United States         Air And
             Environmental Protection    Radiation
             Agency           (ANR-445)
                           EPA 400/1-90-002
                           Februarv 1990
A Summary  of
Available Information

                                      EPA 400/1-90/002
                                      February 1990
Residential  Air-Cleaning Devices

 A Summary of Available Information
            U.S. Environmental Protection Agency
              Indoor Air Division (ANR-445)
         Office of Atmospheric and Indoor Air Programs
               Office of Air and Radiation
                Washington, DC 20460


This document has been reviewed in accordance with      :
U.S. Environmental Protection Agency policy and approved for
publication. Mention of any trade names or commercial products
does not constitute endorsement or recommendation for use.

                       TABLE OF CONTENTS





     Particle Removal	5
     Removal of Gaseous Pollutants	8
     Removal of Radon and its Progeny	,;	11



     Installation, Use, and Need for Maintenance		13
     Cost	13
     Production or Redispersal of Pollutants	14
     Inability to Remove Some Odors	14
     Possible Effects of Particle Charging	14
     Soiling of Walls and Other Surfaces	14
     Noise	15


     Standards for In-duct Devices	15
     Standard for Portable Air Cleaners	16





Indoor air pollutants are unwanted, sometimes harmful, materials! in the air.  They range
from dusts to chemicals to radon. Air cleaners are devices that attempt to remove such
pollutants from the indoor air you breathe.

The typical furnace filter installed in the ductwork of most home healing and/or air-
conditioning systems is a simple air cleaner.  This basic filtering system may be upgraded by
using another filter to trap additional pollutants or by adding additional air-cleaning
devices.  An alternative to upgrading the in-duct air cleaning system is using individual
room, portable air cleaners. Air cleaners generally rely on filtration, or the attraction of
charged particles to the air cleaning device itself or to surfaces within the home, for the
removal of pollutants. The use of "air cleaning" to remove pollutants from the air in
residences is in its infancy; this publication presents the current state of knowledge.

This publication describes the types of air cleaners available to the consumer, provides
available information on their general effectiveness in removing iindoor air pollutants,
discusses some factors to consider in deciding whether to use an air-cleaning unit, and
describes existing guidelines that can be used to compare units. It does not discuss the
effectiveness of air-cleaning systems installed in the central heating, ventilating, and air-
conditioning (HVAC) systems of large buildings, such as apartment, office, or public
buildings, nor does it evaluate specific products.

Because many factors need to be considered in determining whether use of an air cleaner is
appropriate in a particular setting, the decision whether  or not to use an  air cleaner is left to
the individual.  EPA has not taken a position either for or against (the use of these devices in
the home.

For the purposes of discussion, we will divide the pollutants into tlxree groups: particles,
gaseous pollutants, and radon and its progeny.

Particles are very small solid or liquid substances that are light enough to float suspended in
air (e.g., mists, dust, or pollen).  They are composed of diverse materials including inorganic
and organic compounds and dormant and living organisms. Of primary concern from a
health standpoint are: 1) small, invisible respirable-size particles, v/ith a higher probability
of penetrating deep into the lungs, where they may stay a long time and may cause acute or
chronic effects, and 2) larger particles, such as some molds, pollen, animal dander, and
house dust allergens, which do not penetrate as deeply, but may cause an allergic response.

Respirable-size particles include, but are not limited to, those from cigarette smoke;
unvented combustion appliances such as gas stoves and kerosene heaters; viruses, bacteria,
and some molds; and fragments of materials which, when whole, would be considered
larger than respirable-size particles.  Health effects from exposure to respirable-size particles

in the air depend on the types and concentrations of particles present, the frequency and
duration of exposure, and individual sensitivity.  Health effects can range from irritation of
the eyes and/or respiratory tissues to more serious effects, such as cancer and decreased lung
function. Biological particles, such as animal and insect allergens, viruses, bacteria, and
molds, can cause allergic reactions, infectious diseases, and/or can produce toxic products
which may be released into the air.

Gaseous pollutants include combustion gases and organic chemicals which are not
associated with particles. Hundreds of different gaseous pollutants have been detected in
indoor air.

Sources of combustion gases (such as carbon monoxide and nitrogen dioxide) include
combustion appliances, cigarette smoking, and the infiltration of vehicle exhaust gases from
attached garages or the outdoors.

Gaseous organic compounds may enter the air from sources such as cigarette smoking,
building materials and furnishings, and the use of products such as paints, adhesives, dyes,
solvents, caulks, cleaners, deodorizers, personal hygiene products, waxes, hobby and craft
materials, and pesticides.  In addition, organic compounds may originate outdoors or
through cooking of foods and human, plant, and animal metabolic processes.

Health effects from exposure to gaseous pollutants in the air may vary widely depending on
the types and concentrations of the chemicals present, the frequency and duration of
exposure, and individual sensitivity.  Adverse effects may include irritation of the eyes
and/or respiratory tissues; allergic reactions; effects on the respiratory, liver, immune,
cardiovascular, reproductive, and/or nervous system; and cancer.        \

Radon and its progeny are radioactive pollutants  which originate from natural sources such
as rock, soil, groundwater, natural gas, and mineral building materials. These pollutants
have the potential to cause lung cancer in humans.  The risk of lung cancer increases with
the level in the air and the frequency and duration of exposure.

Radon itself is  a gas which produces short-lived progeny in  the form of particles, some of
which become  attached to larger particles.  Radon progeny may deposit in the lungs and
represent the main health hazard from the radon series.

The three strategies (in order of effectiveness) for reducing pollutants in indoor air are
source control, ventilation, and air cleaning.

Source control eliminates individual sources of pollutants or reduces their emissions, and
is generally the most effective strategy.  Some sources, like those that contain asbestos, can

be sealed or enclosed; others, like combustion appliances, can be adjusted to decrease the
amount of emissions.  Unfortunately, not all pollutant sources can be identified and
practically eliminated or reduced.

