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Effectiveness of Home Air Cleaners in
Reducing Indoor Levels of Particles
Health Canada Contract # 4500172935
March 12, 2008
We breathe about 13 cubic meters of air each day, and many of the particles in the air deposit in
our lungs. Hundreds of studies worldwide have documented increases in morbidity and
mortality associated with increases in particle levels outdoors. However, people spend almost
90% of their time indoors, and indoor air has its own list of sources that may be as bad or worse
than outdoors—for example, cigarette smoke. Therefore much interest lies in developing ways
to clean indoor air. One way to improve indoor air quality is to use an air cleaner. Air cleaners
have been sold for many years, but it is important to evaluate their effectiveness in reducing
pollutants in the home. Therefore Health Canada has sponsored this review of the effectiveness
of air cleaners in homes. Although pollutants include gases as well as particles, this review
concentrates on particle air cleaners.
There are several ways to obtain cleaner indoor air in homes. The most effective way is to
remove the source. For certain powerful sources, such as smoking in the home or allergen-
producing pets, removal may be the only effective strategy. A second way to obtain cleaner air
is by ventilation. For indoor sources, increased ventilation will reduce their concentration in
indoor air. However, if outdoor sources are important, increased ventilation will worsen the
situation. Filtering the air, however, can reduce concentrations from both outdoor and indoor
sources. Therefore air filters have been a popular mode of cleaning indoor air, and the associated
industry is in the neighborhood of several hundred million dollars per year.
Types of air cleaners
Air cleaners employing filters are of two general types. Portable air cleaners are self-contained
devices that can be placed in a room, such as the bedroom of an allergic child, and operated there
to reduce particle levels in the room. In-duct air cleaners are installed in the ductwork of a home
with central forced air, and are designed to reduce particle levels throughout the house. This
review will deal with both types.
Types of filters
Filters are of two general types—mechanical and electrical. The mechanical filter simply
intercepts particles in the air passing through the filter. The ordinary furnace filter represents the
most common type of filter: the fibrous filter. Fibrous filters consist of layers of fibers, criss-
crossed in more or less random ways so as to allow air molecules to find a tortuous path through
the filter, while at the same time removing particles that intercept the fibers. The thicker or more
tightly weaved the filter, the more success it will have in removing particles. However,
increased thickness or tightness of the weave requires more energy to move the air through the
filter, so there is an economic cost in going to more efficient mechanical filters.
The second type of filter employs electrical charge to enhance collection of particles. For
example, the fibers can be permanently charged. This type of filter is often called an electret.
Since some air particles are charged, they will be attracted to charges of opposite polarity on the
filter. Even for uncharged particles, the existence of a charge on a nearby fiber will induce a
dipole moment (pushing like charges to the back of the particle and unlike charges to the front,
as seen from the fiber) and the net result is an attraction to the fiber. Since the electric charge on
the filter doesn’t affect the pressure drop, there is no energy penalty. Electrets are widely used in
automobiles to filter air coming in to the passenger compartment.
Another way to use electric charge is to actively add charge to the incoming particles by passing
them through a strong electric field created by multiple wires. The particles are driven to a
number of collecting plates. These air cleaners are called electrostatic precipitators (ESPs).
A third approach using electric charge is to send out large quantities of charged particles (ions)
into a room. The ions collide with particles and stick, thereby charging them and causing them
to be attracted to grounded surfaces such as walls and floors. Such devices are called ion
generators or ionizers.
There can also be hybrid filters. For example, some electrets may also have a continuous electric
field applied to them, in an attempt to reduce the loss of charge over time that has been noted for
These categories and some subcategories are listed in Table 1. Panel filters are flat filters, often
about 1 inch deep, designed typically for insertion into furnaces. They are mainly designed to
protect the furnace from large debris and have virtually no air cleaning capacity for respirable
Pleated or extended surface filters attain better efficiencies by presenting much more surface
area to the particles as they pass through the filter. For a given depth, there are more fibers and
therefore more chance to intercept particles. These filters are often much thicker, 4-6 inches, and
may require cutting sheet metal in the ductwork to install.
HEPA (High Efficiency Particulate Arrestors) filters were originally developed for worker
protection against radioactive elements encountered in certain workplaces. They consist usually
of extremely densely packed fiberglass. To obtain the designation HEPA, they must filter out
99.97% of particles 0.3 μm in diameter. This particle size is normally the hardest to filter, so the
efficiency should be even greater than 99.97% for all other sizes. “HEPA-type” filters meet a
less stringent standard of 95% efficiency for 0.3 μm particles.
Table 1. Types of filters.
fiberglass, many others
Wires at high voltage;
collecting metal plates
The Association of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) has
developed standard tests for classifying and ranking efficiencies of filters. The tests are described
below in the section “Rating filters” and in more detailed fashion in Appendix B.
Effectiveness of air cleaners
The basic question for any air cleaner is “How well does it work?” For many years, there was no
standardized test procedure to answer this question. Eventually, in about 1985, such a test
procedure was developed, but only for portable air cleaners. There is still no standardized
procedure for in-duct air cleaners.
We must first distinguish between two measures: efficiency and effectiveness. The efficiency of
an air cleaner is the fraction of particles that it removes in a single pass through the system. This
is easily measured by measuring the concentration of particles just upstream and just
downstream from the filter. A typical furnace filter has an efficiency well below 10%, whereas
an ESP (when clean) will typically have an efficiency better than 90% for all particle sizes.
Efficiency alone is not enough, however, to characterize an air cleaner. It could have very high
efficiency, but a very low air flow. In that case, it would not be delivering very much clean air.
One must multiply the efficiency by the air flow through the filter to determine how much clean
air it can deliver: the clean air delivery rate, or CADR. The CADR is usually presented as a
volume of clean air delivered per time unit, such as cubic feet per minute (cfm) or cubic meters
per hour (m3/h).
The standardized test procedure mentioned above results in a measure of the CADR for a given
portable air cleaner. The procedure is administered or overseen by the Association of Home
Appliance Manufacturers (AHAM). AHAM certifies those air cleaners that have undergone the
test (AHAM 1988), and maintains a Website listing all brands and models (presently >150) with
their associated CADRs. This is the single most valuable resource for consumers wishing to buy
an air cleaner.
