air filter
because of poor installation
• System or device runtime, which governs
how much air passes through the filter
Particle size greatly affects the removal efficiency
of, and the likelihood of removal by, fibrous
media air filters. Most fibrous media air filters
have a U-shaped removal efficiency curve that
varies by particle size, in which the highest
removal efficiency occurs for both the largest
(e.g., > 3 |jm) and the smallest (e.g., < 0.03 |jm)
particles (Figure 5). However, these same particle
sizes also tend to have the highest deposition
rates indoors, meaning that they deposit onto
surfaces rapidly (U.S. EPA 2008). This means
that deposition to surfaces and removal by filters
compete with each other for particle removal
and that even a very high-efficiency filter may
not have as large of an effect on indoor particle
concentrations as expected (Lee et al. 2015).
Further, because filter removal is a strong
function of particle size, the underlying size
distribution of indoor particles inside the home
can greatly influence the magnitude of reductions
in PM mass concentrations (Azimi et al. 2014,
Stephens, 2018).
MERV16
MERV14
MERV10
MERV12#1
MERV12 #2
MERV7 #2
MERV6
Removal Efficiency of HVAC Filters
HEPA
MERV16
•MERV14
MERV12 #2
MERV12 #1
MERV10
MERV8
MERV7 #2
• • • MERV7 #1
- MERV6
MERV5
0
0.001
0.1
dp ftjm)
Figure 5. Typical size-resolved removal efficiency curves for new (clean) fibrous media air filters rated by
the MERV metric as tested in reported in Azimi et al. (2014). Reprinted from Atmospheric Environment,
Vol. 98, Parham Azimi, Dan Zhao, and Brent Stephens, Estimates of HVAC filtration efficiency for fine and
ultrafine particles of outdoor origin, pages 337-346, copyright (2014), with permission from Elsevier.
22 www.epa.gov/iaq
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RESIDENTIAL AIR CLEANERS
Dust loading will affect the removal efficiency
of fibrous media air filters in different ways for
different particle sizes. Although it is difficult
to make generalizations with available data
because filter products vary so widely, filters that
rely on mechanical means alone for removing
particles typically have an improved efficiency
for some particle size ranges (particularly coarse
particles) as they become loaded with dust over
time (Figure 6) (Hanley et al. 1994; Hanley and
Owen 2003; Owen et al. 2003; U.S. EPA 2008).
Conversely, the removal efficiency of electret
filters sometimes decreases for some particle
sizes (including fine and ultrafine particles) as
the media becomes loaded with particles because
the charge is diminished over time (Figure 7)
(Hanley et al. 1994; Hanley and Owen 2003;
Owen et al. 2003; U.S. EPA 2008). Further, both
the initial and final operating resistance of a fully
dust-loaded filter must also be accounted for in
the design of a system and filter combination
because it is the maximum resistance against
which the fan operates. It is also worth noting
that filter loading and pressure drop increases
are a function of many factors, including filter
type, removal efficiency over time, indoor particle
concentrations, and system runtimes (Stephens et
al. 2010; Waring and Siegel 2008).
Initial
2 weeks
4 weeks
6 weeks
8 weeks
10 weeks
12 weeks
Particle Diameter ((jm)
Figure 6. Example size -resolved removal efficiency curves for new (Ml RV 5 when clean) and loaded non-
eiectret fibrous media air filters as tested in reported in Hanley and Owen (2003). ©ASHRAE from ASHRAE
Research Project Final Report 1190-RP.
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RESIDENTIAL AIR CLEANERS
2 weeks
4 weeks
8 weeks
10 weeks
Particle Diameter (pm)
Figure 7. Example size-resolved removal efficiency curves for new (MERV 11 when clean) and loaded
electret fibrous media air filters as tested in reported in Hanley and Owen (2003). ©ASHRAE from ASHRAE
Research Project Final Report 1190-RP.
Airflow rate, velocity, and resistance to
airflow through the filter media will affect the
performance of fibrous media air filters installed
in any system that has a fan. The pressure drop
across fibrous media filters is typically greater
than that in electronic air cleaners and will
slowly increase over the filter's useful life as
it becomes loaded over time (Stephens et al.
2010). Flat or panel filters are typically only
1-2 inches thick, have low airflow resistance,
and are relatively inexpensive. Pleated or
extended surface filters of the same thickness
will typically have a higher pressure drop and
a higher resistance to airflow. However, deeper
pleated or extended surface filters, which may
be as much as 4-12 inches thick, will increase
the area of the filtration medium and limit
the airflow resistance of the filter. Selection
of any increased efficiency media filter must
also take into account the compatibility of the
filter with the existing ducted HVAC system in
place to ensure that airflow will not be impeded
by the added resistance. Modifications to the
system may be required to install a retrofit to
accommodate a higher efficiency filter media.
Additionally, filters installed at the return grille
rather than at the air-handling unit can also have
a smaller effect on the overall airflow resistance
because they can often be larger in both area
and thickness.
Bypass airflow that flows around an air filter
because of poor installation will reduce air
filter effectiveness. The paths traveled by
the air through a filter installed in a portable
air cleaner or in a central HVAC system are
important in determining effectiveness (e.g., see
Figure 8). Homeowners should install furnace
filters and duct-mounted air cleaners in HVAC
systems such that leakage of air bypassing
the filter is minimized; it is essential to follow
the manufacturer's installation instructions.
Duct-mounted air filter effectiveness can be
substantially reduced because of air leakage
flowing around a filter installed in a poorly
matched or poorly constructed filter frame or
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RESIDENTIAL AIR CLEANERS
gasket (VerShaw et al. 2009). Another form
of bypass airflow also includes flow from
unconditioned spaces through return duct
leakage, which can circumvent the filter if it is
installed at a return grille, rather than at the base
of an air-handling unit.
Figure 8. Example of large amounts of bypass airflow
around a filter in an air-handling unit because of improper
installation combined with excessive loading that increased
the pressure drop across the filter beyond what the filter was
capable of supporting. Photo credit: Brent Stephens.
Electrostatic Precipitators (ESPs) and Ionizers
ESPs and ionizers are electronic air cleaners
that use a powered electrostatic process to
charge particles, which then become attracted to
oppositely charged plates or other indoor surfaces
to remove airborne particles.
ESPs use a high-voltage wire to charge incoming
particles, which are then collected onto oppositely
charged plates inside the air cleaner. ESPs
remove and collect small airborne particles and
often have an initial single-pass removal efficiency
of 60 percent or more for most particle sizes,
increasing to as much as 95 percent depending
on the airflow rate (the lower the airflow rate,
the greater the removal efficiency) (Morawska
et al. 2002), This efficiency will be highest for
clean ESPs, but their efficiency decreases as the
collecting plates become loaded with particles
(Howard-Reed et al. 2003; Wallace et al, 2004).
ESPs can also have different removal efficiencies
for particles with different compositions, as the
electrical properties of some particles will affect
their ability to hold a charge.
Ionizers, or ion generators, use a high-voltage
wire or carbon fiber brush to electrically charge
air molecules, which produces negative ions that
attach to airborne particles. Subsequently, the
charged particles can attach to nearby surfaces
such as walls or furniture (i.e., plate-out), or to
one another, and settle faster. Ion generators are
the simplest form of electronic air cleaner and are
available as tabletop, portable, or ceiling mounted
units. However, because ionizers typically do
not utilize fans to move air past the air cleaner,
ionizers typically have very low CADRs for most
particle sizes (Waring et at, 2008). Additionally,
the charged particles that result from ionizer
operation will deposit on and soil room surfaces
such as walls and curtains (Melandari et al. 1983;
Offermann et al. 1985). Because these deposited
particles remain in the room or area, they may be
resuspended from the surfaces when disturbed by
human activities such as walking or vacuuming,
especially those larger than approximately 2 pm
(Ferro et al. 2004; Qian and Ferro 2008).
Possible Negative Effects of Particle Charging
Another factor to consider related to ion generators
is the effect of particle charging on deposition
in the respiratory tract. Experiments have shown
that particle deposition in the respiratory tract
increases as particles become charged, so using
ion generators may not reduce the dose of particles
to the lungs (Melandari et al. 1983; Offermann
et al. 1985). The effect of charge on very fine
particles results in their higher deposition rate in
the lungs compared to that of uncharged particles.
Additionally, ESPs and ionizers may make a
crackling sound as they accumulate dust, which
may be a nuisance to some occupants.
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RESIDENTIAL AIR CLEANERS
Cautions Concerning Ozone Production by ESPs
and Ionizers
Like fibrous media air filters, ESPs and ionizers
can be installed in HVAC systems or used in
portable units. Although ESPs and ionizers
remove small particles (including ultrafine
particles), they do not remove gases or odors
(Poppendieck et al. 2014; Sultan et al. 2011;
Waring et al. 2008). And because ESPs and
ionizers use high voltage to generate ionized
fields, they may produce ozone, either as a
byproduct or by design (U.S. EPA 2014). Ozone is
a lung irritant that poses risks to health.
Some portable air cleaners that use ESPs and
ionizers produce ozone as a byproduct (Consumers
Union 2005; Waring et al. 2008; Jakober and
Phillips 2008). Some makes and models of
ESPs and ionizers can increase indoor ozone
concentrations that can even exceed public health
standards (Morrison et al. 2014). The California
Air Resources Board, under Title 17 Regulation for
Limiting Ozone Emissions from Indoor Air Cleaning
Devices (California Code of Regulations 2009),
certifies air cleaners in regard to ozone production.
The Title 17 Regulation relies on a test method
for evaluating ozone emissions from air cleaners
described in ANSI/UL Standard 867 (UL 2011),
which is also similar to the method described in
I EC 60335-2-65 (IEC 2015).
Also, even at concentrations below public health
standards, ozone reacts with chemicals emitted
by common indoor sources such as household
cleaning products, air fresheners, deodorizers,
certain paints, polishes, wood flooring, carpets,
and linoleum. The chemical reactions produce
harmful byproducts that may be associated
with adverse health effects in some sensitive
populations. Byproducts that may result from
reactions with ozone include ultrafine particles,
formaldehyde, other aldehydes, ketones, peroxides,
organic acids (Shaughnessy and Sextro 2006; U.S.
EPA 2014; Wechsler 2006). Ozone and ozone-
generating devices are discussed in EPA's "Ozone
Generators that are Sold as Air Cleaners," which
can be found at www.epa.gov/indoor-air-quality-iaq/
ozo n e-ge n erators-a re-so I d -a i r-c I ea n ers.
Ultraviolet Germicidal Irradiation (UVGI) Air
Cleaners
Another type of electronic air cleaner technology,
UVGI, is designed to reduce the number of viable
airborne microorganisms.
UVGI Technology
UVGI air cleaners are designed to use UV
lamps to kill or deactivate microorganisms
such as viruses, bacteria, and fungal spores
and fragments that are airborne or growing on
surfaces (e.g., cooling coils, drain pans, ductwork,
filters). Both UV-A (long wave: 315-400 nm)
and UV-C (short wave: 100-280 nm) are used
in UVGI air cleaners. Most UV lamps that are
used to deactivate microorganisms in residential
settings are low-pressure mercury vapor lamps
that emit UV-C radiation primarily at a wavelength
of 254 nm, which has been shown to have
germicidal effects (VanOsdell and Foarde 2002).
Given sufficient exposure time and lamp power,
UV light can penetrate the outer structure of
a microorganism's cell(s) and alter its DNA,
preventing replication and causing cell death. But
some bacterial and mold spores are resistant to
UV radiation, and to achieve reliable deactivation
of spores, the lighting power must be high and
the exposure times must be long (i.e., on the
order of minutes and hours rather than the few
seconds typical of most UVGI air cleaners).
Types of UVGI Cleaners and Their Effectiveness
There are two types of UVGI applications
in residences: air cleaners designed for
airstream disinfection to reduce the viability of
microorganisms as they flow through the HVAC
system or portable air cleaner, and surface
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cleaners designed for surface disinfection that
are most commonly used to prevent the growth
of microorganisms on cooling coils inside an
HVAC system (Kowalski and Bahnfleth 2000;
VanOsdell and Foarde 2002). UVGI lamps for
airstream or surface disinfection usually are
located in the air duct of an HVAC system
downstream of the filter and upstream of
the cooling coil or in a portable air cleaner
downstream of the filter. Two test standards
are available for objectively evaluating the
effectiveness of UVGI systems and components.
UVGI lamps for in-duct airstream irradiation are
tested using ANSI/ASHRAE Standard 185.1
(ASHRAE 2015b), and UVGI lamps for in-
duct surface irradiation are tested using ANSI/
ASHRAE Standard 185.2 (ASHRAE 2014).
If properly designed, the UVGI cleaner in a
typical airstream disinfection application has
the potential to reduce the viability of vegetative
bacteria and molds and to provide low to
moderate reductions in viruses but little, if any,
reduction in bacterial and mold spores (CDC
2003; Kowalski and Bahnfleth 2000; Levetin
et al. 2001). Spores tend to be resistant to UV
radiation, and killing them requires a very high
dosage (Cundith et al. 2002; VanOsdell and
Foarde 2002; Xu etal. 2002).
UVGI cleaners in a surface disinfection application
are installed in air-handling units to prevent
or limit the growth of vegetative bacteria and
molds on moist surfaces in the HVAC system
(Kowalski and Bahnfleth 1998, 2000; Levetin
et al. 2001; Luongo and Miller 2016). One
study reported a 99 percent reduction in
microbial contaminants growing on exposed
HVAC surfaces but a reduction in airborne
bacteria of only 25 to 30 percent (Menzies et al.
2003). One reason that the surface disinfection
application provides only a slightly noticeable
reduction in airborne microbial concentrations
RESIDENTIAL AIR CLEANERS
may be that microorganisms in the airstream
are exposed to the UV light for a shorter time.
Conversely, microorganisms growing on exposed
HVAC surfaces are given prolonged direct
UVGI exposure. Another study found that UV-C
lamps yielded reduced microbial growth in duct
lining and drain pans from air-handling units
but cautioned that moisture control, properly
designed dehumidifying and cooling HVAC
processes, drain pans designed to drain, and
installing nonporous surfaces downstream of
coils should collectively continue to be the
primary approaches to controlling microbial
growth in air-handing units (Levetin et al. 2001).
Limiting microbial growth on cooling coils has
other benefits, such as improving the heat
transfer rate of the coil, which improves energy
efficiency (Wang et al. 2016a, b).
Prolonged direct UVGI exposure can destroy
vegetative microbial growth—but not most
spores—on the surfaces of forced-ventilation
units, filters, cooling coils, or drain pans. Killing
molds and bacteria while they are still in the
susceptible vegetative state reduces the formation
of additional spores. UV radiation is ineffective in
killing microorganisms if they proliferate inside
the filter media, system crevices, porous thermal
insulation, or sound-absorbing fibrous material
liners (Kowalski and Bahnfleth 2000).