Ventilation brings outside air indoors.  It can be achieved by opening windows and doors,
by turning on local bathroom or kitchen exhaust fans, or, in some situations, by the use of
mechanical ventilation systems, with or without heat recovery ventilators (air-to-air heat
exchangers).   However, there are practical limits to the extent ventilation can be used to
reduce airborne pollutants.  Costs for heating or cooling incoming .air can be significant, and
outdoor air itself may contain undesirable levels of contaminants.

Air cleaning may serve as an adjunct to source control and ventilation. However, the use
of air cleaning devices alone cannot assure adequate air quality, particularly where
significant sources are present and ventilation is inadequate.

Air cleaners are usually classified by the method employed to remove particles of various
sizes from the air. There are three general types of air cleaners* on the market: mechanical
filters, electronic air cleaners, and ion generators.

Mechanical filters may be installed in ducts in homes with central heating and/or air-
conditioning or may be used in portable devices which contain a fain to force air through the
filter.  Mechanical filters used for air cleaning are of two major types.

Flat or panel filters generally consist either of a low packing density of coarse glass fibers,
animal hair, vegetable fibers, or synthetic fibers often coated with a viscous substance (e.g.,
oil) to act as an adhesive for particulate material, or slit and expanded aluminum. (A flat
filter in use in many homes is the typical furnace filter installed in central heating and/or
air-conditioning systems.)  Flat filters may efficiently collect  large particles, but remove only
a small percentage of respirable-size particles.

Flat filters may also be made of "electret" media, consisting of a pennanently-charged plastic
film or fiber. Particles in the air are attracted to the charged materietl.

Pleated or extended surface filters generally attain greater efficiency for capture of respirable-
size particles than flat filters.  Their greater surface area allows the use of smaller fibers and
an increase in packing density of the filter without a large drop in air flow rate.

Electronic air cleaners use an electrical field to trap charged particles. Like mechanical
filters, they may be installed in central heating and/or air-conditioning system ducts or may
       "Because they may reduce some pollutants present in indoor air through condensation, absorption, and
other mechanisms, devices such as air conditioners, humidifiers, and dehumidifiens maiy technically be
considered air cleaners. However, this publication includes only those devices specifically designed and
marketed as air cleaners.

be portable units with fans.  Electronic air cleaners are usually electrostatic precipitators or
charged-media filters. In electrostatic precipitators, particles are collected on a series of flat
plates. In charged-media filter devices, which are less common, the particles are collected
on the fibers in a filter.  In most electrostatic precipitators and some charged media filters,
the particles are deliberately ionized (charged) before the collection process, resulting in a
higher collection efficiency.

Ion generators also use static charges to remove particles from indoor air. These devices
come in portable units only. They act by charging the particles in a room, so they are
attracted to walls, floors, table tops, draperies, occupants, etc. In some cases, these devices
contain a collector to attract the charged particles back to the unit.

(Note: The latter two types of devices may produce ozone, either as a by-product of use or
intentionally.  Concerns about ozone production are discussed in more depth on page 14.)

Some newer systems on  the market are referred to as "hybrid" devices.  They contain two or
more of the particle removal devices discussed above. For example, one or more types of
mechanical filters may be combined with an electrostatic precipitator or an ion generator.

In addition to particle removal devices, air cleaners may also contain adsorbents and/or
reactive materials to facilitate removal of gaseous materials from indoor air.   Air cleaners
which do not contain these types of materials will not remove gaseous pollutants. The
potential effectiveness of air cleaners containing  these materials in reducing levels of
gaseous pollutants in indoor air is discussed on page 8.

The effectiveness of air cleaners in removing pollutants from the air depends on both the
efficiency of the device itself (e.g., the percentage of the pollutant removed as it goes
through the device) and the amount of air handled by the device. For example, a filter may
remove 99% of the pollutant in the air that passes through it, but if the air flow rate is only
10 cubic feet per minute (cfm), it will take a long time to process the air in a typical room of
1000 cubic feet.

Although there is no universally accepted method for comparing air-cleaning devices,
several investigators of portable air-cleaning units have expressed their results as a "clean
air delivery rate" or CADR.  The CADR is the product of the unit efficiency and the air flow
rate, and is a measure of the number of cfm of air it cleans of a specific material. For
example, if an air cleaner has a CADR of 250 for smoke particles, it may reduce smoke
particle levels to the same concentration as would be achieved by adding 250 cubic feet of
clean (ventilation) air each minute.

The CADR can be used to compare removal rates between different devices and to estimate
the removal rate of materials in larger or smaller rooms than those used in the tests.

Knowledge of both the CADR and the unit efficiency may be helpful in choosing a device
for use in removing pollutants from a specific source. For example, a 45 percent efficient
unit operating at a flow rate of 100 cfm has the same CADR as a 90 percent efficient unit
operating at 50 cfm. Nevertheless, the 90 percent efficient unit placed near a specific source
of pollutants would generally provide lower levels of the pollutant in the space away from
the source than the 45 percent efficient unit.

In many cases, especially for in-duct systems and gaseous pollutant; removal, only device
efficiencies are reported, and the total effectiveness of the device would vary based on room
size and air flow rate.

A summary of the results of studies on the effectiveness of air cleaners in removing
particles, gaseous pollutants, and radon and its progeny follows.

Particle Removal

The performance of air cleaners in removing particles from indoor air depends not only on
the air flow rate through the cleaner and the efficiency of its particle capture mechanism,
but also on factors such as:

       •  The mass of the particles entering the device.

       •  The characteristics of the particles (e.g., their size).

       •  The degradation rate of the efficiency of the capture mechanism caused by

       •  Whether some of the air entering the unit bypasses the internal capture

       •  How well the air leaving the device is mixed with air in the room before
          reentering the device.