The entire list can be searched online at
The link has search capabilities allowing quick identification of the companies and products
meeting the user’s specifications. For example, a search for air cleaners exceeding a CADR for
tobacco smoke of 400 cfm identified 10 air cleaners sold by 4 companies. This is extremely
valuable for consumers wishing to cut through the cloudy descriptions of advertisers. The same
search also returned the maximum size room for which the air cleaner would be deemed
effective—in this case, a large room of >679 square feet. Rooms up to this size would, in theory,
have their particle concentrations reduced by 80% or more. However, this would not be true if
the door was open to other rooms in the house, or if the window in the room was open, or if the
house air exchange rate was higher than average.
Very recently, AHAM has added a second criterion to its test—whether or not the air cleaner can
meet a standard for ozone generation. The standard is maintained by the Underwriters
Laboratory (UL) and is UL Standard for Safety 867. The standard specifies that the air cleaner be
operated for at least 8 hours and up to 24 hours (to achieve a steady-state concentration) in a
stainless steel chamber (approximately 10' X 10' X 10' or about 28 m3). The ozone level is
measured at a point about 2 inches from the main outlet (highest ozone emission rate) of the air
cleaner, and must be < 50 parts per billion (ppb).
However, the AHAM certification program is voluntary. Manufacturers need not participate. In
fact, manufacturers of inefficient air cleaners may not wish to participate. Therefore independent
testing of such instruments is also useful. Such testing is done by consumer organizations and
reported in publications such as Consumer Reports Magazine or associated websites.
Even the CADR, however, is not the last word on air cleaners. For one thing, it applies only to
new units. As air cleaners are used, their efficiency may change in different ways. But also the
CADR applies only to the air passing through the filter. Air that reaches the person before
passing through the filter will be unaffected.
Therefore we define effectiveness as the ability of the filter to remove particles from a room or
from the breathing zone of a person. Since all homes have cracks allowing particles to enter,
there is always a fresh supply of particles, some of which will not pass through the filter before
encountering the person. Indoor sources of particles, such as cooking or vacuuming, will add
even more particles that may reach the person before going through the filter. Therefore the
effectiveness of a filter is always less than its efficiency. Effectiveness, like efficiency, may be
reported as a fraction or a percentage of the particles that are removed before reaching the
person. It may also be reported as an effective air change rate, or the number of house volumes
or room volumes of clean air that it can provide in an hour. Effectiveness is much more difficult
to measure than efficiency. However, under certain conditions, effectiveness has a well-
understood mathematical form and can be determined in homes or test houses.
A more mathematical description of efficiency and effectiveness is provided in Appendix A.
The focus of this report is on actual effectiveness of air cleaners as used in homes. The literature
search specified “home” or “residence” as a key word along with “filter” and “air cleaner” or “air
purifier.” Since filters help persons with allergies, we included terms such as “allergy” and
“asthma”; also since studies of effectiveness often use the term “intervention” we included that
as a search term
We believe the literature search is fairly complete for air cleaners used in homes. The earliest
uses of air cleaners may have been by physicians for treatment of acute hay fever or asthmatic
episodes. Some of these air cleaners were set up in hospital rooms or wards, but they have been
included in our report.
We have also identified a number of reports on laboratory or chamber tests of air cleaners.
These are valuable for identifying the upper limit of the effectiveness of an air cleaner, and for
indicating the relative efficiency or clean air delivery rates for a series of filters of different
composition. For example, if a chamber test shows that a furnace filter has an efficiency of 5%,
this is useful information—we don’t need to worry about its ultimate effectiveness, since we
know it will be close to zero once all the other factors such as infiltration and indoor sources
come into play. For this reason, we also provide a summary of the chamber tests that we have
found. We think this summary is also fairly complete.
Air cleaners used in homes
The results of the literature review are summarized in Table 2 (see associated Excel file: Table 2
Residences.xlsx). Many of the early reports were by physicians treating acute asthma or hay
fever. The first we have found is from a Dutch physician (van Leeuwen 1924). He erected a 50-
foot high pipe to sample air from well above the rooftops and bring it to a room for his patients
to breathe. Leopold and Leopold (1925) constructed a room in the Hospital of the University of
Pennsylvania with filtered air. Cohen (1926, 1927a,b, 1928) constructed a portable window-
mounted mechanical filter with a pump pulling 140 cfm (later 250 cfm) into the room. He noted
this made sealing the room unnecessary since the positive pressure would force other dust and
pollen out the cracks in the room. Cohen reported improvement in all 10 of his refractory asthma
patients after spending 12-15 hours per day in the room with the filter. All of these physicians
reported nearly complete relief from symptoms within some period of time ranging from a few to
18 hours. Reported effectiveness of the filters, as determined by counting pollen grains on glass
slides, was very high at better than 95% reduction.
A drawback of the Cohen mechanism was a filter that got clogged after a time in the heavy
ragweed season and was difficult to remove. Peshkin and Beck (1930) improved on this by
developing a device with an easily removable pleated filter (surface area 8 square feet). Since
the ragweed season lasts for some weeks, they noted that their filter needed changing several
times over that period. Their device had a fan capable of pulling 400 cfm at the highest setting.
This would be sufficient to provide a complete air change to a 2000 cubic foot room (14’ X 14’
X 10’) in five minutes. Peshkin and Beck claimed that the device removed 100% of ragweed
pollen as measured by counting grains on a slide downstream of the filter.
Three more articles appeared in 1932-33, each reporting on large numbers of patients treated for
asthma by staying in hospital rooms with filtered air. Beck (1932) reported on 54 patients;
Nelson et al., (1933) on 76; Rappaport et al., (1932) on 105.
The first use of an electrostatic precipitator for patients with asthma was reported by Criep and
Green (1936). The ESP had only just been developed as a prototype model a few years earlier
by Westinghouse. Criep and Green reported >99% effectiveness in removing pollen from their
ESP-equipped hospital room. Improvement in nearly all of their 53 patients was reported within
4-9 hours of entering the room.
Friedlander and Friedlander (1954) studied the clinical effect of a portable ESP on 30 patients.
For most, the ESP was installed in a room of their home. “Excellent” improvement was noted for
12 of 30 patients; no improvement for 7. Earlier tests of the ESP in an empty room showed
better than 90% removal of pollen grains, based on comparisons with a similar room without the
ESP. Lefcoe and Inculet (1971) studied the effect of an ESP installed in the HVAC system of a
20,000 ft3 home with 6 inhabitants. This was also the first to report effectiveness using a
measure other than pollen counts. Lefcoe reported 80-86% reductions in three sizes of respirable
particles between 0.3 and 2 μm, using hourly measures over a 3-day period.