A number of studies report that the most
important performance elements of a UVGI
system are the type of UV lamp and ballast,
relative humidity, temperature, air velocity,
and duct reflectivity (Kowalski and Bahnfleth
1998; Philips Lighting 1985, 1992; Scheir
and Fencl 1996; VanOsdell and Foarde 2002).
The effectiveness of UVGI cleaners in killing
microorganisms may also vary depending on
the UV irradiation dose, system design and
application, system operation characteristics, and
the microorganism targeted for deactivation.
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RESIDENTIAL AIR CLEANERS
Some UVGI cleaners used in HVAC systems or
portable air cleaners are advertised to reduce
dust mite allergens, airborne microorganisms
(e.g., viruses, bacteria, molds) and their spores,
and gaseous pollutants from indoor air. However,
it is likely that the effective destruction of
airborne viruses and fungal and bacterial spores
requires much higher UV exposures than a
typical residential UVGI air-cleaning unit provides
(Kowalski and Bahnfleth 2000; Scheir and Fencl
1996; VanOsdell and Foarde 2002). No research
or studies were found that show UV disinfection
is effective in reducing dust mite and mold
allergenicity or that UV radiation has the potential
to remove gaseous pollutants. Both dead or live
fungal particles can cause allergic reactions in
sensitive populations. Therefore, UVGI cleaners
might not be effective in reducing allergy and
asthma symptoms. If mold is growing indoors, it
should be removed, and the conditions leading to
its growth should be addressed (U.S. EPA 2001).
Regular maintenance of UVGI systems is crucial
and usually consists of cleaning the lamps of
dust and replacing old lamps. Manufacturers'
recommendations regarding safety precautions,
exposure criteria, maintenance, and monitoring
associated with the use of UVGI systems should
be followed.
Disadvantages of UVGI Cleaners
Similar to ESPs, UVGI cleaners can generate
large amounts of ozone as a byproduct of their
operation (Morrison etal. 2014). Uncoated UV-C
lamps that emit UV light with a wavelength of
254 nm and below can generate ozone through
photolysis of oxygen and further reaction
(e.g., 302 -^photolysis ->203). Because of this
issue, some manufacturers apply a special
coating to UV lamps (e.g., doped fused
quartz lamps) to inhibit ozone production.
The California Air Resources Board Title 17
Regulation for Limiting Ozone Emissions from
Indoor Air Cleaning Devices, which relies on
the ANSI/UL Standard 867 test method for
evaluating ozone emissions from air cleaners
(California Code of Regulations 2009) certifies
UVGI air cleaners in regard to ozone. Another test
standard, IEC 60335-2-65, Edition 2.2 2015-
01, documents similar procedures for measuring
ozone production from air-cleaning devices.
There is no specific standard test method to
rate and compare the effectiveness of UVGI
cleaners installed in either residential HVAC
systems or portable air cleaners. Typical UVGI
air cleaners designed for use in homes do not
deliver sufficient UV doses to effectively kill
or deactivate most airborne microorganisms
because the exposure period is too short and/
or the intensity is too low. Thus, UVGI does not
appear to be effective as a sole control device.
When UVGI is used, it should be used in addition
to—not as a replacement for—conventional
particle filtration systems, because UVGI does
not actually capture or remove particles (CDC
2003). Dead or deactivated biological particles
can still contain irritants, allergens, and/or
toxins. Using UVGI in addition to HEPA filters
or other high-efficiency filters (e.g., MERV 13
and above) in HVAC systems or in portable units
offers only minimal infection control benefits
over those provided by the filters alone (CDC
2003; Kowalski and Bahnfleth 1998). However,
UVGI can be effective for inhibiting biological
growth on HVAC cooling coils and drain pans as
a result of longer exposure times.
Air-Cleaning Technologies Used for
Removing Gases
A number of air-cleaning technologies are
designed to either remove gases or convert
them to (ideally) harmless byproducts using a
combination of physical and chemical processes.
Gas-phase air-cleaning technologies include
sorbent media air filters, PCO, plasma, and
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intentional ozone generators sold as air cleaners.
(Note that ozone generators sold as air cleaners
should not be used in occupied spaces. For more
information, visit www.epa.gov/indoor-air-quality-
iaq/ozone-generators-are-sold-air-cleaners.)None
of these technologies are explicitly designed
to remove particles. All in-duct gas-phase air-
cleaning devices can be tested using ANSI/
ASHRAE Standard 145.2, although its use
remains somewhat limited (ASHRAE 2016).
There is no standardized in situ field-testing
method for evaluating gas-phase air cleaner
performance with a metric similar to CADR,
although one study proposed a test method that
used a single VOC (decane) as a representative
gaseous pollutant source in a test house, and
another extended a similar methodology to
evaluate the removal of three VOCs in a test
chamber (Howard-Reed et al. 2008; Kim et al.
2012; Sidheswaran et al. 2012).
Sorbent Media
Sorbent media air filters use a material with a
very high surface area called a sorbent to capture
gaseous pollutants. Two main sorbent processes
can be used to remove gaseous contaminants:
a physical process known as adsorption and a
chemical reaction called chemisorption. Both
types of media can be tested using ANSI/ASHRAE
Standard 145.1 (ASHRAE 2015c).
Adsorption results from the physical attraction of
gas or vapor molecules to a surface. All adsorbents
have limited capacities and thus require frequent
maintenance. An adsorbent will generally adsorb
molecules for which it has the greatest affinity
and will allow other molecules to remain in the
airstream. Adsorption occurs more readily at lower
temperatures and humidity. Solid sorbents such
as activated carbon, silica gel, activated alumina,
zeolites, synthetic polymers, and porous clay
minerals are useful because of their large internal
surface area, stability, and low cost.
RESIDENTIAL AIR CLEANERS
Activated carbon is the most common adsorbent
used in HVAC systems and portable air cleaners to
remove gaseous contaminants. It has the potential
to remove most hydrocarbons, many aldehydes,
organic acids through adsorption, and ozone
through chemisorption. However, activated carbon
is not especially effective against oxides of sulfur,
hydrogen sulfide, low molecular weight aldehydes
(e.g., formaldehyde), ammonia, and nitrogen oxide.
Adsorbent media filters can have high removal
efficiency for many gaseous pollutants, but they
can also have different removal efficiency for
different gases at different concentrations (Kim et
al. 2012). For example, tests performed at EPA
measured the adsorption isotherms for three VOCs
at concentrations of 100 parts per billion (ppb) to
200 ppb using three samples of activated carbon.
The bed depth needed to remove the compounds
was estimated assuming a 150 ppb concentration
in the air, an exit concentration of 50 ppb, and
a flow rate of 100 cfm across a 2-foot by 2-foot
filter. The results of the study suggest that
breakthrough of these chemicals would occur
quickly in 6-inch deep carbon filters used for odor
control (Ramanathan et al. 1988). Therefore, the
thicker the media, the more efficient the filter will
be for longer periods of time.
Adsorbent media can also be impregnated in
thin layers onto fibrous air filter media to remove
both gases and particles. For example, one study
of the effects of various air-cleaner technologies
on the sensory perception of human subjects
demonstrated that an electret filter impregnated
with carbon sorbent received the best ratings
with respect to odor strength, nasal irritation,
eye irritation, and overall air acceptability
(Shaughnessy et al. 1994). However, such thin
layers can become quickly saturated, and the
filter can become a source of previously adsorbed
pollutants (Miller et al. 1991). Gaseous pollutant
adsorption to most adsorbent media does not
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RESIDENTIAL AIR CLEANERS
generate any chemical byproducts, but adsorbent
media filters require regular replacement or
regeneration to restore sorbent sites and avoid
breakthrough. Gas-phase filters that contain
sorbents should generally be located downstream
of particle air filters. The air filter reduces the
amount of PM that reaches the sorbent, and the
sorbent collects vapors that may be generated from
liquid particles that collect on the particle filter.
Chemisorption occurs when gas or vapor
molecules chemically react with sorbent material
or with reactive agents impregnated into the
sorbent. These impregnates react with gases and
form stable chemical compounds that are bound
to the media as organic or inorganic salts, or are
broken down and released into the air as carbon
dioxide, water vapor, or some material more
readily adsorbed by other adsorbents.
A sorbent filter's behavior depends on many
factors that can affect the removal of gaseous
contaminants:
• Airflow rate and velocity through the sorbent
• Concentration of contaminants
• Presence of other gaseous contaminants
• Total available surface area of the sorbent
(Some manufacturing techniques can
significantly reduce a filter's total surface
area.)
• Physical and chemical characteristics of the
pollutants and the sorbent (such as weight,
polarity, pore size, shape, volume, and the
type and amount of chemical impregnation)
• Pressure drop
• Removal efficiency and removal capacity
• Temperature and relative humidity of the
gas stream
Photocatalytic Oxidation (PCO)
PCO air cleaners use a high-surface-area medium
coated with a catalyst such as titanium dioxide
to adsorb gaseous pollutants (Huang et al.
2016; Mo et al. 2009; Wang et al. 2007; Zhong
and Haghighat 2015). When the photocatalyst
is irradiated with UV light, a photochemical
reaction takes place and hydroxyl radicals form
on the media surface. The hydroxyl radicals
oxidize gaseous pollutants adsorbed on the
catalyst surface. This reaction, called PCO,
converts organic pollutants into (ideally) carbon
dioxide and water.
PCO air cleaners can transform a wide array of
gaseous pollutants. However, PCO air cleaners
are often ineffective in completely transforming
gaseous pollutants in indoor air (Henschel
1998; Tompkins et al. 2005a, b) and are also
known to generate harmful byproducts such as
formaldehyde, acetaldehyde, nitrogen dioxide,
and carbon monoxide (Hodgson et al. 2007).
PCO air cleaners can also generate ozone when
used with a UV-C lamp that lacks a coating
to inhibit ozone generation. Therefore, some
PCO air-cleaning devices use adsorbent media
air filters downstream that may adsorb some
of the generated byproducts. There are few
field investigations to validate the performance
of PCO air cleaners, and laboratory studies
demonstrate high variability and often relatively
low removal efficiency for many common indoor
gases. For example, one study reported that PCO
devices installed in portable air cleaners did not
effectively remove any of the test VOCs present at
the low concentrations normally found in indoor
air (Chen at al. 2005). This study compared the
VOC-removal efficiencies of 15 air cleaners that
use different types of technology. A mixture of
16 VOCs commonly found indoors was used. The
report indicated that the PCO devices studied
might not work as advertised.
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The usefulness of PCO air cleaners depends on
the amount of catalyst, the amount of contact
time between gaseous pollutants and the catalyst,
and the amount of UV light that is delivered to
the catalyst surface. If any one of these factors
is not addressed in the design of the device, a
PCO air cleaner may fail to destroy pollutants
completely and instead produce new indoor
pollutants including irritants. PCO of certain VOCs
may create byproducts that are indoor pollutants
if the system's design parameters and catalyst
metal composition do not match the compound
targeted for decomposition, particularly in
the presence of multiple reactive compounds
commonly found in residential settings. One study
reported that no detectable byproducts formed
during the PCO of 17 VOCs using titanium dioxide
under the experimental conditions (Henschel
1998). However, two studies on the degradation
of four chlorinated VOCs found byproducts
including phosgene and chlorides (Alberci et al.
1998; Blake et al. 1993). In addition, the PCO
of trichloroethylene in air using titanium dioxide
as the catalyst yielded as byproducts carbon
monoxide, phosgene, carbon dioxide, hydrogen
chloride, and chlorine. However, these studies
did not report the concentration of chlorinated
precursor compounds or the concentrations of
phosgene formed.
Several other studies have also explored the
following aspects of PCO cleaners, often with
mixed findings and suggestions for further
research:
• How does UV light intensity and residence
time affect PCO performance (Tompkins et
al. 2005a)?
• How does the presence of other compounds
such as toluene, benzene, ethanol, or
siloxanes affect PCO performance (Tompkins
et al. 2005a, b; Turchi et al. 1995; Zorn
2003)?
RESIDENTIAL AIR CLEANERS
• How does the reaction temperature or water
vapor content affect PCO performance (Zorn
et al. 1999)?
• How can PCO systems be best engineered
to optimized performance (DestaiIlats et al.
2012)?
A review of the literature suggests that more
research is needed to further advance PCO as
an effective technology in removing low levels
of gaseous contaminants from the indoor air
of residences (Chen et al. 2005; Tompkins et
al. 2005 a, b). The effectiveness of PCO air
cleaners sold for use in homes remains largely
undocumented. And to date, there is no standard
test method to compare and rate the effectiveness
of PCO cleaners installed in residential HVAC
systems or portable air cleaners.
Plasma
Plasma air cleaners apply a high-voltage discharge
to ionize incoming gases, breaking their chemical
bonds and chemically altering them (Bahri and
Haghighat 2014). Thermal plasma air cleaners
generate a high-temperature plasma flame using
high voltage and high current. Non-thermal
plasma air cleaners accelerate electrons to
generate reactive ions and radicals, which convert
compounds by oxidation reactions. According
primarily to controlled laboratory tests, plasma
air cleaners can have high removal efficiency for
some gases as well as particles, and they can
also kill or deactivate airborne microorganisms.
However, a number of harmful byproducts are
known to form, including particles, ozone, carbon
monoxide, and formaldehyde (Chen et al. 2009;
Van Durme et al. 2009). Moreover, plasma
emitted directly to indoor air contains ozone and
other reactive oxygen species such as hydroxyl
radicals, superoxides, and hydrogen peroxide.
Plasma air cleaners are sometimes combined
with other air-cleaning technologies, such as PCO
www.epa.gov/iaq I 31
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RESIDENTIAL AIR CLEANERS
or adsorbent media, but very little information
exists on the performance of these systems in real
indoor settings.
Intentional Ozone Generators
Ozone generators sold as air cleaners should not
be used in occupied spaces.
Ozone generators sold as air cleaners, which are
typically designed to control odors, use UV lamps
or electrical discharge to intentionally produce
ozone. Ozone reacts with chemical pollutants
to transform them into other compounds at
high concentrations and can kill or deactivate
biological pollutants.
However, ozone is a potent lung irritant. And as
ozone reacts with chemical pollutants, it can
produce harmful byproducts (Shaughnessy and
Sextro 2006; U.S. EPA 2014; Wechsler 2006).