In-duct Systems

Only limited information is available on the performance of whole-house in-duct air-
cleaning systems in removing particles.  Their efficiency for particle removal can be assessed
by three standard methods: the weight arrestance test, the atmospheric dust spot test, and
the OOP method in Military Standard 282.

The weight arrestance test, described in the American Society of Heating, Refrigerating, and
Air-Conditioning Engineers (ASHRAE) Standard 52-761, is generally used to evaluate low
efficiency filters designed to remove the largest and heaviest particles; these filters are
commonly used in residential furnaces and/or air-conditioning systems or as upstream
filters for other air-cleaning devices. For the test, a standard synthetic dust is fed into the air
cleaner and the proportion (by weight) of the dust trapped on the fiilter is determined.

Because the particles in the standard dust are relatively large, the weight arrestance test is of
limited value in assessing the removal of smaller, respirable-size particles from indoor air.

The atmospheric dust spot test, also described in ASHRAE Standard 52-76, is usually used to
rate medium efficiency air cleaners (both filters and electronic air cleaners).  The removal
rate is based on the cleaner's ability to reduce soiling of a clean paper target, an ability
dependent on the cleaner removing very fine particles from the air.  Exhibit 1 shows typical
applications and limitations of filters rated using the ASHRAE Standard 52-76 atmospheric
dust spot test*.

Military Standard 2823 [i.e., the percentage removal of 0.3 micrometer (jam) particles of
dioctylphthalate (DOP)] is used to rate high efficiency air filters, those with efficiencies above
about 98 percent.  [The term "HEPA" (high efficiency particulate air) filter is  commonly
encountered in the marketplace.  These filters are a subset of high efficiency filters and are
typically rated using the DOP method.  One standard-setting organization defines a HEPA
filter as having a minimum particle collection efficiency of 99.97 percent by this testing

Although the above standard tests yield information on the expected efficiency of rated air-
cleaning devices in removing particles from the air flowing through them; few studies
have been conducted to obtain actual effective removal rates in houses in which the devices
were installed.  The efficiency of in-duct devices may vary based on the air flow rate and the
particulate matter load.  Effectiveness may also be decreased if air exiting the heating and/or
air-conditioning system is not well-mixed with room air before reentering the system. This
can happen if air return and intake vents are too closely spaced within the home.  In
addition, the type of device chosen should depend not only on its efficiency but also on its
dust-holding capacity and its resistance to air flow, two additional factors assessed by
ASHRAE Standard 52-76.

Finally, it should be noted that ASHRAE Standard 52-76 addresses the overall efficiency of
removal of a complex mixture of dust.  However, removal efficiencies for different size
particles may vary widely. Recent studies by EPA, comparing ASHRAE ratings to filter
efficiencies for particles by size, have shown that efficiencies for particles in the size range of
0.1 to 1  [OR are much lower than the ASHRAE ratings.  A filter with an ASHRAE dust spot
rating of 95 percent only removed 50-60 percent of particles in the 0.1 to 1 ^m size range.
Many of the respirable-size particles in indoor air (e.g., cigarette smoke) appear to be in this
size range.

In contrast to the ASHRAE Standard 52-76 ratings, efficiencies derived by the DOP method
in Military Standard 282 are expected to be more representative of capture efficiencies for
respirable-size particles.

Portable Units

Studies  have been performed on portable air cleaners assessing particle removal  from the
air in room-size test chambers or extensively weatherized or unventilated rooms. All of
the tests addressed removal of cigarette smoke particles6-1*; some limited testing with larger

 Exhibit 1.  Filter Applications for In-duct Systems Based on ASHRAE Atmospheric
           Dust Spot Test
                          AIR CLEANER EFFICIENCY RATING

window air :
attdheattng i
systems. '

UseMocv ;
Itoiv ]
Somewhat :
Useful CSV
ragweed •

NoHnsry j
useful en :
smckeand •
•• staining
particles, :


Used in air
heating, and
central air

Fairly useful
on ragweed

Not very
useful on
smoke and

Used in i
heating and
systems, and
stspreftJters ;
to high :
efficiency :
eleanets, \

Useful on finer !
airborttedust :
and pollen. \

Reduce :
smudge and
stain; i
materiaOy, j

Slightly :
useful on :
smoke i
particles. i
Not very :
tobacco smoke
Use same as
40%, but

Useful on all
pollens, the
majority of
smudge and
stain, and
coal and oil

useful on
tobacco smoke

CeiEteralfy :
uaedin ;
feoiipitals '
md \

Veiyussful :
on particles i
cauang :
anudgeanct :
stain, arid
ft jU ' tJClfi S»

Use same as
80%, but

against all


  1 Efficiency rating by ASHRAE Standard 52-76 atmospheric dust spot test.

  Adapted from Reference 2.
particles (fine automotive test dust, airborne cat allergen, and pollen) was also
performed9'12/14. The test methods used by each group of investigators varied.

The studies show varying degrees of effectiveness of portable air cleaners in removing
particles from indoor air. In general, units containing either electrostatic precipitators,
negative ion generators,  or pleated filters, and hybrid units containing combinations of
these mechanisms, are more effective than flat filter units in removing cigarette smoke
particles.  Effectiveness within these classes varies widely, however.

Again, important factors/ in addition to the efficiency of the device itself are the air flow
rate; the particle characteristics; the degradation of efficiency with paniculate loading; the
bypass of air around the collection mechanisms used; and the size of the room.

In addition, for negative ion generators, the placement of the device and the air circulation
in the room affect performance. For removal of larger dust particles, negative ion
generators, without additional particle capture mechanisms (e.g., filters), may perform

The general trend in the market over the past few years has been toward larger, more
powerful console-sized models. In recent tests^, the CADRs for 6 table-top units ranged
from about 50 to 100 cfm for smoke  particles, whereas the CADRs for the 21 console units
ranged from about 50 to 250 cfm.  (However, as discussed on page 14, reemission of
chemicals from particles trapped by  these devices is of concern.)