The first report on a HEPA filter, and the first comparing a HEPA filter with an ESP, was
presented by King et al., (1973). He reported reductions of particle concentrations from 106
particles/ft3 to 6000 /ft3 using the HEPA filter, and to 34000 /ft3 using the ESP. He also noted a
decline in the ESP efficiency over only 4 days.
An early study of a negative ion generator found no further reduction of pollen, mold, and
bacterial counts beyond the reduction provided by an air conditioning unit (Spiegelman et al.,
1961). Later, Repace (1983) used a very powerful multi-needle device mounted on the ceiling
and obtained a 96% reduction in environmental tobacco smoke (ETS). Nogrady and Furnass
(1983) tested an ionizer for its effect on bronchial asthma in 20 adults. Sham or active ionizers
were placed in subjects’ bedrooms for two 8-week periods separated by a 4-week “washout”
period. Although the ionizers produced 100-fold increases in ion density, no effects were seen in
lung function, symptoms, or medication use. A decade later, Warner et al., (1993) performed a
similar double blind placebo controlled study on 20 adults, with one difference being the use of
two ionizers, one in the bedroom and one in the living room. Once again there was a major
increase in ion density but no effect on lung function, symptoms, or medication use.
About ten studies have investigated the effect of air cleaners on fungal spores and mold, with
reported effectiveness ranging from 0 (Nelson et al., 1991) to 94% (Richardson, 2000). These
studies include Cheong et al., 2004, Huang 1993, Huang et al., 1995, Jacobs et al., 1989, Li et al.,
1995, Loo et al., 1996, Perraud et al., 1992, Rhame et al., 1984, and Takatori et al., 2001.
Since radioactive radon daughters can attach to particles and be drawn deep into the lungs, an air
cleaner could reduce risk. Several studies investigated this, with general agreement that
exposures could be reduced by substantial amounts (30-60%; Hopke 1993, 1994, 1995a,b; Miles
et al., 1980; Rajala et al., 1984; Wasiolek et al., 1993), but that radioactive dose would be less
affected, partly because although the attached fraction would be reduced, the unattached fraction
(which can also penetrate deep into the lung) would increase.
An idea for increasing the effectiveness of air cleaners by mounting it on the headboard and
blowing the cleaned air directly over sleepers’ heads was tested in several studies (Villaveces et
al., 1977, Zwemer and Karibo, 1973). Although very high effectiveness was claimed in some of
these studies (Verrall et al., 1988, Morris et al., 2006), funding was sometimes supplied by
manufacturers and some authors (Morris et al., 2006, Hacker and Sparrow, 2005) were affiliated
with the companies that manufactured the device. There was also poor study design (no
blinding, no placebo) in some of these studies.
A serious problem in hospitals, particularly in those being renovated, is aspergillosis, an often-
fatal disease striking immunocompromised patients. The renovation disturbs spores of the
common fungus Aspergillus. Intensive filtration procedures are necessary to protect patients, and
several papers have recounted successes and failures in these attempts (Opal et al., 1986, Marieu
& Nelson, 1993, Mahieu et al., 2000, Engelhart et al., 2003). The recent resurgence of
tuberculosis has also sparked interest in filtration to protect against Mycobacterium tuberculosis
(Rutala et al., 1995, Miller-Leiden et al., 1996).
Following the early mostly anecdotal studies of hay fever and pollen allergies, a series of studies,
some with more stringent design (double blind, placebo controlled) were applied to study the
effect of air cleaners on asthmatic and allergic patients (Reisman et al., 1990, Reisman 2001,
Scherr and Peck, 1977, Schwartz et al., 1973). The allergens studied included dust mite
(Antonicelli et al., 1991, Harving et al., 1991, 1993, 1994, Shapiro et al., 1999, Thiam et al.,
1999, van der Heide et al., 1997, 1999, 2000, Warner et al., 1993); cat allergen (Boquete et al.,
1997, Custovic et al., 1998, de Blay et al., 1991, Eggleston et al., 2005, Gore et al., 2003a,b,
Luczynska et al., 1990, Pahdi et al., 1997, Swanson et al., 1985, and Wood et al., 1993, 1997,
1998); and dog allergen (Green et al., 1999). Mixed results ensued. Sometimes the air cleaner
was unsuccessful in reducing allergen levels. Sometimes it did reduce levels considerably, but
symptoms were unaffected. In only a small number of studies were both allergen and symptom
reductions significant (Bascom et al., 1996, Li et al., 1995, van der Heide et al., 1999).
Because of the importance of any treatment that could reduce allergic symptoms, these studies
have been periodically reviewed by committees of physicians and official reports issued by their
associations (e.g., American Lung Association (ALA 1997), American Thoracic Society (1990,
1997), Nelson et al., 1988, National Academy of Sciences (NAS 1993)). The invariable
conclusion has been that there is little to no evidence that air cleaners are effective in reducing
symptoms, and that they cannot be recommended to patients except as additional aids to more
basic strategies such as source removal, isolation of the bedroom from the pet or other source,
encapsulating of bed material in the case of dust mites, etc.
With the recognition of childhood asthma as a fast-growing and serious problem, several large-
scale studies were launched in recent years that included simultaneous approaches toward
reducing allergens: supply of HEPA air filters, HEPA vacuums, impermeable bedding, pest
management procedures, etc. The largest of these studies was the Inner-City Asthma Study
(ICAS), which took place over 3 years in 937 homes in poor neighborhoods in 7 US cities
(Morgan et al., 2004). A cost-benefit analysis of this study concluded that the benefits in
increased symptom-free days, reduced use of inhalers, and fewer unscheduled clinic visits
outweighed the substantial cost of $1469 per family (Kattan et al., 2005). Since the intervention
methods were simultaneous, it is not possible to attribute the benefits to any one of them, but the
HEPA air cleaners were an important part of the strategy. A second study taking this approach of
multiple simultaneous actions, one of which included a HEPA air cleaner in every child’s
bedroom, also resulted in significant improvements (Eggleston et al., 2005).