If ozone concentrations are maintained below
public health standards, it has little potential
to remove indoor air contaminants. However,
even at concentrations below public health
standards, ozone reacts with chemicals emitted
by such common indoor sources as household
cleaning products, air fresheners, deodorizers,
certain paints, polishes, wood flooring, carpets,
and linoleum. The chemical reactions produce
irritating and corrosive byproducts that may
cause adverse health effects and may damage
building materials, furnishings, and wiring. The
ozone reaction byproducts that may result include
ultrafine particles, formaldehyde, ketones, and
organic acids (DestaiIlats et al. 2006; Sarwar et
al. 2003; Waring 2014; Wechsler 2000; Wechsler
and Shields 1999). Do not use ozone generators
sold as air cleaners in occupied spaces. No
federal agency has approved ozone generators for
use in occupied spaces.
Ozone generators sold as air cleaners and
marketed as duct-mounted or portable units use
UV light or corona discharge to produce ozone,
which is dispersed by a fan into occupied spaces
(U.S. EPA 2014). Federal pesticide law requires
manufacturers of ozone generators to list an
EPA establishment number on the product's
packaging. This number merely identifies the
facility that manufactured the product. The
presence of this number on a product's packaging
does not imply that EPA endorses the product, nor
does it imply that EPA has found the product to
be safe or effective.
More information on ozone generators sold as air
cleaners can be found at www.epa.gov/indoor-air-
quality-iaq/ozone-generators-are-sold-air-cleaners.
Practical Considerations for Using Air Cleaners
for Removing Gases
Since many different gas-phase air-filtration
devices are available, comparing and rating
the effectiveness of installed gas-phase filters
is difficult. ASHRAE has developed Standard
145.2 as a standard method for evaluating the
effectiveness of gas-phase filtration devices
installed in the ductwork of HVAC systems,
but it is not widely used at this point in time
(Shaughnessy and Sextro 2006; U.S. EPA 2014;
Wechsler 2006).
Gas-phase filters are much less common than
particle air-cleaning devices in homes because,
currently, a properly designed and built gas-phase
filtration system is too big for a typical residential
HVAC system or portable air cleaner. Other
factors that may contribute to the less frequent
use of gas-phase filters in home HVAC systems
are the filters' limited useful life, the fact that
the sorbent material must be targeted to specific
contaminants, the purchase price of the filters,
and the costs of adapting them to residential
applications, when possible, and of operating
them once they have been installed.
Some gas-phase filters may remove, at least
temporarily, a portion of the gaseous pollutants
in indoor air. Although some gas-phase air
32 www.epa.gov/iaq
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filters—if properly designed, installed, used, and
maintained—may effectively remove specific
pollutants from indoor air, none is expected to
remove adequately all of the gaseous pollutants
in a typical home. For example, carbon monoxide
is not readily captured by adsorption or
chemisorption (Shaughnessy et al. 1994).
Because of their compact design, particle air
filters that use impregnated media for additional
gaseous pollutant removal are available for
residential HVAC systems and portable air
cleaners. They use sorbent particles of carbon,
permanganate alumina, or zeolite incorporated
into fibrous filter media. Such filters generally
range from 1/8 inch to 2 inches thick. They
provide a combination of particulate and gas-
phase filtration with a minor increase in pressure
drop across the filter. Their use in an existing
HVAC system does not require extensive or
expensive modifications to the system. However,
their useful service life varies according to indoor
pollution concentrations and exposure time.
Breakthrough of the contaminants back into the
room can take place very quickly in the thin layer
impregnated with sorbents, resulting in a short
service life for the filter, which must be replaced
frequently. Thus, these devices usually have
limited effectiveness in removing odors.
Removal of Radon and Its Progeny
EPA does not recommend air cleaning to reduce
the health risks associated with radon and the
decay products of radon gas (known as radon
progeny). The Agency recommends the use of
source-control technologies to prevent radon
from entering residential structures. The most
effective radon control technique is active soil
depressurization (ASD) (U.S. EPA 2006). An
ASD system uses an electric fan to minimize
radon entry by drawing air from under the slab/
floor and venting it to the outside above the
building's roofline.
RESIDENTIAL AIR CLEANERS
A limited number of studies have investigated air
cleaners' effectiveness in removing radon and its
progeny. They compared the removal efficiencies
of various air cleaners, including mechanical air
filters, ESPs, and ionizers equipped with fans,
and the risk reduction the air cleaners achieve.
However, the degree of risk reduction found by
these studies has been inconsistent.
SELECTING AND USING A PORTABLE
AIR CLEANER
Key parameters that influence the effectiveness
of portable air cleaners include not only the
fractional removal efficiency for a particular
pollutant, but also the airflow rate through the air
cleaner and the proximity of the air cleaner to the
occupant and any pollutant sources. A helpful
parameter for understanding the effectiveness
of portable air cleaners is the CADR. The CADR
is a measure of a portable air cleaner's delivery
of relatively clean air, expressed in cfm. For
example, an air cleaner that has a CADR of 250
for dust particles can reduce dust particle levels
to the same concentration as would be achieved
by adding 250 cfm of clean air to the space. The
CADR is the product of the fractional removal
efficiency for a particular pollutant and the airflow
rate through the air cleaner. The higher the CADR
the more particles the air cleaner will remove
and the larger the area it can serve. A CADR can
theoretically be measured and calculated for
either gases or particles; however, current test
standards only rate, and most manufacturers only
report, CADRs for the removal of particles.
Consider an example that quantifies the
effectiveness of an air-cleaning device in removing
pollutants from an occupied space. The result
depends on three factors: its fractional efficiency,
the amount of air being filtered, and the path that
the clean air follows after it leaves the filter. For
example, a filter may remove 99 percent of the
www.epa.gov/iaq I 33
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RESIDENTIAL AIR CLEANERS
contaminant from the air that passes through it
(i.e., have 99-percent efficiency). However, if the
airflow rate through the filter is only 10 cfm in a
typical room of approximately 1,000 cubic feet
(e.g., approximately 10 feet by 12 feet by 8 feet),
the filter will be relatively ineffective at removing
contaminants from the air (i.e., 10 times less
effective than if the airflow rate were 100 cfm).
Clean Air Delivery Rates (CADRs) for
Portable Air Cleaners
A voluntary standard is available for comparing
the performance of portable air filters in a room
at steady-state conditions during a controlled
laboratory test: ANSI/AHAM AC-1-2015 (AHAM
2015). It was developed by the Association of
Home Appliance Manufacturers (AHAM), a private
voluntary standard-setting trade association, and
is recognized by the American National Standards
Institute (ANSI). The standard compares the
effectiveness of portable air cleaners in a room
size test chamber, as measured by the CADR.
In addition to developing and maintaining this
standard test method, AHAM has a portable air
cleaner certification program. The organization
lists AHAM certified air cleaners and their CADRs
on its website at www.ahamverifide.org/search-
for-products/room-air-cleaners. AHAM's online
directory of certified portable air cleaners allows
searches by certified CADR ratings, suggested
room sizes, manufacturers, or brand names.
The AHAM CADR rating is based on the removal
of three size ranges of particles as they pass
through the portable air cleaner. These size ranges
span a broad range of actual particle types and
dimensions that overlap with each other, but they
correspond to airborne contaminants that are of
potential interest to consumers. Particles removed
to achieve the "clean air" referred to in the CADR
are described as pollen (particles ranging from
5 to 11 |jm), dust (particles ranging from 0.5 to
3 |jm), and tobacco smoke (particles ranging from
0.09 to 1 |jm). These three pollutants are used
as examples to represent large-, medium-, and
small-sized particles, respectively.
Note that although AHAM uses tobacco smoke
particles to represent smaller airborne particles,
air cleaning is not an effective way to address
environmental tobacco smoke. There are
thousands of particulate and gaseous chemical
compounds, including many known carcinogens,
in tobacco smoke that cannot be removed
effectively by air cleaning.
Also, note that the CADR labeled on product
packaging is typically the highest CADR
achievable, which typically occurs at the highest
airflow setting. While lower airflow settings may
have lower noise production, the CADR may not
be known (but it could be considerably lower than
the highest advertised and thus significantly less
effective at pollutant removal).
Despite their differences, measured CADRs for
each of the three tested particle size ranges are
typically similar to each other for a specific air
cleaner. For example, Figure 9 shows CADRs for
more than 350 individual air cleaners tested by
the AHAM standard and reported on the AHAM
website: www.ahamdir.com. Figure 10 shows the
AHAM recommended maximum room size (in
square feet) for each air cleaner shown in Figure 9
(also as reported on the AHAM website).
On average, CADRs for pollen are typically
approximately 5 percent greater than dust CADRs,
while CADRs for tobacco smoke are approximately
4 percent lower than dust CADRs. Therefore, to
understand how an air cleaner will remove small
particles such as those that make up PM25,
tobacco smoke CADRs should be used as the
most conservative estimate.
34 www.epa.gov/iaq
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RESIDENTIAL AIR CLEANERS
500
y = 1,05x
R* = 0.97
O 300
y = 0.96x
Rz = 0.98
Pollen
= 100
Tobacco Smoke
T
T
T
1
0 100 200 300 400 500
Dust CADR (cfm)
Figure 9. Comparison of CADRs for pollen, dust, and tobacco smoke particles for over 350 air cleaners
tested and reported on the AHAM website: www.ahamdir.com/aham_cm/site/pages/index.html.
The ANSI/AHAM AC 1 test method also provides
a way to recommend what room size an air
cleaner should be specified for. The room size
recommendation is calculated based on an
80-percent reduction in steady-state particle
concentrations in the three size ranges of the
AHAM test. This level of effectiveness assumes a
flow rate of clean air that is four to five times the
volume of the room dimensions used during the
test. Said another way if the unit is placed in a
larger space than specified by its CADR rating,
it can be expected to fall short of 80-percent
reduction, and if placed in a smaller space, the
unit may achieve a higher percent reduction
(assuming in all cases that particle generation
stays at a constant rate).
Based on the removal of tobacco smoke particles
alone, Table 2 summarizes the linear fits to the
data in Figure 10 to approximate the minimum
CADR that would be required for various room sizes
from 100 to 600 square feet. As examples, the
resulting approximations of the maximum room
size that a 20 cfm, 150 cfm, and 300 cfm CADR
portable air cleaner would be most appropriate for
are 30, 225, and 450 square feet, respectively.
For reference, 30 square feet would be equivalent
to a 5-foot by 6-foot room; 225 square feet would
be equivalent to a 15-foot by 15-foot room; and
450 square foot would be equivalent to a 25-foot
by 18-foot room in a typical one-story home.
Table 2. Portable Air Cleaner Sizing for 80% Percent Steady-State Particle Removal
Room area (square feet)
100
200
300
400
500
600
Minimum CADR (cfm)
65
130
195
260
3251
390'
www.epa.gov/iaq I 35
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700
y = 1.55k
R2 = 0.99
600
& 500
y = 1.49x
R2 = 0.98
400
y = 1.41x
R2 = 0.95
« 300
5 200
Pollen
Tobacco Smoke
Dust
100
0 100 200 300 400 500
CADR (cfm}
Figure 10. Maximum room size (in square feet) for which each air cleaner from Figure 9 is most
appropriate (for each tested particle size category), as reported on the AHAM website: www.ahamdir.
com/aham_cm/site/pages/index.html.
Many of the portable air cleaners AHAM has
tested have moderate-to-high CADR ratings for
small particles (Shaughnessy and Sextro 2006).
It is also important to note that a portable air
cleaner's removal rate also competes with other
removal processes occurring in the space,
including deposition of particles on surfaces,
sorption of gases, indoor air chemical reactions,
and outdoor air exchange. Thus, while a portable
air cleaner may not achieve its rated CADR under
all circumstances, the CADR value does allow
comparisons among portable air cleaners.
In addition to evaluating CADRs for the particle
size ranges involved in the AHAM test standard,
studies to date have also assessed portable air
cleaners' performance in removing tobacco smoke
particles; diesel exhaust particles; larger airborne
particles including those that contain cat, dog,
and dust mite allergens; and fine and ultrafine
particles (Bascom et al. 1996; Battistoni and
Fava 1993; Consumers Union 2003; Custovic
et al. 1998; De Blay et al. 1991; Green at al.
1999; Institute of Medicine 2000; Molgaard et
al. 2014; Ongwandee and Kruewan 2013; Peck
et al. 2016; Sultan et al. 2011; Van der Heide et
al. 1999; Waring et al. 2008; Wood et al. 1998).
These studies have generally demonstrated that
CADRs for fine and ultrafine particles commonly
range from less than approximately 20 cfm for
ionizers and PCO portable air cleaners to between
approximately 150 and 300 cfm for many HEPA
and ESP portable air cleaners, depending on the
size of the device.
Portable Air Cleaner Noise
Some intervention studies involving the use of
portable air cleaners have noted that portable air-
cleaning units were used less frequently over time.
Fewer operating hours reduces their effectiveness
and, therefore, their potentially positive effect
on indoor air quality and health outcomes. One
study specifically noted that occupants reported
-------
excessive noise as the reason for turning off air
cleaners at night (Sulser et al. 2009). Further
intervention studies speculated that operating
noise was a reason that air cleaners were turned
off during sleeping hours (Batterman et al. 2012,
2013). That noise is a factor in consumer behavior
is consistent with the observation that consumer
packaging of portable air cleaners frequently
includes descriptors such as "quiet". Intervention
studies do not indicate what noise levels would
encourage more hours of use. However, as of
2017, quantified operating noise is not a factor
in the performance ratings of portable air cleaner
ratings in the United States, nor are the noise
values measured during performance tests
commonly available to consumers on product
packaging. Quantification of the operating noise of
air cleaners could be a useful foundation for better
informed consumer choices.
Practical Considerations for Using Portable
Air Cleaners
Indoor particle concentrations are not
constant over time. Some indoor pollutants
are periodically generated from sources such
as hobby and craft materials or cooking food,
and high concentrations can continue to last
for long periods of time even after the source
is gone. Others may infiltrate from episodic
outdoor sources such as wildfire emissions (for
recommendations on air cleaning to reduce
exposure to wildfire smoke inside homes, refer
to the EPA's 2016 document Wildfire Smoke: A
Guide for Public Health Officials, available online
at www3.epa.gov/airnow/wiIdfire_may2016.pdf).
Therefore, a portable air cleaner would need to be
operating both during and after these intermittent
pollutant sources to have a meaningful effect on
pollutant concentrations and exposures.
The placement of any portable air cleaner will
affect its performance. For example, if there is a
RESIDENTIAL AIR CLEANERS
specific, identifiable source of pollutants, such as
office appliances or other point sources, the unit
should be placed so its intake is near that source.
If there is no specific source, the air cleaner
should be placed where it will direct clean air into
the breathing zone of the occupants.