In general, placement of any portable device may affect its performance. If there is a specific,
identifiable source of pollutants, the unit should be placed so that its intake is near that
source.  If there is no specific source, the air cleaner should be placed to force cleaned air into
occupied areas.  In addition, the air cleaner should be located where the inlet and outlet are
not blocked by walls, furniture, or other obstructions.

Effectiveness of a unit may also be decreased if air exiting the air cleaner outlet is not
adequately mixed with room air before reentering the device.

The use of a single portable unit would not be expected to be effective in large buildings
(e.g., apartments or office buildings) with central heating, ventilating, and air-conditioning
(HVAC) systems. Portable units are designed to filter the air in a limited area (e.g., up to
several connected rooms without obstructions to air flow). Air circulated within central
HVAC systems may have large effective volumes (e.g., several floors of a building). To
clean air in these situations requires the use  of either multiple portable units or in-duct
systems designed for the building by HVAC engineers.

Removal of Gaseous Pollutants

Some air cleaners are designed to remove gaseous pollutants  as well as particles.  However,
studies on the effectiveness of portable or residential in-duct air cleaners in removing
gaseous pollutants are limited.

Sorption on solid sorbents is the most frequently used process for removing such
contaminants from indoor air. The  performance of solid sorbents is dependent on several
factors, including:

       • The air flow rate through the  sorbent.

       • The concentration of the pollutants.

       •  The presence of other gases or vapors (e.g., humidity).

       •  The physical and chemical characteristics of both the pollutants and the sorbent
          (e.g., weight, polarity, size, and shape).

       •  The configuration of the sorbent in the device.

       •  The quantity of sorbent used and the sorbent bed depth,

Because the rate of sorption (i.e., the efficiency) decreases with the amount of pollutant
captured, gaseous pollutant air cleaners are generally rated in terms of the sorption capacity
(i.e., the total amount of the chemical that can be captured) and penetration time (i.e., the
amount of time before capacity is reached) is.

Activated Carbon

Activated carbon will adsorb some pollutants even in humid em/ironments15'16 such as
those found indoors. However, it does not efficiently adsorb certain pollutants such as
volatile, low molecular weight gases16/17.

Sometimes, relatively small quantities of activated carbon will reduce odors in a residence
to imperceptible levels. However, because many chemicals produce health effects at levels
below those where odors are perceived, removal of odors alone is not an indicator of a
healthful environment.

Tests of gaseous pollutant removal by activated carbon have generally been performed
using only high concentrations of pollutants, so little information is available on the
effectiveness of carbon in removing chemicals present at the low  (part per billion, or ppb)
concentrations normally found in indoor air. Recent tests performed at EPA measured the
adsorption isotherms for three volatile organic chemicals (VOCs)  in the 100 to 200 ppb
concentration range using three samples  of activated carbon. Estimates of the bed depth
needed to remove the compounds were made assuming a 150 ppb concentration in the air,
an exit concentration of 50 ppb, and a flow rate of 100 cfm across a 2' X 2' filter. The results
of the study suggest that these chemicals would quickly penetrate the 6-inch deep carbon
filters currently marketed for odor control in in-duct systems18.  Therefore, the useful
lifetime of these filters in removing many indoor  air pollutants may be short.

The ability of carbon to reemit pollutants it has trapped from indoor air is also of concern.
The National Institute of Standards  and Technology (NIST), formerly the National Bureau
of Standards (NBS), is currently developing a standard method to be used in evaluating the
effectiveness of media used for gaseous pollutant removal19.  They have reported the
results of a study using activated carbon, in which the concentration of toluene in the air
flowing into the carbon was varied during the test (from 150 to 0 to 340 to 26 to 0 ppm). The
experiment simulates the variations in pollutant levels which would be expected in indoor
air situations.  They found that toluene initially adsorbed by the media was slowly


reemitted each time the pollutant level entering the media dropped.  The amount of
toluene emitted by the media during the 45-hour experiment was approximately equal to
that adsorbed.

Special Sorbents

Special sorbents have been developed to remove specific gaseous pollutants such as
formaldehydeis/zo. Many of these are chemisorbents, impregnated with chemically active
materials/ such as potassium permanganate or copper oxide, which will react with one or a
limited number of different reactive gaseous pollutants.

Several studies have focused on the removal of formaldehyde in homes using such
chemisorbents. These data suggest that large quantities of sorbent and high air flow rates
may be required to effectively reduce formaldehyde levels2**.

In addition, because chemisorbents are specific for one or a limited number of reactive
pollutants, they should not be expected to efficiently reduce pollutants for which they are
not specifically designed.

Tests of Portable Units

Testing has been performed recently on gaseous pollutant removal by several portable air
cleaners containing activated carbon and/or additional specialized sorbents10'11'13'21. The
CADRs calculated for "hydrocarbons" or individual VOCs (excluding formaldehyde) in
these studies were generally low, ranging from 0 to 30 cfm. None of four units tested for the
removal of dichloromethane removed any of this compound.  Lower molecular weight
gases/ including nitrogen oxides, sulfur dioxide, formaldehyde, hydrogen cyanide, and
ammonia, were generally removed at greater rates than the higher molecular weight
organic compounds.  Nitrogen dioxide removal for eight units where CADR values were
reported ranged from 3 to about 94 cfm11'13'21. CADRs were available for only two units for
each of the remaining lower molecular weight gases; the highest CADRs reported were for
nitrous oxide and formaldehyde (approximately 120 cfm in one unit).

In general, units containing specialized sorbents performed better in the removal of gaseous
pollutants than those containing activated carbon alone.  However, as suggested by the
above results, removal rates varied widely between units,  m addition, widely differing
removal rates were found for the pollutants tested in the same unit; some models that
removed larger quantities of one pollutant did not remove much of another.