Beyond asthma, two groups of illnesses have been shown to be exacerbated by particles: chronic
obstructive pulmonary disease (COPD) and cardiovascular disease. Since particles are a mixture
of many elements and chemical compounds, it may be that certain components of the particles
are more toxic than others. Suspicion has focused in the past on combustion particles, and more
recently on ultrafine particles, which are more numerous than larger particles and may be more
toxic on an equal-mass basis (Oberdorster et al., 2005). A recent study employed filtration of
ultrafines to detect an effect of auto exhaust on oxidative stress of 29 healthy subjects (Bräuner et
al., 2007). The subjects were exposed in a chamber in random order for 24 hours to unfiltered
air and for 24 hours to filtered air from a nearby street with traffic. The HEPA filter reduced
ultrafine particle levels by 97.7%. The unfiltered air exposures resulted in significantly elevated
levels of oxidative stress as measured by DNA damage. Bräuner et al. (2008) extended these
studies to a set of 42 elderly healthy subjects, who were exposed to filtered and unfiltered air for
48 hours. The HEPA filter reduced the ultrafine particle number by about 68%, and the fine
particle mass by 63%. There was a significant 8% improvement in microvascular function
following exposure to the filtered air. However, in this study, the fine particle mass was more
highly associated with damage than the ultrafine particle number.
Long-term studies of air cleaners used in homes
A second question for the prospective buyer of an air cleaner after “How well does it work?” is
“How long will it work?” A weakness of the AHAM certification program is that it applies only
to new air cleaners. Therefore we searched for studies looking at long-term use within a home.
The Canadian Mortgage and Housing Corporation (CMHC) has provided a valuable series of
studies on filters as used in homes (CMHC 1998, 1999, 2001, 2005, 2007; Bowser et al., 1999;
Bowser and Fugler 2002; Fugler et al., 2000; Fugler and Bowser 2002). These studies have
looked at the effectiveness of portable filters, in-duct filters, and various determinants of particle
levels such as penetration factors, effects of floor cleaning, etc. In many cases, the CMHC has
looked at the effectiveness of the filter as actually used and with varying lengths of use. The
results from one study of five homes with in-duct ESPs is particularly relevant to the question
above, and returned rather disturbing answers. In several cases, leaks or gaps allowing air to
avoid the filter were present. In most cases, the filter performance degenerated within days after
cleaning. The best-performing filter averaged about 80% efficiency, but others were as low as
35% or 40%. These efficiencies would translate to even lower effectiveness, as determined by
measurements of actual indoor concentration reductions.
In several long-term studies in a home, Wallace et al. (2002, 2004) and Howard-Reed et al.
(2003) documented the behavior of an ESP and a good-quality mechanical filter over 6 months
to a year of continual use in a forced-air system that was nearly always on. Since this is almost
the only study documenting long-term (up to one year) behavior in a real-life situation, the
results of the study will be examined in detail. The ESP was installed in the return air duct of the
forced-air system. Probes were inserted upstream and downstream and particle levels measured
every minute using Climet monitors with 6 size categories: 0.3-0.5 μm, 0.5-1, 1-2.5, 2.5-5, 5-10,
and > 10 μm. Monitor results were automatically logged and inspected periodically. Thus the
changing efficiency over time could be monitored. The ESP when new was extremely efficient
for all particle sizes: 98-99% for the three largest sizes (2.5-10 mm), and 95-98% for the smallest
sizes (0.3-2.5 mm). However, after several weeks constant use, the efficiency for the smallest
sizes began to decline. The manufacturer recommended cleaning every month or two by soaking
in dishwasher detergent. This was done, and resulted in restoring the very high efficiency for the
coarse particles, but the efficiency for the fine particles reached about 95%, not the 95-98% that
had been achieved before. Therefore each of the 20 or so collector plates was wiped with a
sponge to remove collected dust. This resulted in bringing the efficiency back to the high levels
observed before. The sponge-wiping step was added to the cleaning routine from then on. (It has
subsequently been learned that the reason for the falloff in efficiency of ESPs is probably due not
to deposition of dust on the collector plates, but rather to the wires themselves being coated with
a silicon-based molecule (Chen and Davidson, 1999; Davidson and McKinney, 1998), perhaps
from household use of deodorants or other products; the reason for the positive effect of the
sponge wiping was probably that the narrowness of the opening between plates meant that each
wire was cleaned by the sponge at the same time the associated plate was being cleaned.) Over
the course of almost a year, the ESP was cleaned five times (approximately every 2 months).
The results of these long-term efficiency tests for the ESP and mechanical filter are provided in
Figures 1 & 2. Figure 1 documents the repeated declines and restoration of the ESP efficiency
for both fine and coarse particles provided by the cleaning routine. Although the ESP was
operating at reduced efficiency much of the time, overall it was providing a major air cleaning
benefit for both fine (65% reduction) and coarse (75%) particles. An even greater benefit would
have been obtained had the cleaning been done more often (every 3 weeks or so, instead of the
actual rate of every 6 weeks or so.)
Figure 1. Long-term Efficiency of the Electrostatic Precipitator (ESP). Over nearly one year of
constant air flow (2400 cfm) over the ESP, the average efficiency for fine particles was about 65%,
a reduction of particle concentrations by a factor of 3. For coarse particles the reduction averaged
about 75% (a factor of 4). After a month or two without cleaning, however, the efficiency was
reduced from > 90% to about 20-50%. The five cleanings over the year restored the efficiency to
>90% for both fine and coarse particles.
Figure 2 shows the long-term performance of the mechanical filter. A great contrast is seen
between its efficiency for coarse particles (80%) and that for fine respirable particles (near zero).
As dust collected on the filter, the efficiency for the coarse particles showed a slight
improvement, but the near-zero efficiency for fine particles was unchanged.
Figure 2. Long-term Efficiency of the Mechanical Filter. The mechanical filter increased its
efficiency for coarse particles (>2.5 μm) from less than 60% to nearly 80% over the course of 1600
hours of operation. However, the efficiency for fine particles hovered near 0 at all times.
Howard-Reed et al. (2003) extended these results to look at the overall effectiveness of both
filters by calculating the increase in deposition rate when the filter was on compared to when it
was off. The effective decay rate produced by the ESP was several times the natural decay rate
due to air exchange and deposition, indicating an effectiveness of better than 60% for both fine
and coarse particles (0.3-10 μm). The mechanical filter had somewhat smaller effectiveness for
the coarse particles and was ineffective for the fine particles.