The air cleaner should not be situated where
walls, furniture, curtains, and other obstructions
will block the intake and outlet. Manufacturer
instructions may indicate that the air cleaner be
placed a certain distance from any objects that
might obstruct airflow. Additionally, a portable air
cleaner will be much more effective for a specific
room when any exterior doors and windows in a
room are closed.
Regular filter media replacement and/or cleaning
are essential for ensuring performance. Follow
manufacturer's instructions for filter replacement
and/or cleaning.
Some portable air cleaners sold to consumers are
ENERGY STAR® qualified. Earning the ENERGY
STAR® means that a product meets strict energy
efficiency guidelines set by EPA and the
U.S. Department of Energy. The ENERGY STAR®
disclaimer label, which includes the following
statement, is placed on the product packaging
of ENERGY STAR® qualified air cleaners: "This
product earned the ENERGY STAR® by meeting
strict energy efficiency guidelines set by the
U.S. EPA. EPA does not endorse any manufacturer
claims of healthier indoor air from the use of this
product." For more detailed information on the
approximate energy costs of operating portable air
cleaners, refer to Table 3.
Information about portable air cleaners is
available from the Consumer Reports magazine/
website. Consumers Union is a nonprofit
organization that provides product reviews and
ratings. The details of the test method(s) used by
Consumers Union to evaluate the performance of
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RESIDENTIAL AIR CLEANERS
air-cleaning devices are not publically available.
Consumers Union rates air cleaners based on a
variety of criteria including noise.
Caution should be exercised during replacement
and cleaning of filter media and other air cleaner
components. During cleaning or replacement of
air cleaners, an effort should be made to ensure
that pollutants are not re-emitted into the air and
do not come into contact with skin. To minimize
exposures, excessive movement or air drafts
should be avoided when filters are removed.
Using an N-95 respirator (such as those sold for
home improvement projects) and gloves can help
provide additional protection during cleaning or
filter replacement. Used filters should be placed
in sealed plastic bags or containers for disposal.
Noise may also be a consideration in selecting a
portable air cleaner that contains a fan. Portable
air cleaners that do not have fans typically are
much less effective than units that have them. In
tests by Consumers Union, the largest portable air
cleaners were the noisiest on their most effective
high-speed settings (Consumers Union 2002).
Recent peer-reviewed studies have also confirmed
this same finding (Peck et al. 2016). However,
some performed more quietly at low speed than
many smaller cleaners do on high. Some larger
portable units operating at low speed were
found to be quiet enough for most households
(Consumers Union 2003).
SELECTING AND USING A FURNACE
FILTER OR OTHER IN-DUCT AIR
CLEANER
In addition to fractional removal efficiency
metrics for furnace filters such as MERV, MPR,
FPR, or HEPA, the effectiveness of furnace filters
and in-duct air cleaners is influenced by several
other key parameters and practical design and
operation considerations.
Practical Considerations for Using In-Duct
Air Cleaners
Removal of pollutants is often limited by system
operation. Although fractional removal efficiency
ratings are an important indicator of potential
performance, reduction of pollutant concentrations
is a strong function of system effectiveness. The
effectiveness of an in-duct filter or other air cleaner
is a function of many parameters in addition to
the fractional removal efficiency of the filter or
air cleaner, including the airflow rate through the
system relative to the size of the space and the
HVAC system runtime. In most homes, central
forced air heating and cooling systems only operate
to meet heating and cooling needs. Although
quite limited to date, experimental studies have
demonstrated that typical central HVAC runtimes
average less than 20 to 25 percent in most
residential building types in most climate zones
(James et al. 1997). Also, in some locations,
such as where air-conditioning is not needed or
where air-conditioning is provided by window
air conditioners, central HVAC systems may not
operate at all for many months of the year. Low
system runtimes can greatly limit the effectiveness
of an in-duct air cleaner simply by not passing
air through it long enough to yield substantial
reductions in indoor pollutant concentrations
(Stephens 2015; Zhao et al. 2015). Because
of low system runtimes, experimental data and
theoretical predictions indicate that for particle
removal, medium- to high-efficiency furnace filters,
such as some MERV 12 filters and most MERV 13
to 16 filters, are likely to be almost as effective as
HEPA filters in reducing the concentrations of most
sizes of indoor particles, including those linked to
health effects (Fisk et al. 2003; Zhao et al. 2015).
Continuous operation of the HVAC fan will improve
air circulation and air cleaning, but this operation
mode also increases electrical energy consumption
and its cost (NAFA 2007). For more detailed
information on the approximate energy costs of
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RESIDENTIAL AIR CLEANERS
operating HVAC systems with in-duct air cleaners
and filters, refer to Table 3.
Not all HVAC system fans can accommodate
high-efficiency filters without affecting system
performance. Existing residential HVAC systems
may not have enough fan or motor capacity to
accommodate higher pressure drop filters without
reducing airflow to the point where cooling
or heating capacity is lost or good air mixing
is sacrificed. These shortcomings can lead to
increased risk of component failure and/or comfort
problems in the space (Proctor 2012; Proctor et
al. 2011; Walker et al. 2013). Therefore, in new
installations, the HVAC manufacturer's information
should be checked to determine whether it is
feasible to use high-efficiency (and high pressure
drop) filters, given the intended design, size, and
velocity of the supply and return duct systems. In
existing homes, performance of the entire installed
system with respect to airflow rate versus equipment
airflow and pressure capabilities can be measured
to ensure that the system can accommodate the
increased pressure drop imposed by adding a
high-efficiency air filter; this should be done by a
professional. Simply installing a high-efficiency
filter is no guarantee that it will work as intended.
Concerns about HVAC system performance are
lessened or eliminated by use of high-efficiency
filters with low airflow resistance, due to extensive
pleating of filter media, increased filter thickness,
and the use of electrostatically charged media. Such
filters are increasingly available.
In-duct air-cleaning devices should be installed
such that bypass airflow is prevented. Air filters
should be installed so that the directional arrow
printed on the side of the filter points in the
direction of airflow within the system. Incorrectly
designed or installed filter frames can cause
bypass airflow, which significantly decreases filter
effectiveness. Bypass airflow can also result from
return duct leakage. If air from unconditioned
spaces enters through the return duct, it can
circumvent the filter, if the filter is installed at a
return grille, rather than at the base of an air-
handling unit. It is recommended that HVAC
ducts be well sealed for return grille installations.
High-efficiency filters require well-sealed frames
to prevent leaks.
Table 3. Approximations of Annual Electricity Use and Electricity Costs for Operating Several Portable Air Cleaners Based on Power Draw Measurements
Reported in the Literature and Assumptions for 20-, 50-, or 100-Percent Runtime
Annual electricity use (kWh)
Annual electricity costs1
Air cleaner type
Reference
Power draw
(W)
Airflow rate
(cfm)
Assumed runtime
20%
50%
100%
20%
50%
100% 1
ESP
102
500
179
447
894
$21
$54
$107
HEPA 1
Waring et al.
(2008)
206
182
361
902
1,805
$43
$108
$217
HEPA 2
103
340
180
451
902
$22
$54
$108
Ion generator 1
8
36
14
35
70
$2
$4
$8
Ion generator 2
5
<18
9
22
44
$1
$3
$5
HEPA 1
167
267
293
731
1,463
$35
$88
$176
HEPA 2
226
571
396
990
1,980
$48
$119
$238
Fibrous electret
135
463
237
591
1,183
$28
$71
$142
HEPA 3 + activated carbon
98
146
172
429
858
$21
$52
$103
ESP
Sultan et al.
(2011)
98
473
172
429
858
$21
$52
$103
Ion generator 1
46
112
81
201
403
$10
$24
$48
Ion generator 2
45
382
79
197
394
$9
$24
$47
Plasma + HEPA
110
344
193
482
964
$23
$58
$116
PCO 1
444
913
778
1,945
3,889
$93
$233
$467
PCO 2
14
8
25
61
123
$3
$7
$15
IJVGI
16
12
28
70
140
$3
$8
$17
Assuming $0.12/kWh constant electricity cost.
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RESIDENTIAL AIR CLEANERS
For existing systems, installing a higher efficiency
or HEPA filter may require modifications to the
existing ductwork to permit the installation of the
thicker filter. In addition, a more powerful fan
may be needed to overcome the higher pressure
drop. Electronic air cleaners and UV lamps
should have an accessible power supply and an
indicator showing when electrical service is off.
The installation of UV lamps requires the addition
of access holes into the duct, and the holes must
be properly sealed to maintain HVAC efficiency.
To avoid electrical and mechanical hazards, make
sure air-cleaning devices that require an electrical
power supply are listed on the Underwriters
Laboratories website (www.ul.com) or with another
independent safety testing laboratory.
In-duct air-cleaning devices require sufficient
access for inspection during use, repair, and
maintenance. In-duct air cleaners should
be selected to match operating conditions,
such as type of pollutant to be removed and
allowable pressure drop. Filters and sorbents
must be replaced regularly, in accordance with
manufacturer's specifications. Electronic air
cleaner efficiency decreases as the collecting
plates become loaded with particles, so the
plates must be cleaned, sometimes frequently,
as required by the manufacturer. The cleanings
should be scheduled to keep the unit operating at
peak efficiency. Special attention must be given
to cleaning the ionizing wires of electronic air
cleaners designed to target specific contaminants.
Turn the power off while servicing or cleaning
powered in-duct air cleaners and central HVAC
systems. During cleaning or replacement of air
cleaners or filters, an effort should be made to
ensure that pollutants are not re-emitted into
the air and do not come into contact with skin.
To minimize exposures, excessive movement
or air drafts should be avoided when filters are
removed. Using an N-95 respirator and gloves
can help provide additional protection during
cleaning or filter replacement. Used filters
should be placed in sealed plastic bags or other
containers for disposal.
APPROXIMATIONS OF OPERATIONAL
ELECTRICITY COSTS OF PORTABLE
AND IN-DUCT AIR CLEANERS
Detailed life cycle cost analyses of all types of
portable and in-duct air cleaners and systems
described in this document were not found
in the literature, although Table 3 provides
approximations of the operational electricity
costs of using various portable and in-duct
air cleaners. Waring et al. (2008) and Sultan
et al. (2011) reported electrical power draw
measurements for several types of portable air
cleaners, including HEPA air cleaners (average
of approximately 160 watts [W)], ion generators
(average of approximately 25 W), ESPs (average
of approximately 100 W), plasma (one unit
combined with HEPA at approximately 110 W),
PCO (average of approximately 229 W with a wide
range), and UVGI (one small unit at 16 W). In
general, the units with higher airflow rates also
had higher power draws and were more effective
air cleaners for removing ultrafine particles
compared to the units with lower airflow rates
and lower power draws. These air cleaners, power
draws, and airflow rates are summarized in the
first portion of Table 3.
Also shown in Table 3 is an approximation of the
number of kilowatt-hours (kWh) and the annual
electricity costs required to power selected
portable air cleaners for 20, 50, and 100 percent
of the hours of the year, assuming constant power
draws and an average electricity cost of $0.12 per
kWh. The estimated annual electricity costs for
running these portable air cleaners 100 percent
of the time range from less than $10 per year
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for a small ionizer unit to more than $450 per
year for a large PCO unit. The average annual
electricity costs for running portable HEPA air
cleaners 100 percent of the time are just under
$200 per year, with individual units ranging from
just over $100 to nearly $250 per year.
For comparison, the blower fan in a typical central
air-handling unit in a residential HVAC system,
which commonly moves between 500 and
2,000 cfm when operating, draws between about
250 and 600 W (with an average of approximately
450 W) (Stephens et al. 2010). Running an
average air-handling unit drawing approximately
450 W for 100 percent of the year would cost
approximately $475, which is approximately $380
higher than the cost of running the same unit for
a typical fractional runtime of approximately 20
percent to meet only heating and cooling needs
(which is approximately $95). Although these
cost estimates do not consider filter replacement
costs, maintenance costs, or the incremental costs
of changes in HVAC energy use (based on other
aspects such as changes in air cleaner pressure
drop over time, fan airflow rates, or heating and
cooling system runtimes (Fazli et al. 2015),
typically, the operational electricity cost of most
portable air cleaners will likely be lower than
operating central HVAC fans for the same amount
of time. Note that these are approximations and
the power draw of specific air cleaners and air-
handling units will vary.
WILL AIR CLEANING REDUCE
HEALTH EFFECTS FROM INDOOR AIR
POLLUTANTS?
In 2000, the Institute of Medicine Committee
on the Assessment of Asthma and Indoor Air
of the National Academy of Sciences reviewed
literature on the effects of particle air cleaners
on allergy and asthma symptoms and concluded
RESIDENTIAL AIR CLEANERS
that: "The results of existing experimental studies
are inadequate to draw firm conclusions regarding
the benefits of air cleaning for asthmatic and
allergic individuals... Air cleaners are helpful in
some situations in reducing allergy or asthma
symptoms, particularly seasonal symptoms, but
it is clear that air cleaning, as applied in the
studies, is not consistently and highly effective
in reducing symptoms" (Institute of Medicine
2000). Since the year 2000, technologies have
advanced, and several additional studies have
further investigated the impact of portable air
cleaners on health outcomes or biomarkers of
cardiovascular and respiratory health outcomes.
Several of these studies were originally
summarized in detail in Fisk (2013).
This document includes a modified version of
the summary and a subjective assessment of
the strength of the study design for residential
air cleaner and health intervention studies from
Fisk (2013). In addition, several more recent
studies have also been summarized. Only those
studies that focused on air cleaner interventions
in residences were included in this document;
studies of interventions in commercial buildings
(Skulberg et al. 2005; Wargocki et al. 2008) were
excluded because of the differences in the nature
of indoor pollutant sources and HVAC system
technologies in commercial buildings.
Evidence for the Impacts of Air Cleaners on
Indoor Pollutant Concentrations
Several recent studies have shown that the use
of portable air cleaners with CADRs of about 100
to 300 cfm in living rooms and/or bedrooms can
substantially reduce indoor concentrations of PM
of both indoor and outdoor origin, often reducing
indoor PM25 concentrations by around 50 percent
on average (e.g., Allen et al. 2011; Barn et al.
2008; Brauner et al. 2008; Butz et al. 2011;
Chen et al. 2015; Cui et al. 2018; Kajbafzadeh
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RESIDENTIAL AIR CLEANERS
et al. 2015; Karottki et al. 2013; Lanphear et
al. 2011; Park et al. 2017; Shao et al. 2017;
Weichenthal et al. 2013; Xu et al. 2010).