Several factors were not assessed in the tests of the portable units, making evaluations of
the effectiveness of these devices in indoor air environments incomplete. For example,
because these tests did not determine the sorption capacity or penetration rates for the air
cleaners, it is not known how long the filters would remain effective. Preliminary tests
were performed on one air cleaner to assess long-term efficiency in removing NOa (260 ppb)
and six VOCs. The VOCs chosen were representative of six classes of VOCs found in indoor
air, and the concentrations and relative proportions of the  six VOCs were selected to reflect
those reported for their respective classes in indoor air.  Following testing in a test chamber


to determine the initial removal efficiencies for these compounds, the air cleaner was
operated intermittently in a home over a two-and-a-half-month period.  Follow-up testing
in the test chamber showed a decrease in efficiency of 50 percent or more for each chemical
after 160 hours of use (i.e., 15 percent of the manufacturer's recommended filter lifetime)2i.

Another factor that was not assessed was the effect of additional chemicals in the air (e.g.,
water) during the removal process. Since indoor air is a complex mixture of chemicals, tests
on one or a mixture of several pollutants may not adequately represent removal rates in
indoor environments.

In summary, data are too limited at present to assess the overall effectiveness of air-cleaning
devices in removing gaseous pollutant mixtures.  Although some of the devices which are
designed to remove gaseous pollutants may be effective in removing specific pollutants
from indoor air, none are expected to adequately remove all of the gaseous pollutants
present in the typical indoor air environment.  In addition, information is limited on the
useful lifetime of these systems.

Removal of Radon and its Progeny

Air cleaning is generally not the preferred approach to reducing health risks associated with
radon.  When source control techniques  are not possible, or do  not result in acceptable
radon levels, air-cleaning techniques are available to reduce levels of radon gas and its
progeny.  Studies on the effectiveness of air cleaners in removing Ithese pollutants have
focused on either removing radon gas itself or removing the short-lived progeny produced
by radon.

Some limited research on the effectiveness of carbon in removing radon gas itself from
indoor air suggests that extremely large quantities of carbon would be required.  However,
some radon removal units which are specifically designed to regenerate the carbon media
that they contain can increase the range  of situations (area and  radon concentration to be
treated) where this technique is applicable.

Since the health hazard from radon is associated with the radon progeny, rather than radon
gas itself, the effectiveness of air cleaners in removing radon progeny has also been assessed.
Although some radon progeny are removed by filtration or electrostatic precipitation, the
types of radon progeny not removed from the air may be of relatively greater concern from
a health standpoint.  In addition, radon  gas concentrations are unaffected, and can continue
to be a source of radon progeny in areas of the structure that are noi: effectively treated by the
air cleaner.  Because uncertainty exists concerning the effectiveness of air cleaners in
reducing the health risks associated with radon, EPA neither currently endorses nor
discourages their use as a method of reducing radon progeny in indoor air22.


As previously discussed, no air-cleaning system is available that will effectively remove all
pollutants from indoor air.  As such, the use of air cleaners should only be considered when
the use of other methods to reduce indoor air pollutants (e.g., controlling specific sources of
pollutants or increasing the supply of outdoor air) are not successful in reducing pollutants
to acceptable levels.

Under the right conditions, some air-cleaning systems can effectively remove certain
particles, although the particles must be suspended in the air as discussed below. Some of
the air cleaners containing sorbents may also remove a portion of the gaseous pollutants in
indoor air, and may help eliminate some of the hazards from these pollutants, at least on a
temporary basis. However, air-cleaning systems are not expected to totally eliminate all of
the hazards from gaseous pollutants;  In addition, gaseous pollutant removal systems may
have a limited lifetime before replacement of the sorbent is necessary.  It should also be
noted that although some air-cleaning devices may be effective at reducing tobacco smoke
particles, many of the gaseous pollutants from tobacco smoke are not expected to be
effectively eliminated. In addition, gases may be reemitted from tobacco smoke particles
trapped by the air cleaner17.

The typical air cleaner which does not contain a specialized carbon regenerating device
would appear to be ineffective in removing radon gas and, because many questions exist
concerning the relative health risks of radon decay products, there are  insufficient data to
quantify the impact of air cleaning on reducing the risks of lung cancer caused by radon

There is currently some controversy about how effectively air cleaners alleviate allergic
reactions produced by larger particles such as pollen, house dust allergens,; some molds, and
animal dander. In February 1987, an ad hoc committee convened at the request of the Food
and Drug Administration and several manufacturers of air-cleaning devices met to
determine whether standards could be recommended for portable air cleaners and
concluded that "the data presently available are inadequate to establish the utility of these
devices in the prevention and treatment of allergic respiratory disease."23

Pollen and house dust allergens settle out rapidly from the air if not disturbed and
suspended in the air again. Because only a small  proportion of these allergens is generally
suspended in the air, air cleaners may be relatively ineffective in their  removal.

Although other allergen  particles, such as animal dander, do not settle  as rapidly as pollen
and house dust allergens, the amount of allergen associated with surfaces either due to
direct deposition or to settling will generally far exceed that in air.  However, because larger
quantities of these allergens may remain in air, air cleaning may be more effective in
reducing these particles under some circumstances23. On the other hand, use of an air
cleaner may disturb allergen which has settled on surfaces, resulting in a decrease in overall
allergen removal from the

Published reports reviewed by the ad hoc committee were limited in scope, but indicated
that the exposure to allergens originating outdoors during the warm months (i.e., pollen
and some molds) can best be prevented by the use of an air conditioner, with only minimal
additional benefit from an air cleaner. The effectiveness of air conditioning in reducing
these pollutants was related to the exclusion of outdoor air (often 10 percent of the output of
chilled air) and, in the case of molds, also to a reduction in humidity.