Wallace et al. (2004) extended the results further to the realm of ultrafine particles. Figure 3
shows the increase in decay rates for the entire range of particle sizes provided by the ESP
compared to the mechanical filter, the furnace filter, and no filtration. It should be noted that the
large increases in the decay rates provided by the ESP is an average over all periods including
those when its efficiency was impaired. As an aid to estimating the effectiveness associated with
these decay rates, one can take the ratio of the decay rate with a filter to that with no filtration. A
ratio of 2:1 means the filter is reducing particles twice as fast as no filtration: a reduction of 50%.
A ratio of 3:1 corresponds to an effectiveness of 2/3 (about 67%), 4:1 an effectiveness of ¾
(75%), and 5:1 corresponds to an effectiveness of 4/5 (80%). The latter 80% figure is the figure
used by AHAM in calculating the size of the room that can be cleaned by a given portable air
cleaner. This figure is much more difficult to attain in a whole-house situation, where the
volume to be cleaned may be an order of magnitude larger than a room, and the number of
infiltration locations and indoor sources another order of magnitude larger. Nonetheless, it is
possible to estimate that the overall effectiveness across all particle sizes of the ESP in this real-
life situation was better than 50%. Theoretical calculations (Riley et al., 2002) suggest that a
whole-house effectiveness of about 65% could be expected for a well-performing and well-
maintained in-duct ESP.
A recent study of long-term (2 months) filter performance was carried out by Batterman et al.,
(2005). Four single-family homes with cigarette smokers were equipped with portable HEPA
filter units (CADR rating of 330 cfm). Over the course of the study, the airflow dropped by 7-
14%, largely due to particle accumulation on the prefilters. The effectiveness of the filters
ranged from 30-70%. Presumably this could be improved by a few percent with more frequent
changeout of the prefilters.
Shaughnessy et al. (1993) tested two filters (HEPA and ESP) for 800 hours over a 6-month
period. The HEPA was tested in a nonsmoking bedroom, the ESP in an office with smoking.
Both filters lost efficiency, with the CADR reduced by 25% for the HEPA filter and 38% for the
ESP. The difference might not be significant, since the ESP no doubt dealt with much greater
Figure 3. Effectiveness of an ESP compared to a good-quality mechanical filter, a furnace filter,
and no filtration. In the 0.1 to 1 μm range (where most particle mass resides) the ESP outperforms
the other filters by a ratio of about 3:1, corresponding to a reduction of particle levels by about
0.01 0.1 110
Particle diameter (
Fan on, no filter
Laboratory or chamber studies of air cleaners can provide precise measurements of their
efficiencies and clean air delivery rates. They are most valuable in providing relative rankings of
different air cleaners. They can also be used to rule out all air cleaners below some desired clean
air delivery rate. For this purpose, the AHAM web site is the most useful and up-to-date
compilation of air cleaner CADRs.
However, earlier chamber studies also have their uses. For example, the detailed information on
efficiency, flow rates, and clean air delivery rates allows a calculation not only of the cost of
buying and maintaining any air cleaner tested, but also of calculating the cost of the clean air
provided and comparing that value across different air cleaners.
Therefore we also include in this report the chamber studies that were identified by our literature
search (Table 3; see associated Excel file:table 3—chamber studies.xls).
One of the earliest and most influential chamber studies was provided by Offermann et al.,
(1985). He studied 10 air cleaners including all major types (panel filters, pleated filters, HEPA
filters, ESPs and ionizers). A room in a test house operated by Lawrence Berkeley National
Laboratory provided excellent control over all important environmental variables. To compare
the devices on a single metric, Offerman developed the concept of the Effective Cleaning Rate
(ECR), which was later adopted by AHAM and renamed the Clean Air Delivery Rate (CADR).
(Offermann was a consultant to AHAM when they developed their test method.) For most of the
air cleaners mentioned, the CADR is the product of the air flow through the device and its
efficiency. For example, if the device pulls air at 300 cubic feet per minute across the filter, and
the filter has an efficiency of 50%, the device will produce clean air at the rate of 150 cubic feet
per minute: a CADR of 150 cfm. The units of CADRs are normally either cfm or m3/h.
The study of 10 air cleaners showed that the HEPA filter (ECR about 300 m3/h) and the two
ESPs (about 200 m3/h each) were by far the best performers. The ion generators, panel filters,
and electrets ranged between 0 and about 50 m3/h, while the combination extended surface
electret/ionizer attained about 100 m3/h cleaning rate. Shortly thereafter, Humphreys (1987)
tested 3 HEPAs, two ESPs and one ionizer, with similar results: the HEPAs and ESPs ranged
from about 30-120 cfm of clean air whereas the ionizer provided only 30 cfm. In Sweden,
Olander et al., (1987, 1988) tested all 31 brands of air cleaners on the Swedish market, again
showing ESPs near the top and ionizers at the bottom of the rankings.
The first test of a duct-mounted filter, an ESP, seems to have been performed by Hanley et al.,
(1990). The device was tested for eight size fractions at three flow rates (250, 500, 1000 cfm)
with efficiencies ranging from 70-90% at low flow and 45-90% at high rates. The higher
efficiency at lower flow rates is expected, since the particles spend longer in the region affected
by the electric field and have more time to deposit. However, Hanley also suspected that some
particles might flow through areas unaffected by the electric field (“sneakage”). By masking the
extremities of the device, he was able to improve the efficiency even at high flow to 60-100%.
Offermann et al., (1991, 1992) then extended the in-duct testing to six air cleaners, including two
panel filters, one pleated filter, a HEPA filter, and two ESPs. Testing was done in a 3-room test
house at LBNL. The HEPA filter, pleated filter and one of the two ESPs accounted for the
highest clean air delivery rates (400-600 cfm) compared to a range of only 14-24 cfm for the
other three filters. Offermann calculated the cost of clean air delivery (taking into account both
retail cost and upkeep) and found it was 28-30 cents per cfm for the ESP and pleated filter, and
56 cents per cfm for the HEPA filter, but $4-7 per cfm for the other three in-duct filters.
Hanley et al., (1994) summarized tests at Research Triangle Institute on 12 in-duct air cleaners.