Fewer studies have investigated the impact
of portable air cleaners on gaseous pollutant
concentrations or portable air cleaner use patterns
over time. One study demonstrated that the use
of a portable HEPA air cleaner with an activated
carbon media filter also reduced indoor nitrogen
dioxide concentrations in residences immediately
after follow-up, although the reductions
diminished over time, likely as occupants began
to operate the air cleaners less often (Paulin et
al. 2014). The same type of behavior was also
observed in another study in which most people
used their portable air cleaners when researchers
were visiting often early in the study, but usage
declined to only about one-third of households
after researchers stopped visiting (Batterman
et al. 2012). These results further confirm the
importance of maintaining and actually operating
any type of air-cleaning device.
A few experimental studies have also
demonstrated that higher efficiency central HVAC
fibrous media air filters such as MERV 13 or
above can reduce indoor particle concentrations
(Heroux et al 2010; Singer et al. 2016).
Although they remain limited in number, they
tend to confirm several existing modeling
studies that demonstrate similar predicted
outcomes (Azimi et al. 2016; Brown et al. 2014;
Macintosh et al. 2010; Myatt et al. 2008; Zhao
et al. 2015).
Overall, field-testing and simulation studies
show that high-efficiency duct-mounted and
high-CADR portable air cleaners can reduce
levels of airborne particles and, in some cases,
gaseous pollutants in a home. High-efficiency
fibrous media filters (e.g., with high MERV or
HEPA rated) and activated carbon sorbent media
filters have generally been shown to be the most
effective while having the fewest limitations or
adverse consequences.
Evidence for the Impacts of Air Cleaners
on Health Outcomes and/or Biomarkers of
Health Outcomes
Studies investigating the impact of air cleaners
on health outcomes and/or biomarkers of health
outcomes are divided into two categories:
(1) intervention studies of respiratory health
outcomes in homes with subjects with allergies or
asthma and (2) intervention studies of primarily
cardiovascular health outcomes in homes not
targeting subjects with allergies or asthma. The
first group of studies is summarized in Table 4 and
the second group of studies is summarized in Table
5. Each study is also summarized in more detail in
a subsequent section at the end of this document.
Summary of the Impacts on Allergy and Asthma
Health Outcomes
A total of eight intervention studies that
investigated the impact of using air cleaners
in homes on respiratory health outcomes and/
or changes in allergy or asthma symptoms in
subjects with allergies or asthma are summarized
in Table 4. Table 4 includes five studies reported
in Fisk (2013) as well as two additional studies
published since then and one prior study that
was not included in Fisk (2013). Six studies
investigated portable high-efficiency (typically
HEPA) particle filters, one study investigated a
bedroom outdoor air supply unit without a filter,
and one study investigated a central in-duct
UVGI unit. All eight studies reported statistically
significant improvements in at least one health
endpoint, including but not limited to objective
and self-reported outcomes such as peak
expiratory flow, bronchial inflammation markers,
medication use, or symptoms scores. However,
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the magnitudes of improvements were often
modest, and typically a number of other measured
health outcomes were either not affected or the
observed changes were not statistically significant.
Changes in indoor pollutant concentrations, when
measured, were generally large and statistically
significant for measures such as PM2 5 or total VOC
(typically approximately 50-percent reductions
in concentrations), but not for allergen or other
microbial counts. These studies and others also
suggest that the delivery of filtered air close to
the breathing zone (for example, operating an
air cleaner in a bedroom of sleeping allergic or
asthmatic occupants) appears to be more effective
than central HVAC or living room air filtration
(Sublett 2011). Despite some effectiveness
limitations for allergenic particles, the evidence
indicates that air cleaners can be somewhat
effective for reducing allergy or asthma symptoms
in susceptible populations, although the magnitude
of possible improvements is not very large.
Summary of the Impacts on Cardiovascular
Health Outcomes
A total of 11 intervention studies that
investigated the effect of using air cleaners
in homes on primarily cardiovascular health
outcomes and markers of these same health
outcomes in subjects without allergies or asthma
are summarized in Table 5. Measured health
outcomes include lung function, exhaled breath
condensate, blood pressure, and/or heart rate,
while markers of health outcomes include
biomarkers of microvascular endothelial function,
inflammation, oxidative stress, and/ or lung
damage. Table 5 includes four studies reported
in Fisk (2013) and seven additional studies
published since then. Eight studies investigated
portable high-efficiency (typically HEPA) particle
filters, two studies investigated either central in-
duct or window air-conditioner mounted particle
filters with recirculation air, and one study
RESIDENTIAL AIR CLEANERS
investigated a window-mounted unit with outdoor
air ventilation supply. Ten studies involved short-
term health outcomes, while only one study
involved long-term (yearlong) health outcomes.
Ten of the 11 intervention studies found a
significant improvement in at least one measured
cardiovascular health outcome or marker of
cardiovascular health outcomes, including all of
the studies with strong experimental designs. The
magnitude of measured improvements in short-
term health outcomes or markers was typically
between 5 and 10 percent compared to control
groups or conditions. The evidence of a beneficial
effect was generally stronger and more consistent
for studies in locations with higher particle
concentrations. It should be noted that health
benefits from lower exposure to airborne particles,
even in healthy people, are more clearly accrued
over long periods of time (years) rather than
during the short duration (days to weeks) of these
intervention studies (Pope and Dockery 2006). The
results of these short-term studies are therefore
likely capturing only a fraction of the expected
benefits. In fact, in the one long-term study, some
changes in health outcomes (e.g., blood pressures)
were of similar magnitude to those observed
in short-term studies, while changes in other
health outcomes (e.g., markers of inflammation
and oxidative stress) were much greater (e.g.,
approximately 50 percent) (Chuang et al. 2017).
Summary of Health Intervention Studies and
Their Limitations
Of the 20 residential intervention studies
reviewed, 19 found statistically significant
reductions in indoor exposures to indoor PM25,
PM10, and/or particle number counts with the
use of air cleaners, while levels of allergens in air
or in dust were reduced in only one out of three
studies reviewed that measured allergens. Most of
the airborne PM exposure reductions with HEPA
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RESIDENTIAL AIR CLEANERS
or other high-efficiency portable air cleaners
were on the order of approximately 50 percent or
higher. Only three studies investigated the use of
central in-duct air cleaners, and reductions in PM
exposures were not as consistently large.
Nineteen out of 20 residential intervention
studies also found statistically significant
associations between the introduction and use
of air cleaners (and typically reduced indoor
exposures) and at least one measure of health
outcomes or marker of health outcomes. However,
most of the health improvements were relatively
modest in magnitude and, when multiple
outcomes were measured, typically only a fraction
of health outcomes or biomarkers of health
outcomes were impacted.
Although these intervention studies suggest
positive effects of air cleaners on health
outcomes, caution must be taken when
interpreting many of their results. For one, some
studies on the health benefits of air cleaning
involve multiple interventions such as use of
mattress and pillow covers, exclusion of pets from
the bedroom, weekly baths for pets, or vacuum
cleaning, and thus are not necessarily useful in
determining the effects of air cleaners alone.
Additionally, multiple objective health outcomes
typically were measured, but typically only a
fraction of measured outcomes had significant
changes, and sometimes with inconsistent
diurnal patterns or lag periods between exposures
and outcomes, whereas the others were either
unchanged or the changes were non-significant.
Nevertheless, results from the studies reviewed
herein continue to suggest, similar to Fisk
(2013), that particle filtration in homes (primarily
by portable air cleaners with appropriately
sized CADRs) can typically reduce indoor PM
concentrations of various sources and sizes by
an average of approximately 50 percent, whereas
allergen levels in dust are less affected. Using
air cleaners has also been linked to reductions in
some allergy and asthma symptoms, and lowering
indoor PM concentrations with air cleaners has
been shown to beneficially impact some markers
of cardiovascular effects associated with exposure
to indoor PM of both indoor and outdoor origin.
In addition to these intervention studies, there
is sufficient evidence that reducing exposure
to airborne particles in outdoor air has long-
term and short-term benefits to cardiovascular
and respiratory health, among others (U.S. EPA
2009). Given what is known, it is reasonable
and logical to assume that, because much of
human exposure to particles of outdoor origin
actually occurs indoors and because air cleaning
can substantially reduce indoor exposures to
these particles, reduced mortality and morbidity
associated with outdoor particle exposure could
be achievable with the use of improved air
cleaning. Several studies have estimated that
potential health benefits of using particle filtration
to lower indoor exposures to PM of outdoor
origin, including wildfire emissions, are large,
and the estimated financial benefits far exceed
the estimated costs (Fisk and Chan 2017a, b;
Montgomery et al. 2015; Zhao et al. 2015).
Another recent modeling study came to similar
conclusions for using activated carbon filters in
homes to reduce indoor ozone of outdoor origin
(Aldred etal. 2015).
No intervention studies to date were found that
investigated the effects of gas-phase filtration,
ESPs, ionizers, PCO, or plasma systems in portable
or in-duct air cleaners in homes on indoor pollutant
concentrations and associated health symptoms.
The scarcity of data results in little scientific
evidence to evaluate whether these devices are
associated with a reduction in health symptoms.
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Table 4. Intervention Studies of Primarily Respiratory Health Outcomes in Homes With Subjects With Allergies or Asthma
Study
Brehler etal. (2003)
Francis et al. (2003)
Bernstein et al. (2006)
Sulser et al. (2009)
Subjects
44 adults with allergies and/or asthma
30 adults allergic to cats or dog allergen
19 mold-sensitized asthmatic children,
age 5 to 17 years
30 asthmatic children sensitive to pet
allergen
Type of building
Homes (24 rural, 20 urban)
Homes with cats or dogs
Homes with central forced air HVAC
systems
Homes with high cat or dog allergen levels
in dust
Exposures focus
General particles, pollens
Pet allergen
Allergens in dust, bacterial, and fungal
counts in air and dust
Pet allergen
First filter location, type, and
CADR
Bedroom outdoor air supply (fresh air,
no filter)
Bedroom (HEPA, unknown CADR)
In-duct central HVAC (CREON2000 UVGI
with HEPA pre-filter)
Bedroom (220 cfm)
Second filter location, type,
and CADR
n/a
Living room (HEPA, unknown CADR)
n/a
Living room (220 cfm)
Gas-phase filtration
No
No
No
No
Intervention period
2 weeks
12 months
8 weeks
12 months
Reduction in exposures
Not reported
* SS and substantial reductions in
airborne cat and dog allergen in both
groups
* Reductions in intervention group not SS
relative to reductions in control group
* Small but not SS reduction in mold and
bacterial counts in indoor air with UVGI
unit versus placebo
* No SS difference in allergens or molds
in house dust samples
No SS change in cat and dog allergen
concentration in dust
Change in allergy and asthma
symptoms
Subjects with seasonal allergy:
• Nose3 J, (30%) ^
* Eyes3 J, (42%)
* Lung <->•
Subjects with perennial allergy:
• Nose
• Eyes
* Lung <->•
n/a
First treatment period only:
* Asthma symptoms J,
* Asthma medication use J,
Nasal J,
Nocturnal J,
Pediatric quality of life score <->•
Change in objective health
outcomes
* Peak expiratory flow (PEF, a measure
of how fast a person can exhale) in
morning J, (5%)
* PEF in daytime <->•
• Bronchial hyper-reactivity and/or
asthma treatment requirements J,
• Forced expiratory volume (FEV, how
much air a person can exhale during a
breath) <->•
• Forced vital capacity (total amount of
air exhaled during an FEV test) <->•
Both treatment periods:
* Peak expiratory flow (PEF) rate variability
J, (-2% mean; -59% median)
• Forced expiratory volume (FEV) <->•
• Eosinophil cationic protein
(inflammation marker) <->•
• Non-SS trend toward improved
bronchial hyper-responsiveness
Assessment of study strength
Strong (crossover, placebo, randomized
order of exposure)
Moderate (random assignment to
intervention vs. control group, no placebo)
Moderate (random assignment, placebo,
crossover design), but small sample size
Strong (control group with placebo,
random assignment to groups)
Author(s) main conclusion(s)
Recommends fresh air filtration systems
in bedrooms.
"Small but significant improvement in
combined asthma outcome."
"Central UV irradiation was effective at
reducing airway hyper-responsiveness
manifested as peak expiratory flow rate
variability and some clinical symptoms."
"Although HEPA air cleaners retained
airborne pet allergens, no effect on
disease activity...was observed."
>
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Table 4 (continued). Intervention Studies of Primarily Respiratory Health Outcomes in Homes With Subjects With Allergies or Asthma
Study
Xu et al. (2010)e
Butzetal. (2011)
Lanphear et al. (2011)
Park et al. (2017)e
Subjects
30 children with asthma
85 children with asthmab
215 children with asthma
16 children with asthma and/or allergic
rhinitis
Type of building
Homes in New York state
Homes with smokers
Homes with smokers
Homes in California
Exposures focus
General particles and gases
Environmental tobacco smoke
Environmental tobacco smoke
General particles
First filter location, type, and
CADR
Bedrooms (HEPA, -150 cfm, with -3
air changes per hour of outdoor air
ventilation)
Bedroom (HEPA, 225 cfm)
Bedroom (HEPA, 220 cfm)
Living room (HEPA with activated
carbon, -600 cfm)
Second filter location, type,
and CADR
n/a
Living room (HEPA, 225 cfm)
Main activity room (HEPA, 220 cfm)
Bedroom (HEPA with activated carbon,
-450 cfm)
Gas-phase filtration
No
Yes (activated carbon)
Yes (activated carbon and potassium
permanganate zeolite)
Yes (activated carbon)
Intervention period
6 weeks
6 months
12 months
12 weeks
Reduction in exposures
• 72% (PM2E_10)
• 59% (TVOC)
• Intervention group: SS 19.9 and 8.7
^jg/m3 (59% and 46%) decreases in
PM25 and PM10, respectively versus
control group
• Control group: 3.5 and 2.4 ^jg/m3
(9% and 14%) increases in PM25 and
PM10, respectively
• No SS changes in air nicotine or urine
cotinine concentrations
• SS 25% reduction in particle counts >0.3
|jm in intervention group relative to 5%
reduction in control group
* No SS reductions in particle counts >5 pm
or airborne nicotine
43% (PM25)
Change in allergy and asthma
sym ptoms
n/a
• Symptom-free days0 \ (10%)
• Slow activity days <->•
• Nocturnal cough <->•
• Wheeze
• Tight chest <->•
• Asthma symptoms <->•
• Asthma control test scores t (-45%)
* Nasal symptom scores J, (-30%)
Change in objective health
outcomes
• Peak expiratory flow (PEF) t
• Exhaled breath nitrate concentration
(pulmonary inflammation marker) j
• Exhaled breath condensate pH
(pulmonary inflammation marker) t
n/a
• Unscheduled asthma-related visits to a
healthcare provider J, (25%)
• Exhaled nitric oxide (inflammation
indicator) <->•
• Medication use <->•
• Peak expiratory flow (PEF) t (-100%)
Assessment of study strength
Weak (all participants received crossover
intervention, with randomized different
timings; effect size is difficult to
interpret)
Moderate (random assignment to
intervention vs. control group, no
placebo)
Strong (control group with placebo, random
assignment to groups)
Weak (randomized control and
intervention groups, small sample size
of 8 homes per group, no placebo, no
crossover)
Author(s) main conclusion(s)
"Air cleaning in combination with
ventilation can effectively reduce
symptoms for asthma sufferers."d
Air cleaners reduce particles and
symptom-free days but do not prevent
exposure to secondhand smoke.