With subjects sensitive to house dust allergen, the use of impermeable coverings on the
mattresses appeared to be as effective as the use of a laminar flow air-cleaning system above
the bed.  Based on these results, the committee felt that "air-cleaning devices should be
considered only if symptoms remain severe despite other avoidance measures and there is
reason to believe that a significant load of airborne allergens is present"23

Several factors other than the ability of air-cleaning devices to reduce airborne pollutant
concentrations should be considered when making decisions about using air cleaners. These

Installation, Use, and Need for Maintenance

The air-cleaning unit may have certain installation requirements that must be met, such as
an adequate and accessible power supply or the need for access during use, repairs, or

After installation, operating and maintenance procedures specified by the manufacturer
need to be followed to assure adequate performance from the air cleaner. Filters and
sorbents must be cleaned or replaced and plates or charged media of electronic air cleaners
must be cleaned, sometimes frequently.  To avoid electrical and mechanical hazards, the
purchaser should ascertain that the unit is listed with Underwriters Laboratories (UL)  or
another recognized independent safety testing laboratory.

In addition, during cleaning an effort needs to be made to ensure pollutants do not get
reemitted back into the air.  For example, when filters are removed., excessive movements
or air currents should be avoided to prevent redistribution of particles into the air.


Cost may also  be a consideration.  Major costs include the initial purchase of the unit,
maintenance costs (i.e., cleaning and/or replacement of filters and other parts), and
operating costs (e.g., costs for electricity).

In general, the most effective units (e.g., those with high air flow rates and efficient particle
capture systems) are also the most costly. Maintenance costs vary depending on the device,
and should be considered before choosing a particular unit. In comparison to purchase and
maintenance costs, operating costs for portable units (e.g., costs for electricity) are

Production or Redispersal of Pollutants

Another consideration is whether some units will produce new pollutants or redisperse old
ones. The potential for ion generators and electronic air cleaners to produce ozone, a lung
irritant, may be of concern, particularly if electronic air cleaners are not properly installed
and maintained7'15/16. This requires further study.  At least two manufacturers of portable
units advertise that their products produce ozone to facilitate removal of harmful gases, but
the levels produced by these devices and the possible health effects are not known.
Measurable levels of ozone were produced by one portable and two in-duct electrostatic
precipitators in tests by EPA5, and the Agency is conducting research to determine if the
concentrations produced by the in-duct air cleaners are potentially harmful.

The production of fine particulate material by electronic air cleaners has also been
reported8'11'24.  Also, filters and other particulate control devices may remove particles from
air and then may reemit gases and odors from the collected particles1?, and materials used in
the construction of air cleaners may themselves emit chemicals to indoor air (e.g.,
formaldehyde may be emitted if particleboard is used in the air cleaner housing21).

Inability to Remove Some Odors

A number of air cleaners tested were found to reduce the levels of cigarette smoke particles
in the air.  However, the odor of cigarette smoke remained because many of the devices do
not contain effective systems to remove the gaseous products of cigarette smoke and
because the gaseous products may be adsorbed and later reemitted by articles in the home8-9.
To overcome this, some devices scent the air to mask odors, which may lead the occupants
of the home to believe that the odor-causing pollutants have been removed.

Possible Effects of Particle Charging

Another factor with respect to ion generators, particularly those that do not  trap some of the
charged particles, is the effect of particle charging on deposition in the respiratory tract.
Experiments have shown a linear increase in particle deposition with charge; therefore, the
use of ion generators may not reduce the dose of particles to the lungs.

Soiling of Walls and Other Surfaces

Ion generators are  generally designed not to remove particles from the air but to deposit
them on surfaces around the room. This results  in soiling of walls and other surfaces,
especially if the particles charged by the apparatus are not collected on a filter9.


Noise may be a problem with air cleaners containing a fan7,9,i2.  Some portable units
operating at high speed can produce noise equivalent to a small vacuum cleaner19 or that
made by light traffic at 100 ft7. Even at low speed, some models produce an annoying hum
or whine12.

With the exception of the DOP method in Military Standard 2823, used only to rate particle
reduction by high efficiency filters, the federal government has not published any
guidelines or standards for use in determining how well an air cleaner works in removing
pollutants from indoor air. However, standards for rating particle removal by in-duct or
portable air cleaners have been published by two private standard-setting trade
associations ^s. These estimate the efficiency or effectiveness of an air-cleaning device in
removing particles from indoor air, and can be used for comparisons among different

Standards for air cleaners now focus only on particle removal. No guidelines or standards
are available for use in assessing the comparative ability of air cleamers to remove gaseous
pollutants or radon and its progeny, and research is currently inadequate to draw firm
conclusions regarding the relative effectiveness  of air-cleaning devices in removing such

Standards for In-Duct Devices

ASHRAE Standard 52-761 and the DOP method in Military Standard 2823 may be used to
estimate the efficiency of in-duct devices in removing particles.  Using the ratings of the
ASHRAE Standard 52-76 atmospheric dust spot test, Exhibit 1 can give a general indication
of the types of particles which should be removed by a specific air (leaner. These standards
can generally be used to compare the performance characteristics of one device with
another, but cannot by themselves predict the actual effectiveness of a given unit in use in a
residence or its useful lifetime. In addition, as discussed on page 6, the efficiency of these air
cleaners may vary by air flow rate and particle load, and removal of some small respirable-
size particles may actually be lower than assessed by the ASHRAE atmospheric dust spot

(Note: In examining information on ASHRAE ratings, be aware of differences in results
from the weight arrestance test and the atmospheric dust spot test.  For example, a filter
with a weight arrestance of 90 percent may have an atmospheric dust spot efficiency below
40 percent. The ASHRAE weight arrestance test is of limited value; in assessing the removal
of respirable-size particles from indoor air.)