The best performers were ESPs (80-90% efficient at lower face velocities, 70% at higher
velocities); and ASHRAE 95% “pocket filters” (70-98% new; > 95% loaded). The furnace filter
had an efficiency of about 2% and a “self-charging” filter an efficiency of about 60%.
Shaughnessy et al., (1993, 1994) summarized experiments on 12 air cleaners in his test chamber
at Tulsa University. The rank order was ESP >HEPA >extended surface (electrets) >ionizers
>ozone generators. A more recent review is Shaughnessy and Sextro (2005, 2006).
In an extensive study at the 3M company, Kinzer and Moreno (1997) measured the efficiency of
27 air cleaners, including panel filters, pleated filters, “washable/reusable” filters, “deep” pleated
filters, and ESPs. Of them all, only the ESPs were able to achieve better than 50% efficiency
(range of 44-94%).
One chamber study (Faulkner et al., 1999) dealt with a “personal air supply system” designed for
use in offices. This is an attempt to deliver filtered air directly to the breathing zone of the
worker, thus avoiding the common problem of pollutants contaminating the filtered air before it
reaches the worker. The particular system tested appeared capable of reducing particle levels by
a substantial amount, thus reducing the amount of outdoor air necessary to bring in by 23-47%;
this would account for a major saving in the expense of conditioning the outdoor air.
Although we did not seek studies of filtration in offices or other buildings, a number of such
studies showed up in our literature search. These studies are listed in Table 4 (see associated
Excel file:table 4--offices.xls). The list is not complete, but does serve to give an indication of
the type of filtration studies that have been performed in offices, schools, day care centers, and
similar buildings. Over the past two decades, a series of studies by a group in Denmark
(Wargocki et al., 2002, 2004, Wyon 1992, 2004) have concentrated on the effects of ventilation
on productivity, with fairly consistent indications that productivity is often improved by 6-9%
when conditions are improved (e.g., newer or better filters are installed). Only one study in
schools has been carried out (Wargocki and Wyon, 2007) but it showed a similar improvement in
objective performance on tests of various types when ventilation was increased (but no effect of
replacing an old filter with a new one was seen). However, the old filter had seen little use so the
difference between filters was small.
Fisk et al. (1987, 2000, 2002, 2003) have concentrated on performance and cost of air filtration
technologies. Clausen (2004), Clausen et al., (2002a,b) and Hytinnen et al., (2002, 2003a,b,
2006) have focused on the effects of used filters on odor and health. Seppanen and Fisk (2002,
2004), Seppanen et al. (2006), and Jaakola et al. (1991) have investigated the role of filters in
worker performance and the Sick Building Syndrome. Hänninen et al. (2005) investigated the
reduction potential of mortality risk attributed to filtration. A specific study of the effect of ESPs
on airborne dust and health of employees was carried out by Skyberg et al. (2000, 2003) and
Skulberg et al. (2005). The ESPs succeeded in reducing particle levels by 46% but improvements
in symptom prevalence were not significant.
Combinations of measurement and models have been used by Emmerich and Nabinger (2001),
and Jamriska et al., (2000, 2002, 2003) to determine the impact of air filtration in buildings.
“Personalized” ventilation systems (individual supply of filtered air contained in or attached to
the desks of office works) have been investigated by Hedge (1993), Kaczmarczyk (2002 a,b,
2004), Melikov (2000, 2002), and Zeng et al., (2002). In most cases, these systems succeed in
providing considerable reductions of particle exposures as long as people remain at their desks.
Special attention to ionizers may be warranted. As mentioned above, most comparative studies
show poor results from ionizers, with CADRs typically ranging between 0 and 50 cfm. In a
highly visible controversy, Sharper Image, Inc sued Consumer Reports over their low rating of
the Ionic Breeze, a best-selling ionizer. The courts supported the Consumer Reports testing
methods and conclusions. (And in February 2008, in the face of several class actions brought
against Sharper Image regarding the Ionic Breeze, and sharply declining sales of the Ionic
Breeze, Sharper Image declared bankruptcy.) However, several studies such as Repace et al.,
(1983) and a series of studies from the laboratory of Sergei Grinshpun (Lee et al., 2004a,b;
Grishpun et al., 2005) have shown excellent results from ionizers. This apparent contradiction
may simply be due to the relative strength of the ionizers tested. The ceiling-mounted system
Repace tested and the series of ionizers tested in the Grinshpun chamber (funded by Wein
Products, a manufacturer of ionizers) are apparently much stronger than those typically available
to the consumer. A major reason for this is that the ionizers produce ozone, and the stronger
ionizers produce more ozone. Several tests indicate that a medium-strength ionizer will produce
ozone at levels sufficient to exceed the 50 ppb level that is a recommended upper limit (Britigan
et al., 2006, Mullen et al., 2005, Niu et al., 2001a,b, Shaughnessy and Oatman, 1991, Phillips et
al., 1999). Therefore most ionizers available to the public are rather weak, and probably unable
to maintain high levels of ions throughout a medium-sized room. In addition, the Grishpun tests
are done in a sealed chamber with no air change and therefore no constant supply of particles to
be charged; Mayya et al., (2004) showed that such a supply of particles very quickly reduced the
efficiency of the ionizers tested.
Both ionizers and ESPs produce ozone as a byproduct. Several studies have documented the
amount of ozone produced (Table 5; see associated Excel file:table 5—ozone.xls). In some
cases, the resulting ozone levels would exceed various standards such as the FDA standard of 50
ppb. Most existing studies of ozone production have looked at ozone generators and ionizers.
Whether ESPs would have equivalent ozone production rates is not well known. One study of
five Canadian homes with duct-mounted ESPs showed an average ozone increase of 9 ppb
(CMHC 2007). While well below the FDA standard, this could add to existing ozone levels to
cause concentrations exceeding the standard in some cases.