Air cleaners promising "as part of multi-
faceted strategy to reduce asthma morbidity."
"Reducing indoor PM25with air purifiers
may be an effective means of improving
clinical outcomes in patients with
allergic diseases."
SS = statistically significant; Symbols: t Increase (SS unless otherwise noted), J, Decrease (SS unless otherwise noted), <->• No change
improved in morning log but not subsequently in daytime log.
Excluding subjects in group with air cleaners plus health coach.
CSS improvement in symptom-free days when subjects with air cleaners, both with and without a health coach, were compared to controls.
dln reality, the study did not report changes in asthma symptoms, but rather indicators of asthma symptoms.
eNot reviewed in Fisk (2013).
Table adapted from Fisk (2013) with permission from the publisher.
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Table 5. Intervention Studies of Primarily Cardiovascular Health Outcomes in Homes Not Targeting Subjects With Allergies or Asthma
Study
Brauner et al. (2008)
Allen etal. (2011)
Lin etal. (2011)
Weichenthal etal. (2013)
Subjects
41 healthy non-smoking adults age
60-75
45 adults
60 healthy non-smoking young adults
(students)
37 adults and children, 6 with asthma
Type of building
Urban homes within 350 m of a major
road in Denmark
25 homes in a small city in Canada
Homes in Taiwan
First Nations homes in Canada, most
with smoking
Exposures focus
General particles
Wood smoke
General particles
General particles, tobacco smoke
First filter location, type, and
CADR
Bedroom (HEPA, -320 cfm)
Bedroom of each home (HEPA, 150 cfm)
Central HVAC filter (3M Filtrete)
Main living area (224 cfm)
Second filter location, type,
and CADR
Living room (HEPA, -320 cfm)
Living room (HEPA, 300 cfm)
n/a
n/a
Gas-phase filtration
No
No
No
No
Intervention period
2 days
1 week
4 weeks
1 week
Exposure concentration without
treatment
12.6 ^jg/m3 (PM25 geometric mean)
9.4 ^jg/m3 (PM2510 geometric mean)
10,016 cm 3 (count 10-700 nm)
11.2 [jg/m3 (PM25 mean)
22.8 ± 12.2; 24.5 ± 13.0 \ig/m3 (PM2 5
mean)
49.0 Mg/m3 (PM10)
42.5 Mg/m3 (PM25)
37.5 Mg/m3 (PM:)
Reduction in exposures
63% (PM25 geometric mean)
51% (PM10 geometric mean)
68% (count 10-700 nm)
60% PM25
74% levoglucosan (wood smoke marker)
-20% reduction in PM25
54% (PM10)
61% (PM25)
62% (PMxj
Change in objective health
outcomes
Microvascular function (coronary event
predictor) J, (8%)
Hemoglobin J, (1%)
Inflammation biomarker <->•
Biomarker of coagulation <->•
Reactive hyperemia index (coronary
event predictor) J, (9%)
C-reactive protein (inflammation marker)
| (33%)
Oxidative stress <->•
Systolic blood pressure J, (11%)
Diastolic blood pressure J, (7%)
Heart rate J, (7%)
Systolic blood pressure J, (7%)
Diastolic blood pressure J, (6%)
Forced expiratory flow (PEF) J, (6%)
Forced vital capacity <->•
Peak expiratory flow J, (8%)
Reactive hyperemia index (coronary event
predictor)
Assessment of study strength
Strong (blinded, placebo-controlled
intervention, within-subject, randomized
order of exposure
Strong (crossover, placebo, randomized
order of exposure)
Weak (intervention periods always followed
periods without intervention)
Strong (randomized double blind
crossover with placebo)
Author(s) main conclusion(s)
Filtration of recirculated air may be
a feasible way of reducing the risk of
cardiovascular disease.
Predictors of cardiovascular morbidity
can be favorably influenced by reducing
particles with air cleaners.
Air filtration can reduce indoor PM25
concentrations and modify the effect of
PM25 on blood pressure and heart rate in a
healthy, young population.
Reducing indoor PM may contribute to
improved lung function in First Nation
communities.
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Table 5 (continued). Intervention Studies of Primarily Cardiovascular Health Outcomes in Homes Not Targeting Subjects With Allergies or Asthma
Study
Karottki et al. (2013)e
Chen et al. (2015)e
Kajbafzadeh et al. (2015)e
Padro-Martmez et al. (2015)e
Subjects
48 elderly nonsmoking adults
35 healthy university students
83 healthy adults
20 non-smoking adults
Type of building
27 homes in Denmark
Dormitories in Shanghai, China
Homes in Vancouver, British Columbia,
Canada
Public housing units within 200 m
of major interstate in Somerville,
Massachusetts
Exposures focus
General particles
Indoor particles of outdoor origin
Traffic and woodsmoke particles
Traffic-related and general indoor
particles
First filter location, type, and
CADR
Living room (HEPA, unknown CADR)
Center of the room (Filtrete, 141, 116,
and 97 cfm for pollen, dust, and smoke)
Living room (HEPA, 300 cfm for smoke)
Window mounted in living rooms (MERV
17, 170 cfm with outdoor air ventilation)
Second filter location, type,
and CADR
Bedroom (HEPA, unknown CADR)
n/a
Bedroom (HEPA, 150 cfm for smoke)
n/a
Gas-phase filtration
No
No
No
No
Intervention period
2 weeks
2 days
1 week
3 weeks
Exposure concentration
without treatment
8 LJg/m3 (PM25 median)
7,669 cm 3 (count)
96.2 ^jg/m3 (PM25 mean)
7.1 [jg/m3 (PM25 mean)
11,660 cm 3 (count, mean of medians)
Reduction in exposures
-50% (PM2 5)
-30% (10-300 nm particle number)
57% (PM2 5)
40% (PM2 5)
47% (7 nm to 3 ^jm number
concentrations, or PNC)
Change in objective health
outcomes
Microvascular function ta +-*¦
Lung function
Biomarkers of systemic inflammation <->•
Circulatory inflammatory markers:
* Monoctyle chemoattractant protein-1
I (18%)
• Interleukin-lp J, (68%)
* Myeloperoxidase J, (33%)
Circulatory coagulation markers:
• Soluble CD40 ligand j (65%)
Systolic blood pressure J, (3%)
Diastolic blood pressure J, (5%)
Fractional exhaled nitrous oxide J, (17%)
Several other biomarkers of
inflammation, coagulation,
vasoconstriction or lung function <->•
Biomarkers of systemic inflammation:
• C reactive protein jb
• lnterleukin-6 <->•
• Band cells
Microvascular endothelial function <->•
Reactive hyperaemia index <->•
Biomarkers of systemic inflammation and
coagulation:
• lnterleukin-6 (IL-6) t
• C reactive protein <->•
• Tumor necrosis factor alpha-receptor II
(TNF-RII) ++
• Fibrinogen <->•
Systolic blood pressure <->•
Diastolic blood pressure <->•
Assessment of study strength
Strong (randomized, double-blind,
crossover intervention)
Strong (randomized, double-blind
crossover with placebo)
Strong (randomized, single-blind
crossover with placebo)
Moderate (randomized, double-blind
crossover with placebo; small sample
sizes)
Author(s) main conclusion(s)
"Substantial exposure contrasts in the
bedroom" observed.
The study "demonstrated clear
cardiopulmonary benefits of indoor air
purification among young, healthy adults
in a Chinese city with severe ambient
particulate air pollution."
The "association between C-reactive
protein and indoor PM25 among healthy
adults in traffic-impacted areas is
consistent with the hypothesis that
traffic-related particles (even at low
concentrations) play an important role in
the cardiovascular effects of the urban
PM mixture."
"HEPA filtration remains a promising,
but not fully realized intervention."
Associations between decreased PNC
and increased IL-6 could be due to
confounding factors, interference with
anti-inflammatory medication use, or
exposure misclassification due to time-
activity patterns.
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Table 5 (continued). Intervention Studies of Primarily Cardiovascular Health Outcomes in Homes Not Targeting Subjects With Allergies or Asthma
Study
Chuang et al. (2017)d
Shaoet al. (2017)d
Cui et al. (2018)e
Subjects
200 healthy adults aged 30 to 65 years
35 elderly adults
70 non-smoking healthy adults aged 10
to 26 years
Type of building
Homes in Taipei
Homes in Beijing
Homes in a Shanghai suburb
Exposures focus
General particles and gases
General particles (much from outdoors)
General particles
First filter location, type, and
CADR
Living room (3M Filtrete MPR 1000/MERV 11
in window air-conditioners)
Living room (Philips AC4374, HEPA and
activated carbon with CADR of 215 cfm)
Living area (mostly dorms) (Amway
Atmosphere, HEPA, and activated carbon
with airflow rate of 100 cfm)
Second filter location, type, and
CADR
Master and guest bedrooms (3M Filtrete MPR
1000/MERV 11 in window air-conditioners)
Bedroom (Philips AC4016, HEPA and
activated carbon with CADR of 177 cfm)
n/a
Gas-phase filtration
No
Yes
Yes
Intervention period
1 year
2 weeks
1 day (overnight)
Exposure concentration without
treatment
* 21.4 [jg/m3 (PM25 mean)
* 1.22 ppm (TVOC mean)
60 [jg/m3 (PM25 mean)
* 33.2 ^jg/m3 (PM25 mean)
* 5938 #/cm3 (count mean)
Reduction in exposures
* -40% (PM25 mean)
• -65% (TVOC mean)
-60% (PM25 mean)
* -72% (PM25 mean)
* -59% (PM count mean)
Change in objective health
outcomes
* Systolic blood pressure J, (7%)
* Diastolic blood pressure J, (6%)
* High sensitivity-C-reactive protein (hs-CRP, a
marker of inflammation) J, (50%)
* 8-hydroxy-2'-deoxyguanosine (8-OHdG, a
marker of oxidative stress) J, (53%)
* Fibrinogen (marker of blood coagulation) <->•
* IL-8 (systemic inflammation) J, (58%)d
* Exhaled breath condensate measures <->•
* Lung function measures <->•
* Blood pressure <->•
* Heart rate variability <->•
Airway impedance J, (7%)
Airway resistance J, (7%)
Small airway resistance J, (20%)
Von Willebrand factor (vWF) j (27%)
FEV1 and FVC ^
Blood pressure <->•
IL-6 ^
Assessment of study strength
Strong (randomized, blind, crossover
intervention with large sample size and long
sample duration)0
Moderate (randomized, blind, crossover
intervention), but short duration and small
sample size
Strong (randomized, blind, crossover
intervention with medium/large sample
size but short duration)
Author(s) main conclusion(s)
"...air pollution exposure was associated with
systemic inflammation, oxidative stress and
elevated blood pressure." And "the long-term
filtration of air pollution with an air conditioner
filter was associated with cardiovascular health
of adults."
"...results showed that indoor air filtration
produced clear improvement on indoor air
quality, but no demonstrable changes in
the cardio-respiratory outcomes of study
interest observed in the seniors living with
real-world air pollution exposures."
"A single overnight residential air
filtration, capable of reducing indoor
particle concentrations substantially, can
lead to improved airway mechanics and
reduced thrombosis risk."
SS = statistically significant, m3 = cubic meters; Symbols: \ Increase (SS unless otherwise noted), j, Decrease (SS unless otherwise noted), No change
aSS effects on microvascular function (~6% improvement on average) were observed among subjects not taking any vasoactive drugs when controlling for decreases
indoor PM25 concentrations, suggesting that improvements in vascular function were linked to the effectiveness of the air purifiers in each bedroom.
bA SS increase only occurred in the traffic-impacted homes, not in woodsmoke-impacted homes.
cThe authors noted that while the intent was to blind the intervention (Filtrete) and control (coarse gauze) filters, the participants were not entirely blinded because
the two filters looked very different.
dMeasured in the combined group (both chronic obstructive pulmonary disease [COPD] and non-COPD); the COPD group also experienced a 70% reduction in IL-8.
eNot reviewed in Fisk (2013).
Table adapted from Fisk (2013) with permission from the publisher.
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RESIDENTIAL AIR CLEANERS
Detailed Descriptions of Health Intervention
Studies
Each of the intervention studies summarized in
Table 4 that investigated the effects of using air
cleaners in homes on objective respiratory health
outcomes and/or changes in allergy or asthma
symptoms in subjects with allergies or asthma is
described in more detail below.
1. Brehler et al. (2003) conducted a
randomized, controlled, double-blind,
two-period crossover study to investigate
the effectiveness of fresh air filtration
systems installed in the bedrooms of 44
adult volunteers in Germany suffering
from hay fever. The filtrations systems
were used for a total of 4 weeks in each
home: 2 weeks with an active filter and
2 weeks with a placebo. The combined
ventilation and filtration systems used
a fan ducted to the outside to bring in
outdoor air to provide ventilation, and the
outdoor air was filtered using a European
F7 filter class (approximately equivalent
to MERV 13). Outdoor air ventilation flow
could be controlled between approximately
500 and 2,000 cfm. No indoor exposure
measurements were made. There was a
significant decrease in nighttime hay fever
symptoms and an increase in morning
peak expiratory flow rates, although no
effects were observed in volunteers who
also had perennial allergies.
2. Francis et al. (2003) conducted a
randomized parallel-group study in
the United Kingdom to investigate the
clinical effects of placing portable air
cleaners in the living room and bedroom
of 30 asthmatic adults sensitized to and
sharing a home with cats or dogs for
12 months. The study group included
air cleaners and the use of HEPA filter
vacuum cleaners, while the control group
included the use of HEPA filter vacuum
cleaners alone. The air cleaners had
HEPA filters with an unknown CADR
(Honeywell Model DA-5018). Measured
clinical effects included measures
of airway responsiveness, treatment
requirement, lung function, and peak
flow, results of which were combined
into a single combined asthma outcome.
Measurements of reservoir and airborne
allergen were taken before and after the
interventions. A statistically significant
improvement in combined asthma
outcomes was observed in 10 out of 15
subjects in the active group compared
to only three out of 15 subjects in
the control group after 12 months of
intervention. There were no significant
differences between the active and
control groups for changes in measures
of lung function, reservoir pet allergen, or
airborne pet allergen.