Because higher efficiency pleated filters are much thicker than filters generally used in
standard home heating and/or air-conditioning  systems, their use results in substantial air

resistance, so they cannot be directly incorporated into the standard residential system.
Instead/ a system must be specially designed with a fan of sufficient power to create the
necessary air pressure and with one or more efficient prefilters. Costs for installation of the
system/ replacement of prefilters and filters, and system operation should be considered
before deciding whether to purchase higher efficiency filters. Again, the purchaser should
be aware of the difference between high "arrestance" and high "efficiency," as provided by
the standard tests.
Further information on standards for in-duct air cleaners can be obtained through a local
heating/air-conditioning contractor or from:

      Air-Conditioning and Refrigeration Institute (ARI)
      1501 Wilson Blvd., 6th Floor
      Arlington, VA 22209
Standard for Portable Air Cleaners

The Association of Home Appliance Manufacturers (AHAM) has developed an American
National Standards Institute (ANSI)-approved standard for portable air cleaners
(ANSI/AHAM Standard AC-1-1988)25. This standard may be useful in estimating the
effectiveness of portable air cleaners. Under this standard, room air cleaner effectiveness is
rated by a clean air delivery rate (CADR) for each of three particle types in indoor air: tobacco
smoke, dust, and pollen.

Only a limited number of air cleaners have been certified under this program at the present
time. A complete listing of all current AHAM-certified room air cleaners and their CADRs
can be obtained by sending a stamped, self-addressed envelope to AHAM at:

       Association of Home Appliance Manufacturers
       Air Cleaner Certification Program
       20 North Wacker Drive                                         i
       Chicago, IL 60606

Exhibit 2 shows the percentage of particles removed from indoor air in rooms of various
size by rated CADR, as estimated by AHAM.  Because CADR values on air cleaners in the
market will vary from the five in the exhibit, the figures are to be used only as a guide to a
model's performance. The exhibit provides estimates of the percent of particles removed by
the air deaner and the total removal by both the air cleaner and by natural settling.

There are other factors to consider in using the ANSI/AHAM ratings.  The CADR values
reported are based on reducing particle levels from sources which emit the particles
intermittently rather than continually. If the source  is continual, the devices would not be
expected to be as effective as suggested by Exhibit 2. In addition, the values represent
performance that can be expected during the first 72 hours of use. Subsequent performance
may vary depending on conditions of use.  Use and care directions should be followed
routinely to get adequate performance from the air cleaner.


Exhibit 2.   Estimated Percentage of Particle Removal for Portable Units by CADR and
            by Room Size
                                       PERCENTAGE OF PARTICLES REMOVED:

Room Size I I CADR I

5X6 10

9X12 40
12 X 18 80


18X24 150
20X30 300

•4- •:•:•, b
j. &&""•<• T
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V ,,,,V,,M, „
"••'• 4ssjnfc[?"" ' 68%
*• v.\QjEl''Qb< jf f Q7QS*
-if*?8 ,,,„ *' ">
"-"$mg%£ 100%
, ™"*?^£;£
, SSfcfe'''^-. 71%
I - H% :" 89%
; \M% C 98%
i' $$£ "- 71%

! 74%" x\- 87%
K.r$£&- " 97%
| ^t^A,. _
IN -^3^-l _
! ^""^Vl
^SJ*^;| 70%
f ••""•?$%v\! 87%
^-VV_ x,^| _
""••v. '•'•'• ""
•^ ^^-tf^^-^ TO or
••s-. 8^ w*" f /V%
^--^wS.^S —
XV^^ ^ •. v^v»»
.•xxsVw^ "^ v---\0;

4 • b
s AID * T

&& 70%
88% 98%
$m 100%

- iSt^;\, 72%
f&& ^ 89%
— §Wfc 98%
^KM&. *• "j^ ^»
MW 21? i £• /Q

^ n% 88%
•^\ Y>u[[
"%¥ * 99%
-«v^^ - '_
%% 'X'^W'-A's "•
§dj^x« 71%
' 1
*f«& . 91%
^V _
•^- 7^ :
^%X £14%
\ s*1 "•

- ^ > 6
f 'AC - - T
% \
1^^ sv
s^^> - " 93%
•; "t^, "%
•'^ ~-^ ••••'•'•'• <.
!' M% 78%
: >IB% ;f 86%
5 ^^.^ 94%
T^'&'T 78%

"™a®% 85%
% ">Sff f^fff f "•
''V. ''
--m%:-- 97%
" ™"C'X
i^2$|^ s ; 78%
"*"- lilH. ^""^ — ri-.-J
'-V-.^_4."r,, _
^l^ ^%
i $|lk ^ 86%
5 ^-.v. -.^ ^••
^ >-. "^:\% w..^
 Removal by the air-cleaning device
 Removal by the air-cleaning device plus natural settling

Note: Estimates ignore the effect of incoming air. For smoke and, to a lesser extent, dust, the more drafty
the room, the smaller the CADR required. For pollen, which enters from outdoors,, a higher CADR is
needed in a drafty room.

Source: Reference 26.


Three strategies (in order of effectiveness) that may be used to reduce indoor air pollutants
are source control/ ventilation, and air cleaning.  Air cleaning may achieve an additional
reduction in the levels of certain pollutants when source control and ventilation do not
result in acceptable pollutant concentrations. However, air cleaning alone cannot be
expected to adequately remove all of the pollutants present in the typical indoor air

Air cleaners are usually classified by the method employed for removing particles of
various sizes from the air. There are three general types of air cleaners on the market:
mechanical filters, electronic air cleaners, and ion generators. Hybrid units, using two or
more of these removal methods, are also available. Air cleaners may be in-duct units
(installed in the central heating and/or air-conditioning system) or stand-alone portable

The effectiveness of air cleaners in removing pollutants from the air is a function of both
the efficiency of the device itself (e.g., the percentage of the pollutant removed as it goes
through the device) and the amount of air handled by the device.  A product of these two
factors (for a given pollutant) is expressed as the unit's clean air delivery rate (CADR).