Ozone can react with some organic gases to produce ultrafine particles (Liu et al., 2004, Mullen
et al., 2005, Siegel et al., 2006, Tung et al., 2005, Waring et al., 2007, 2008). The most
productive reactions are with terpenes, including alpha-pinene (the pine scent) and limonene
(lemon scent), common constituents of many consumer products. Therefore most homes with
ozone levels indoors will have some ultrafine particles produced when these consumer products
Another problem with some of the ionizer studies are that results are not reported as CADRs,
which are comparable across different room volumes and ventilation rates. However, in her
Master’s thesis, Yu was able to use reported results to calculate CADRs for all of these cases
(Yu, 2005). (Her calculations are included in Table 3). The CADRs in general range from about
10 to about 70 m3/h, which puts nearly all of the ionizers studied near the bottom of the
effectiveness ratings. Considering that even these values were obtained under conditions of no
supply of new particles, it is clear that the effectiveness of ionizers will be even lower under
Ozone generators have been marketed to the public as both air cleaning devices and mood
elevators. However, ozone has generally deleterious effects on organisms (it can kill microbes at
high concentrations) and few if any positive effects. The state of California has recently passed
legislation to stop the sale of ozone generators and other products that will create ozone in excess
of 50 parts per billion (ppb) according to a standardized test (Underwriters Laboratory Standard
for Safety 867).
A few studies have provided data on the cost of the air cleaners studied. Offermann et al., (1985,
1992) detailed the cost of 10 portable air cleaners (and one circulating fan) in 1985, and did the
same for 6 in-duct air cleaners in 1991-92. We have applied Offermann’s cost figures and his
calculations of the CADR for these 16 air cleaners to calculate the cost per cubic meter per hour
of clean air supplied (Table 6). We have also calculated the effectiveness of the air cleaners for a
50 m3 room and a 350 m3 house, using several assumptions regarding air movement within
homes and air change rates for Canadian homes.
Table 6. The Cost of Clean Air
Air cleaner type
replacement d Power
Portable air cleaners
ESP 1 (metal plates)
ESP 2 (foam pad)
In-duct air cleaners
pleated bag filter
ESP 1 (metal plates)
ESP 2 (foam pad)
a Cost figures for portable air cleaners from Offermann (1985); for in-duct air cleaners from
b Cost per m3/h calculated over 20-year period with one filter change per year
c Effectiveness calculated by (CADR/V)/(CADR/V + a + k), where
a (room) = 0.5 h
) = 0.4 h
V (room) = 50 m
a (house) = 0.2 h
V (house) = 350 m
d For in-duct cleaners, includes cost of running the central fan at all times during the year
The results on effectiveness are in complete agreement with the findings of Offermann et al.,
(1985, 1992), Shaughnessy et al., (1994), Kinzer and Moreno (1997), and Hanley et al., (1994)—
that is, the best air cleaners, whether portable or in-duct, are the ESP and HEPA devices.
However, considering cost in addition, the ESPs have a distinct advantage over the HEPA
devices, because they require no changes in filters (which can cost nearly $100 for the HEPA
cleaners) and do not increase the electric bill in the way that HEPA devices do. Most of the
mechanical filters tested do well on coarse particles but not on the fine particles that may have
more serious health effects. Many of the electrets lose efficiency for reasons that are not fully
known, but may relate to shielding of the fibers by the collected particles. And most of the
ionizers have low CADRs to begin with, while those with stronger ion generation powers have
other undesirable attributes such as unacceptable ozone production.
However, it is not possible to wholeheartedly recommend the ESPs either. For one thing, they
require a commitment on the part of the homeowner to keep a regular maintenance schedule.
The evidence suggests that monthly cleaning is required. Although the cleaning regimen is not
very demanding (soaking in detergent, cleaning each wire and plate with a sponge, and hosing
off), if neglected it will not be long before the efficiency dips well below 50%. Therefore the
prospective buyer should be honest with himself about how well he can maintain a regular
Secondly, there is sobering evidence from the CMHC study of five homes with installed in-duct
ESPs that the installation process itself may often be unsatisfactory. For the ESP to work
properly, it needs to be tightly fitted into the duct to avoid bypass. The installation problem
extends to all in-duct air cleaners, but is perhaps worst for the HEPA filters, since they present
the greatest resistance to air flow. This means the air is even more likely to find small defects or
openings in the fitted housing and be diverted around the filter. There are engineering
approaches to finding and filling these cracks, with liquid-based sealant, but this may often be
beyond the ability of the homeowner himself to carry out.
Often a family with an allergic child will consider an air cleaner for the child’s room. They
might be swayed by the argument that a less powerful air cleaner will be sufficient for a small
room. However, there are several problems here. For one thing, the allergen may not be
confined to the child’s room. If the allergy is to cockroaches or furry pets, the allergen will be
found throughout the house. Secondly, the less powerful air cleaner, supposedly suited to the
room, will not perform as well if the room door is open much of the time. In that case, there will
be considerable air exchange with the rest of the house, reducing the effectiveness of the air
cleaner. (Even if the door is closed, if there is a forced-air system, it will bring in air from other
parts of the house unless care is taken to close the register in the room.) Finally, most allergens,
such as dust mite fecal allergen, are contained in relatively large particles and are mostly found
in bedding and house dust, which the air cleaner cannot clean.
The high-efficiency in-duct pleated filter tested by Offermann et al., (1991, 1992) did about as
well as the ESP and HEPA filter. These filters have advantages as well. They require little or no
maintenance, unlike the ESP, and they don’t have the energy penalty of the HEPA filter. They
could be a good choice for the homeowner who does not wish to bother with the maintenance of
the ESP. However, care would need to be exercised regarding the rating of the filter. An
ASHRAE Dust Spot Efficiency rating of 80% to >95% or a MERV rating of 13-16 would be
required to clean the air effectively over the entire house.
In summary, the CADR is a controlling factor determining the upper limits of the cleaning ability
of a portable filter. But other factors, including particularly the tightness of the house and
whether interior doors are open or closed will affect the degree of cleaning the portable filter can
provide. For in-duct filters, no CADR test is available, so the buyer needs to determine the
rating of the filter to make an informed decision. This is the subject of the next section of this
Before the CADR was developed, filters were rated by a standardized test—in the U.S., this test
was known as ASHRAE Standard 52.1 (ASHRAE 1992). A test dust was introduced to the filter
at a certain spot and the amount of dust intercepted was measured by the color of the spot (the
Dust Spot rating, on a percentage scale from <20% to > 95%). A second test measured the
fraction (by weight) of the test dust that came through the filter after a given amount of exposure
time (the Arrestance rating, also on a percentage scale, from <65% to >98%). However, these
tests had major deficiencies. First, the test dust was not very similar to the type of dust that is
found in modern-day outdoor air. Secondly, measuring by weight of the dust trapped gave high
values for larger particles (which weigh more)—yet research indicates that the smaller particles,
which can penetrate deep into the lung, may be more harmful. Thirdly, these tests did not
provide an indication of how well the filter worked for different sizes of particles, since only a
total amount was considered. Fourthly, the tests did not have some of the practical problems of
installing filters in homes, such as fitting the filter tightly into its assigned space to eliminate
bypass. Fifthly, these short-term tests gave no indication of how the filter would perform over an
extended period of time.