3. Bernstein et al. (2006) conducted a
double-blind, placebo-controlled crossover
trial to investigate the effects of UV
irradiation units with a HEPA pre-filter
(CREON2000 units) installed in central
forced air HVAC systems in the homes of
19 mold-sensitized asthmatic children,
age 5 to 17 years. The study lasted
28 weeks involving 8 weeks with the
UVGI unit operating and 8 weeks with
the placebo operating. The order in which
the systems were used was randomized
among the study group. Clinical outcome
measurements included morning and
evening peak expiratory flow rates and
variability, changes in forced expiratory
volume in 1 second, changes in total
rhino-conjunctivitis and asthma symptom
scores and quaIity-of-life scores, and
changes in medication use. Airborne mold
and bacterial counts were also measured.
Controls had a sham blue light installed
in the HVAC system. There was a small
50 www.epa.gov/iaq
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but not significant reduction in mold
and bacterial counts in indoor air with
the UVGI unit operating, while there was
no significant difference in allergens
or molds in house dust samples. The
authors reported a statistically significant
improvement in peak expiratory flow rate
variability with the UVGI unit compared
to the placebo for both treatment periods
(the mean improvement was 2 percent,
whereas the median improvement was
approximately 59 percent). Also, during
only the first treatment period, there was
a statistically significant improvement in
asthma symptom scores, the number of
days with asthma symptoms, total asthma
medication use, and peak expiratory flow
rate variability in subjects receiving the
UVGI units compared to the placebo
units. No significant differences were
observed between the UVGI units and
placebo units from other clinical or
environmental outcome measurements.
The authors concluded that "central UV
irradiation was effective at reducing airway
hyper-responsiveness manifested as peak
expiratory flow rate variability and some
clinical symptoms."
4. Sulser et al. (2009) conducted a
randomized controlled trial of 30
asthmatic children with sensitization to
cat and/or dog allergens to test the effect
of HEPA air cleaners (IQ Air Allergen 100
with a CADR of approximately 220 cfm)
placed in the living room and bedroom on
pulmonary function, allergy symptoms,
and allergen levels in house dust (Sulser
et al. 2009). After 6 to 12 months, there
was no significant change in lung function
(as measured by peak expiratory flow) or
in the use of medication; however, there
was a slight improvement in bronchial
sensitivity. There was no change in
allergen concentrations in dust samples.
RESIDENTIAL AIR CLEANERS
Overall, the study concluded that the
effectiveness of these air cleaners as
asthma therapy is doubtful.
5. Xu et al. (2010) conducted a field
study in which a combined outdoor air
ventilation supply and HEPA filtration
unit was installed in the bedrooms
of children with physician-diagnosed
asthma for a period of 6 weeks at a time.
The unit provided approximately three
air changes per hour of ventilation air
from outdoors and approximately nine
air changes per hour of recirculation
flow through the filter (i.e., a CADR of
approximately 150 cfm). Exhaled breath
condensate was collected every sixth day
and analyzed for nitrate and pH, and
peak expiratory flow was also measured.
Indoor air measurements included PM10,
carbon monoxide, carbon dioxide, and
TVOC in each bedroom. Indoor PM10 and
TVOC concentrations decreased with the
operation of the device by an average of
72 and 59 percent, respectively. Exhaled
breath condensate nitrate concentrations
decreased significantly and peak
expiratory flows increased significantly
with operation of the unit.
6. Butz et al. (2011) tested the use of a
portable air cleaner and a health coach
intervention to reduce secondhand
smoke exposure in children with asthma
residing with a smoker. The air cleaners
used were Holmes Harmony Air Purifier
HAP650 with an activated carbon filter
and a pleated HEPA filter and a CADR of
225 cfm. The portable air cleaners were
installed in the child's bedroom and in
the living room of his or her home. They
measured indoor PM, nicotine, and urine
cotinine concentrations and tracked the
number of days with asthma symptoms
in the children. Randomly assigned study
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RESIDENTIAL AIR CLEANERS
groups included those that received only
air cleaners, air cleaners plus a health
coach, or a delayed air cleaner installation
(i.e., the control). Each group contained
approximately 40 children, and the
study lasted 6 months. PM25and PM10
concentrations were significantly lower
in both air cleaner groups compared to
the control group, but no differences
were found in indoor air nicotine or urine
cotinine concentrations. The introduction
of a health coach provided no additional
reduction in PM concentrations.
Symptom-free days were significantly
increased in both air cleaner groups
compared with the control group, by an
average of approximately 10 percent.
The study concluded that although the
use of air cleaners reduced indoor PM
concentrations and increased symptom-
free days, it was not adequate to prevent
exposure to secondhand smoke.
7. Lanphear et al. (2011) conducted a
double-blind, randomized trial to test
the effects of HEPA air cleaners on
unscheduled asthma visits and symptoms
among children with asthma exposed to
secondhand smoke. The HEPA air cleaners
(Austin Healthmate, with what appears
to be a CADR of 220 cfm) also contained
carbon-potassium permanganate-zeolite
filter inserts to adsorb gases. Two air
cleaners were installed; one in the child's
bedroom and one in the main living area.
A total of 225 children were enrolled
in the study; 110 were assigned to the
intervention group with an active HEPA
air cleaner, and 115 to the control group
with a sham air cleaner. Children in the
intervention group had approximately
18 percent fewer unscheduled asthma
visits than the control group, corresponding
with a 25-percent reduction in particle
concentrations (>0.3 |jm) in the
intervention group compared to a 5-percent
reduction in the control group. There were
no statistically significant differences in
parent-reported asthma symptoms, exhaled
nitric-oxide levels, air nicotine levels, or
cotinine levels between groups.
8. Park et al. (2017) evaluated the
effectiveness of portable air cleaners for
reducing indoor PM25 concentrations
and health outcomes in children with
asthma and/or allergic rhinitis in 16
homes in California. Air cleaners were
installed in the living room and bedrooms
of the subjects during a 12-week study
duration. The air cleaners used a three-
step filtering system: dust filter, activated
carbon filter, and HEPA filter (Samsung
Models AX7000 and AX9000, with
CADRs of approximately 450 cfm and
approximately 600 cfm). Eight homes
received air purifiers, and eight homes
did not. The average indoor PM25
concentration was 43 percent lower in
the air cleaners group (from 7.4 to 4.3
|jg/m3). At 12 weeks, the air cleaners
group showed improvements in childhood
asthma control test scores and mean
evening peak flow rates, whereas the
control group showed deterioration in the
same measures. Total and daytime nasal
symptoms scores were also significantly
lower in the air cleaners group.
Each of the intervention studies summarized in
Table 5 that investigated the effects of using air
cleaners in homes on primarily cardiovascular
health outcomes and/or markers of cardiovascular
health outcomes—including lung function, exhaled
breath condensate, blood pressure, heart rate, and/
or several biomarkers of microvascular endothelial
function, inflammation, oxidative stress, and/or
lung damage—is described in more detail below.
52 www.epa.gov/iaq
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1. Brauner et al. (2008) investigated the
effects of controlled exposure to indoor air
particles on microvascular function and
biomarkers of inflammation and oxidative
stress in a healthy elderly population living
in apartments in Denmark. A total of
21 non-smoking couples participated in a
randomized, double-blind, crossover study
with two consecutive 48-hour exposures to
either particle-filtered or non-filtered air.
HEPA filter air cleaners with a flow rate of
540 m3/hr (approximately 320 cfm) were
placed in the living room and bedroom
in each apartment. Indoor air filtration
significantly improved microvascular
function by approximately 8 percent,
and the mass concentration of PM2 5 was
more important than the total number
concentration of particles 10 to 700 nm.
2. Allen et al. (2011) deployed portable
HEPA air filters and placebo filtration in a
randomized crossover intervention study
of 45 healthy adults in a woodsmoke-
impacted community during consecutive
7-day periods of filtered and non-filtered
air each. The air cleaners were installed
in the main activity room of the house
(with a CADR of 300 cfm for tobacco
smoke) as well as in the participants'
bedrooms (with a CADR of 150 cfm for
tobacco smoke). They measured indoor
PM25 concentrations using integrated
gravimetric sampling and evaluated
endothelial function and measures of
oxidative stress and systemic inflammation
as markers of cardiovascular health. HEPA
filters reduced indoor PM25 concentrations
in 24 of 25 homes, with a mean
reduction of 60 percent. Concentrations
resulting from both indoor and outdoor-
infiltrated sources were significantly
reduced. Air filtration was associated
with improved endothelial function and
RESIDENTIAL AIR CLEANERS
decreased concentrations of inflammatory
biomarkers but not markers of oxidative
stress. Specifically, HEPA filtration was
associated with a 9.4 percent increase in
reactive hyperemia index, an indicator of
microvascular endothelial function, and
a 32.6 percent decrease in C-reactive
protein, an indicator of inflammation.
3. Lin et al. (2011) evaluated whether the
use of improved central air conditioner
filters (3M Filtrete) would reduce indoor
PM25 and impact blood pressure and heart
rate in a young, healthy population of
60 students in Taiwan. Blood pressure and
heart rate were monitored continuously
for 48 hours at approximately 2-week
intervals over the course of four home
visits within a 1.5-month period each.
PM25 concentrations were measured at
1-minute intervals during each study
period. TVOCs were also measured. Indoor
PM2 5concentrations and participant blood
pressure and heart rate were higher during
the first two visits without a filter than the
last two visits with the filter.
4. Weichenthal et al. (2013) conducted
a crossover study on a First Nations
reserve in Manitoba, Canada, of portable
electrostatic air cleaners installed in the
main living area of 20 homes with
37 residents. Lung function, blood
pressure, and endothelial function
measures were collected at the beginning
and end of each week-long measurement
period. The air cleaners were 3M Filtrete
FAP03-RS Ultra Clean Air Purifiers
with a CADR of 224 cfm for smoke. A
placebo was installed for the control
weeks. Indoor pollutant measurements
included integrated PMj, PM25, and
PM10; polycyclic aromatic hydrocarbons;
several VOCs; and nitrogen dioxide.
Average indoor PM25 concentrations were
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RESIDENTIAL AIR CLEANERS
almost 50 percent lower with the filters
installed, although concentrations were
still much higher than outdoors because
of a high prevalence of indoor smoking.
Portable air cleaner use was associated
with statistically significant increases
in lung flow, decreases in systolic blood
pressure, and decreases in diastolic blood
pressure. Consistent inverse associations
were also observed between indoor PM25
and lung function. The study concluded
that commercially available portable air
cleaners may offer substantial reductions
in indoor PM concentrations and that
such reductions may be associated with
improved lung function, but that efforts
aimed at improving indoor air quality
should begin with reducing indoor sources
such as smoking in these communities.
5. Karottki et al. (2013) conducted a
randomized, double-blind crossover
intervention study with consecutive
2-week periods with or without a portable
HEPA air cleaner (with an unknown
flow rate and CADR) installed in the
living room and bedroom of 48 elderly
nonsmoking adults in 27 homes to
investigate their effects on respiratory
and cardiovascular health by measures of
inflammation and vascular dysfunction.
Health outcome measures included blood
pressure; microvascular and lung function;
and hematological, inflammation,
monocyte surface and lung cell damage
markers measured from collected blood
samples. The air cleaners reduced indoor
PM25 mass concentrations and particle
number concentrations by approximately
50 and 30 percent on average,
respectively, although the effectiveness
varied by home. There were no statistically
significant differences in microvascular
and lung function or the biomarkers of
systemic inflammation with and without
the HEPA filter installed. However, there
was a small impact when filtration was
considered in conjunction with indoor
PM25 concentrations, resulting in
improved microvascular function in homes
with lower indoor PM25 concentrations.
6. Chen et al. (2015) conducted a
randomized, double-blind crossover
trial of short-term portable air cleaner
interventions in the dormitories of
35 healthy college students in Shanghai,
China. Students were randomized into
two groups and alternated the use of
true or sham air purifiers for 48 hours
with a 2-week interval in between.
The air cleaners had a CADR of 141
for pollen, 116 for dust, and 97 for
smoke and three fan speeds. Fourteen
biomarkers of inflammation, coagulation,
and vasoconstriction; lung function;
blood pressure; and fractional exhaled
nitric were measured as markers of
cardiopulmonary impacts. On average,
air purification resulted in a 57 percent
reduction in PM25 concentrations when
filters were operating. Air purification was
significantly associated with decreases
in several circulating inflammatory and
thrombogenic biomarkers, decreases in
systolic and diastolic blood pressure, and
decreases in fractional exhaled nitrous
oxide. The effects on lung function
and vasoconstriction biomarkers were
beneficial but not statistically significant.
7. Kajbafzadeh et al. (2015) conducted
a randomized, single-blind crossover
intervention study to evaluate the
effects of portable HEPA air cleaners on
indoor PM25 concentrations, endothelial
function, and systemic inflammation
among 83 healthy adults in Vancouver,
British Columbia, Canada, living in traffic-
or woodsmoke-impacted areas. HEPA
filtration, including one located in the
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living room (Honeywell Model 50300 with
a CADR of 300 cfm for smoke) and one
located in the bedroom (Honeywell 18150
with a CADR of 150 cfm for smoke), was
associated with a 40 percent decrease
in indoor PM25 concentrations, but
there was no relationship between PM25
exposure and endothelial function. There
was an association between indoor PM25
concentrations and a measure of systemic
inflammation in homes in areas affected
by vehicle traffic but not by woodsmoke.
8. Padro-Martinez et al. (2015) conducted
a randomized, double-blind crossover
trial of the effects of HEPA air filter units
in the living rooms of 19 public housing
units located within 200 m of a highway
on particle number concentrations,
blood pressure, and blood biomarkers of
cardiovascular health. The air cleaners
were HEPAiRx units with a MERV 17
filter and an airflow rate of approximately
170 cfm. Particle number concentrations
were reduced by 21 to 68 percent in the
apartments, but there were no significant
differences in blood pressure or three
of four biomarkers, while one biomarker
actually increased with the filtration
units. The study noted the importance
of using larger sample sizes and better
understanding time-activity patterns that
also contribute to exposures.
9. Chuang et al. (2017) conducted a
randomized, blind crossover trial of the
effects of high-efficiency window-mounted
air-conditioning filters (3M Filtrete with
1000 MPR/MERV 11) installed in
200 homes in Taipei. One hundred adult
participants were randomly assigned to an
air filtration or control group, and six home
visits were conducted per year. The control
and intervention groups were then switched
after 1 year. Indoor pollutant measurements
RESIDENTIAL AIR CLEANERS
included 24-hour monitoring of PM25 and
TVOC concentrations. Blood pressure was
monitored for each participant during each
visit. The morning following air pollution
monitoring, blood samples were collected
and analyzed for biological markers of
cardiovascular health, including high
sensitivity-C-reactive protein (hs-CRP),
8-hydroxy-2'-deoxyguanosine (8-OHdG, a
marker of oxidative stress), and fibrinogen.