Portable air cleaners vary in size and effectiveness in pollutant reduction capabilities.  They
range from relatively  ineffective table-top units to larger, more powerful console units.  In
general, units containing either electrostatic precipitators, negative ion generators, or
pleated filters, and hybrid units containing combinations of these mechanisms, are more
effective than flat filter units in removing tobacco smoke particles. Effectiveness within
these classes varies widely, however.  For removal of larger dust particle^, negative ion
generators, without additional particle capture mechanisms (e.g., filters), may perform

Pollutants in indoor air may be divided, for convenience, into three groups: particles,
gaseous pollutants, and radon and its progeny. Some air cleaners, under the right
conditions, can effectively remove small particles which are suspended in air. However,
controversy exists as to the efficacy of air cleaners in removing larger particles such as
pollen and house dust allergens, which rapidly settle from indoor air. In assessing the
potential efficacy of an air cleaner in removing allergens, one should consider the relative
contribution of airborne to surface concentrations of the allergens, particularly in the case of
pollen and house dust allergens where natural settling may be so rapid that air cleaners
contribute little additional effect.  Animal dander may settle more slowly although, again,
the surface reservoir far exceeds the amount in the air.  Furthermore, control of the sources
of allergens and, where allergens do  not originate outdoors, ventilation should be stressed
as the primary means of reducing allergic reactions.

Some of the air cleaners containing sorbents may also remove some of thje gaseous
pollutants in indoor air.  However, no air-cleaning systems are expected to totally eliminate
all hazards from gaseous pollutants and these systems may have a limited lifetime before

replacement is necessary. In addition/ air cleaning may not be effective in reducing the risks
of lung cancer due to radon.

In choosing an air cleaner, several factors should be considered. These include:

       •  The potential effectiveness of the device under the conditions it will be used.

       •  The need for routine maintenance, including cleaning and replacement of filters
          and sorbents.

       •  The estimated capital and maintenance cost.

       •  The installation requirements (e.g., power, access).

       •  The manufacturer's recommended operating procedures.

       •  The possible production or redispersal of pollutants, such as ozone, particles,
          formaldehyde, and trapped gaseous pollutants.

       •  The inability of air cleaners designed for particle removal to control gases and
          some odors, such as those from tobacco smoke.

       •  Possible health effects from charged particles produced by ion generators.

       •  Possible soiling of surfaces by charged particles produced by ion generators.

       •  The noise level at the air flow rates that will be used.
Finally, one Federal standard, addressing only high efficiency air filters;, and two standards
provided by independent standard-setting trade associations outside the Federal
government may be useful as guidelines in choosing an air cleaner for reduction of particles
in indoor air. For in-duct systems, the atmospheric dust spot test of ASHRAE
Standard 52-76 and the DOP method in Military Standard 282 may tie used, respectively, to
estimate the performance of medium and high efficiency air cleaners. For portable air
cleaning systems, ANSI/AH AM AC-1-1988 may be useful in estimating the effectiveness of
the units.  Similar standards are not currently available to compare the performance of air
cleaners in removing gaseous pollutants or radon and its progeny.

 i ASHRAE. 1976.  ASHRAE standard 52-76. Method of testing air-cleaning devices used in
   general ventilation for removing particulate matter.  New York, NY: American Society
   of Heating, Refrigerating, and Air-conditioning Engineers, Inc.

 2ASHRAE. 1979.  Air cleaners.  In: ASHRAE handbook and product directory. 1979
   equipment.  Atlanta, GA: American Society of Heating, Refrigerating, and Air-
   conditioning Engineers, Inc. As cited in reference 16.

 3U.S. DOD. 1956. MIL-STD-282.  Military Standard.  Filter units, protective clothing, gas-
   mask components and related products: Performance-test methods. Washington, DC:
   U.S. Department of Defense.

 4Institute of Environmental Sciences. 1986. Recommended practice for HEPA filters. IES-
   RP-CC-001-86. Mt. Prospect, IL: Institute of Environmental Sciences.

 SEnsor DS, Viner AS, Hanley JT, Lawless PA,  Ramanathan K, Owen MK, Yamamoto T,
   Sparks LE.  1988.  Air cleaner technologies for indoor air pollution. In:  Engineering
   solutions to indoor air problems.  Proceedings of the ASHRAE conference IAQ'88, April
   11-13, 1988, Atlanta, GA. Atlanta, GA:  American Society of Heating, Refrigerating and
   Air-Conditioning Engineers, Inc. pp. 111-129.

 SRepace JL, Seba DB, Lowrey AH, Gregory TW. 1983. Effect of negative ion generators on
   ambient tobacco smoke.  Journal of Clinical Ecology 2(2): 90-94.
       Shelter.  1983. A test of small air cleaners. In: Home products report. Emmaus, PA:
   Rodale Press.

 sOffermann FJ, Sextro RG, Fisk WJ, Grimsrud DT, Nazaroff WW, Nero AV, Rezvan KL,
   Yater J.  1985.  Control of respirable particles in indoor air with portable air cleaners.
   Atmospheric Environment 19(11): 1761-1771.

 ^Consumers Union. 1985.  Air cleaners. Consumer Reports 50(1): 7-11.

 lOCanine C. 1986.  Clearing the air.  Rodale's New Shelter, January 1986, p. 64-67.

 "Olander L, Johansson J, Johansson R.  1987. Air cleaners for tobacco smoke. In:  Seifert B,
   et al., eds. Indoor air '87. Proceedings of the 4th Conference on Indoor Air Quality and
   Climate, West Berlin, August 17-21, 1987. Vol. 2. Berlin: Institute for Water, Soil and
   Air Hygiene, pp. 39-43.

 ^Consumers Union.  1989. Air purifiers. Consumer Reports, February 1989, p. 88-93.

^Humphreys MP. 1987. Performance testing of residential indoor 
25Association of Home Appliance Manufacturers. 1988. American national standard
  method for measuring performance of portable household electric cord-connected room
  air cleaners. ANSI/AHAM AC-1-1988. Chicago, IL:  Association of Home Appliance

26Association of Home Appliance Manufacturers. 1989.   Comparing room air cleaners.
  An AHAM buying guide.  Chicago, IL: Association of Home Appliance Manufacturers.