Indoor PM25 and TVOC concentrations
were lower in the filtration intervention
groups by approximately 40 and
65 percent, on average. Lower PM25
and TVOC concentrations were also
correlated with lower blood pressure and
lower levels of hs-CRP and 8-OHdG (with
no statistically significant changes in
fibrinogen levels).
10. Shao et al. (2017) conducted a
randomized crossover trial of the effects
of portable air filtration units on indoor
PM2 5 and biomarkers of respiratory and
systemic inflammation, oxidative stress,
lung function, and blood pressure and
autonomic nervous system function in
35 non-smoking elderly participants with
and without chronic obstructive pulmonary
disease (COPD) in Beijing. Portable
air cleaners with HEPA and activated
carbon filters (Philips AC4374 with a
CADR of 215 cfm in the living room and
Philips AC4016 with a CADR of 177
cfm in the bedrooms) were installed for
a 2-week period in addition to a 2-week
sham installation period. Measurements
were conducted in 20 households.
Pollutant monitoring included 10-day
integrated indoor PM25 and black carbon
concentrations, along with elemental
analysis of PM25 concentrations. Clinical
outcomes included measures of respiratory
inflammation and oxidative stress
(e.g., exhaled breath condensate),
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RESIDENTIAL AIR CLEANERS
systemic inflammation (e.g., fibrinogen,
C-reactive protein, interleukin-6,
interleukin-8), lung function (e.g., forced
expiratory volume), and blood pressure
and heart rate variability. The 10-day
average indoor PM25 concentrations
were approximately 60 percent lower
in the intervention group. The only
significant change in health endpoints
was that interleukin-8, a measure of
systemic inflammation, was reduced in
the filtration group (both the total group
and the COPD group). There were no
significant improvements in lung function,
blood pressure, or heart rate variability
the following short-term air cleaner
interventions.
11. Cui et al. (2018) conducted a double-
blind, randomized crossover study of the
effects of portable air filtration on markers
of cardiopulmonary health outcomes in
70 non-smoking healthy adults, aged 19
to 26 years, during overnight (-13 hour)
periods in homes in a suburb of Shanghai,
China. Each participant received both true
and sham indoor air filtration, with true
and sham sessions separated by a 2-week
washout interval. Participants received a
commercially available air purifier with
a HEPA and activated carbon filters and
an airflow rate of approximately 100 cfm.
Participants were a combination of healthy
adults and nursing students living in
dormitory rooms. Each session started at
6 p.m. on a Saturday, and participants
stayed and slept in their dorms with doors
and windows closed until the next morning.
The ordering of true and sham filtration
was randomly assigned. Pollutant exposure
measurements included PM25, particle
number (i.e., 10 nm to 1 |jm), ozone, and
N02. Measured markers of health outcomes
included: lung function by spirometry
and impulse oscillometry; respiratory
inflammation by fractional exhaled nitric
oxide; cardiovascular function by pulse
wave analysis and systolic and diastolic
blood pressure; systemic inflammation
and coagulation by blood sampling
and analysis for interleukin-6, soluble
P-selectin (sCD62P), and von Willebrand
factor (vWF); and systemic oxidative stress
by urine sampling and analysis for urinary
free malondialdehyde (MDA). Outdoor
PM25 concentrations ranged from 18.6 to
106.9 |jg/m3 during the study. Compared
to sham filtration, true filtration decreased
the indoor PM25 and total particle number
concentrations by 72.4 percent and
59.2 percent on average, respectively.
True filtration significantly improved lung
function measured immediately after
the end of filtration, as measured by
lowered airway impedance and resistance
as indicators of airway mechanics. No
significant improvements for spirometry
indicators were observed. True filtration
also significantly lowered vWF by 26.9
percent on average 24 hours after the
end of the filtration period, indicating
reduced risk for thrombosis. Finally, in
analyses stratified by male and female
participants, vWF and interleukin-6 were
both significantly reduced in males while
pulse pressure was significantly decreased
in females. The authors concluded that a
single period of overnight residential air
filtration was capable of reducing indoor
particle concentrations substantially and
led to improved airway mechanics and
reduced thrombosis risk.
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RESEARCH NEEDS
Research needs on duct-mounted and portable
air-cleaning technology effectiveness:
• Conduct long-term health intervention
studies of portable and in-duct air cleaners.
• Collect field measurements of pollutant
removal effectiveness and conduct health
intervention studies for those air-cleaning
technologies that have not yet been
comprehensively studied, such as PCO,
plasma, UVGI, sorbent technologies, and
other technologies that are currently being
marketed to consumers.
• Investigate what aspects of product
design and operation affect how and why
consumers operate portable and in-duct air
cleaners along the entire life cycle of an air
cleaner (e.g., runtimes, noise, maintenance,
filter changes) and how that impacts
effectiveness.
• Develop and validate air cleaner test
standards that specifically address indoor
PM,,, ultrafine particles, and speciated
VOCs.
• Develop and validate accurate pollutant
sensors for incorporating into effective and
economical consumer-grade holistic air-
cleaning systems (e.g., ability to accurately
measure concentrations of ultrafine
particles, PM25, and speciated VOCs over
long periods of time).
• Assess the pollutant removal effectiveness
(e.g., for fine and ultrafine particles) of
ductless residential HVAC systems such as
mini-split systems.
• Collect field measurements of pollutant
removal effectiveness and conduct health
intervention studies on emerging air-
cleaning technologies such as passive
material coatings and bio-walls.
RESIDENTIAL AIR CLEANERS
FURTHER RESOURCES
Association of Home Appliance Manufacturers
(AHAM): www.aham.org
ASHRAE position document on filtration and
air cleaning: www.ashrae.org/about/position-
documents
CADR information: www.ahamverifide.org/search-
for-products/room-air-cleaners/what-is-the-clean-
air-delivery-rate-cadr
California Air Resources Board Certified Air
Cleaning Devices: www.arb.ca.gov/research/indoor/
aircleaners/certified.htm
Consumer Reports-, www.consumerreports.org
EPA's Indoor Air Quality website: www.epa.gov/
indoor-air-quality-iaq
EPA's Radon website: www.epa.gov/radon
EPA's "Ozone Generators that are Sold as Air
Cleaners": www.epa.gov/indoor-air-quality-iaq/
ozone-generators-are-sold-air-cleaners
EPA's "Should You Have the Air Ducts in Your
Home Cleaned?": www.epa.gov/indoor-air-quality-
iaq/should-you-have-air-ducts-your-home-cleaned
EPA's "Wildfire Smoke: A Guide for Public
Health Officials": www3.epa.gov/airnow/wildfire_
may2016.pdf
National Air Filtration Association (NAFA):
www.nafahq.org
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RESIDENTIAL AIR CLEANERS
ACRONYMS AND ABBREVIATIONS
8-OHdG
8-hydroxy-2'-deoxyguanosine
AHAM
Association of Home Appliance Manufacturers
ANSI
American National Standards Institute
ASD
active soil depressurization
CADR
clean air delivery rate
CDC
Centers for Disease Control and Prevention
cfm
cubic feet per minute
COPD
chronic obstructive pulmonary disease
EPA
U.S. Environmental Protection Agency
ESP
electrostatic precipitator
FPR
Filter Performance Rating
hs-CRP
high sensitivity-C-reactive protein
HVAC
heating, ventilating, and air-conditioning
IEC
International Electrotechnical Commission
ISO
International Organization for Standardization
kWh
kilowatt-hour
|jm
micrometer
m3
cubic meter
MERV
Minimum Efficiency Reporting Value
MPR
Microparticle Performance Rating
NAFA
National Air Filtration Association
nm
nanometer
PCO
photocatalytic oxidation
PM
particulate matter
pm,5
fine particulate matter smaller than 2.5 |jm in diameter
PM10
coarse particulate matter smaller than 10 |jm in diameter
PPb
parts per billion
RRNC
radon-resistant new construction
TVOC
total volatile organic compounds
UV
ultraviolet
UVGI
ultraviolet germicidal irradiation
VOC
volatile organic compound
vWF
von Willebrand Factor
W
watts
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RESIDENTIAL AIR CLEANERS
GLOSSARY
Acute
Adsorption
Air cleaner
Air filter
Airflow resistance
Allergen
Allergic respiratory disease
Allergy
Having a rapid onset and following a short but potentially severe course.
The physical process that occurs when liquids, gases, or suspended matter
adhere to the surfaces or in the pores of a material.
A device used to remove particulate or gaseous impurities from the air;
examples include fibrous filter media combined with a fan, sorbent media
combined with a fan, electrostatic precipitator, ion generator, ultraviolet
germicidal irradiation cleaner, and photocatalytic oxidation cleaner.
A device that removes particulate material from an airstream.
See pressure drop.
A chemical or biological substance (e.g., pollen, animal dander, house
dust mite proteins) that can cause an allergic reaction characterized by
hypersensitivity (an exaggerated immune response).
A collection of health conditions, including allergies and asthma, that are
characterized by nasal or bronchial symptoms that can be triggered by
environmental exposures.
An exaggerated or pathological immune reaction to breathing, eating,
or touching substances that have no comparable effect on the average
individual.
American Society of Heating, ASHRAE is a global professional society that focuses on building systems,
Refrigerating and Air-Conditioning energy efficiency, indoor air quality, refrigeration, and sustainability
technologies.
Engineers (ASHRAE)
Asthma
Bacterial spore
Chemisorption
Chronic
Clean air delivery rate (CADR)
A usually chronic inflammatory disorder of the airways characterized by
intermittent episodes of wheezing, coughing, and difficulty breathing,
sometimes associated with an allergy to inhaled substances.
Inactive stage of bacteria, with a thick protective coating that allows the
bacteria to survive harsh environmental conditions.
A process whereby a chemical substance adheres to a surface through the
formation of a chemical bond.
Marked by long duration, by frequent recurrence over a long time, and often
by slowly progressing seriousness.
A measure of air cleaner performance, defined as the amount of
contaminant-free air delivered by the device, expressed in cubic feet per
minute (cfm). CADRs are always the measurement of a unit's performance as
a complete system.
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RESIDENTIAL AIR CLEANERS
Corona discharge
Dander
Disinfection
Double-blind study
Effectiveness (of an air cleaner)
Efficiency (of an air cleaner)
Electret media
Electrostatic precipitator (ESP)
Fibrous media air filter
Filter Performance Rating (FPR)
HEPA (high-efficiency particulate
air) filter
Ionizer (air cleaner)
Minimum Efficiency Reporting
Value (MERV)
Mold spore
Microparticle Performance Rating
(MPR)
An electrical discharge brought on by the ionization of a fluid surrounding a
conductor, which occurs when the potential gradient exceeds a certain value.
Minute scales of skin. Dander also may contain hair or feathers.
The process of any reduction or prevention of growth in a microbial
population with no percentage efficiency specified.
A type of clinical trial study design in which the study participants and the
investigators do not know the identity of the individuals in the intervention
and control groups until data collection has been completed.
A measure of the ability of an air-cleaning device to remove pollutants from
the space it serves.
A measure of the ability of an air-cleaning device to reduce the concentration
of pollutants in the air that passes once through the device. Also referred to
as "single-pass" efficiency.
Fibrous filter media with an electrostatic charged initially applied to enhance
particle removal.
A type of air cleaning technology that removes particles by an active
electrostatic charging process that requires electricity to charge particles
that become attracted and adhere to oppositely charged plates.
A type of air filter that removes particles by capturing them onto fibrous fiber
materials.
A proprietary filter efficiency rating metric.
An extended surface mechanical air filter having a minimum fractional
particle removal efficiency of 99.97 percent for all particles of 0.3 |jm
diameter, with high efficiency for both larger and smaller particles.
An air-cleaning device that uses a high-voltage wire or carbon fiber brushes
to electrically charge air molecules, which produces negative ions onto which
airborne particles attach and become charged. The charged particles can
attach to nearby surfaces such as walls or furniture, or to one another, and
settle faster. Also called an "ion generator."
A filter efficiency rating metric resulting from laboratory testing following
ASHRAE Standard 52.2.
Tiny reproductive structures produced by vegetative mold.
A proprietary filter efficiency rating metric.
60 www.epa.gov/iaq
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Ozone
Particle
Photocatalytic oxidation (PCO)
Placebo effect
Plasma air-cleaning technology
Pressure drop
Radon
Rhinitis
Sorption
Ultrafine particles
RESIDENTIAL AIR CLEANERS
Chemical symbol 03; An unstable allotrope of oxygen that is formed
naturally from atmospheric oxygen by electric discharge or exposure to
ultraviolet radiation and is also produced in the lower atmosphere by the
photochemical reaction of certain pollutants. It is poisonous at sufficiently
high concentrations.
A small discrete mass of solid or liquid matter that remains individually
dispersed in gas or liquid emissions (usually considered to be an
atmospheric pollutant).
An air cleaner technology that uses a high-surface-area medium coated with
a catalyst such as titanium dioxide that adsorbs and reacts with gaseous
pollutants when irradiated with UV light.
A usually, but not necessarily, beneficial effect attributable to an expectation
that an action such as a treatment will have a desired outcome.
An air-cleaning technology that uses a high-voltage discharge to ionize
incoming gases, which breaks their chemical bonds and chemically alters
gaseous pollutants.
The difference in pressure between two points of a fluid (such as air) in a
system. Pressure drop occurs when frictional forces act on a fluid as it flows
through a system.
A colorless, odorless, radioactive gas that can be found in indoor air. It
comes from radium in natural sources such as rock, soil, ground water,
natural gas, and mineral building materials (e.g., granite countertops).
As uranium breaks down, it releases radon, which in turn produces short-
lived radioactive particles called "progeny," some of which attach to dust
particles.
Inflammation of the mucous membrane lining of the nose.
The common term used for adsorption or chemisorption interactions.
Particles smaller than 0.1 |jm.
Ultraviolet (UV) light
UV-A
UV-B
UV-C
Vegetative bacteria and molds
Volatile organic compounds
(VOCs)
An electromagnetic radiation with a wavelength from 10 nm to 400 nm,
shorter than that of visible light but longer than X-rays.
Long-wave UV radiation (315 to 400 nm).
Mid-wavelength UV radiation (280 to 315 nm).
Short-wave UV radiation (100 to 280 nm).
Microorganisms that are in the growth and reproductive stage (i.e., not
spores).
Chemicals that contain carbon and are vaporous at room temperature and
pressure.
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RESIDENTIAL AIR CLEANERS
*
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