Therefore ASHRAE sponsored research to get a better idea of how filters actually worked with
different sizes and compositions of particles (Hanley 1993a,b). In general terms, there are two
processes at work: diffusion and interception/impaction. In diffusion, or Brownian motion, the
particle moves in random directions as it is hit by molecules of air. The smaller the particle, the
farther and faster it will move in these random directions, and the more likely it is to hit and be
trapped by a fiber on the filter. A large particle has more inertia, and as the airstream changes
direction to get through the cross-hatchings of the filter, the large particle keeps right on going
and impacts (or is intercepted by) the fiber. Therefore in very general terms, it must be that
filters are most effective for the smallest and largest particles, and will reach a minimum at some
intermediate size. This size turned out to be in the neighborhood of 0.1-0.5 µm .
With this knowledge, it was realized that a useful indicator of the efficiency of a filter would be
its efficiency at this minimum level. Therefore ASHRAE Standard 52.2 was developed, and
ranks filters according to their minimum efficiencies (MERV, for Minimum Efficiency Rated
Value) (ASHRAE , 2007). There are 20 categories within the MERV appellation, with MERV-1
being the least and MERV-20 the most efficient. ASHRAE Standard 52.2 did not replace
Standard 52.1, so that filter ratings may use either the newer MERV scale or the older ASHRAE
Dust Spot Efficiency and/or Arrestance ratings. The relationship between these three systems is
shown in Table 7.
The MERV 17-20 ratings apply to HEPA and ultra-low penetration air (ULPA) filters used in
clean rooms, hospital surgery rooms, laboratories dealing with radioactive materials, etc. MERV
13 -16 rated filters are also used mainly in commercial locations requiring very low particle
levels, such as hospital inpatient rooms, general surgery, smoking lounges, and high-end
Residences were classified according to their filter materials in an Appendix to ASHRAE
Standard 52.2 as “superior” (MERV 9-12), or “better” (MERV 5-8).
A more detailed discussion of the history of the ASHRAE filter tests is provided in Appendix B.
Table 7. MERV ratings from ASHRAE Standard 52.2 compared to the ASHRAE
Standard 52.1 Dust Spot Efficiency and Arrestance Tests
ASHRAE Dust Spot Efficiency
Recommendations on Choosing Air Cleaners
About 60% of Canadian homes have central forced air systems (CMHC 1999). For homes with
forced air, Consumers Union recommends whole-house (in-duct) air cleaners as the “only
sensible” choice (ConsumerReports.org 2008). Within that category, they state that electrostatic
precipitators (ESPs) “have worked best overall and restricted airflow least among whole-house
models.” They also state that although ESPs may cost more at first than mechanical filters, they
may cost less overall because of the need to replace the filters every one to three months.
Although they also note that ESPs emit small amounts of ozone, the seven models that were
rated in 2008 all produced less than the 50 ppb that is the FDA guideline. Finally, they state that
the top-performing models that they tested had a MERV rating between 11 and 13. These
recommendations are in general agreement with the scientific studies reviewed in this report.
However, as the CMHC study of 5 homes with existing in-duct ESPs makes clear, they often
perform at a much lower effectiveness than expected. One suspected cause of this is faulty
installation. If a gap exists, some of the air will not pass through the working parts of the ESP
and will not be cleaned. Even if it appears that no gap exists, some researchers have noted that
the air can avoid the best cleaning zone (“sneakage”) and reduce the effectiveness of the air
cleaner. These researchers masked the outer perimeter of the air cleaner, forcing the air through
the central regions, and measured improved efficiency.
The problem of faulty installation is not restricted to ESPs. In fact, HEPA or “HEPA-type”
filters have even more of a problem, since the increased resistance of the filter makes it more
likely that the air will find a way to avoid going through. For filter-based in-duct systems, there
is a need to determine whether the system is matched to the existing air handling unit capacity,
both at the expected pressure drop for a clean filter and the higher manufacturer-specified
pressure drop for a used filter. This means that the buyer must have his system capacity checked
before making a final decision on the filter-based system to buy. Because installation is so
important, a careful investigation of the reputation of the installer, including references from
other customers or appearance on lists of high-quality performers would be important.
Assuming that the filter-based system is properly matched to the existing forced-air system, and
has been properly installed, and has an efficiency equivalent to a MERV 13-16 rating, such a
system should give good performance. The homeowner will have to remember to change filters
every few months.
For all in-duct systems, ESP and filter-based alike, there remains the problem of testing whether
the installed system is working as designed. A possible way for the homeowner to test whether
his in-duct system is working properly is to place a mechanical filter behind it. The mechanical
filter should remain perfectly clean for a long time if the in-duct filter is working. Often the in-
duct system is installed in the return air duct, leaving a space for the furnace filter, so this space
could be utilized for the “test” filter.
For portable units, as described above, the AHAM website is always kept up-to-date and its
searchability makes it the most convenient and efficient way to compare all models by the single
best unit of comparison, the CADR. Therefore the first step in buying a portable air cleaner is to
determine the size of the space for which cleaning is desired. If the space is open to other areas
in the home, or if it has a door that will sometimes be open, the homeowner should multiply the
volume by some factor greater than 1 and look for a portable air cleaner rated for that size on the
AHAM Website. This will provide a list of air cleaner manufacturers and the model numbers
that will meet these specifications. A reasonable second step is to investigate the Consumer
Reports ratings of air cleaners. Consumer Reports.org maintains a website that provides access
to all previous ratings reports from the magazine. (This requires a subscription.) These reports
provide information on aspects other than the CADR, such as unit cost, annual energy cost,
replacement filter costs, noise of operation, ozone emission rates, and an overall rating. These
two objective sources of information should provide a trustworthy guide to obtaining the
portable air cleaner suited to the homeowner’s needs.
Air cleaners have been employed for more than 80 years. The first practitioners (in the 1920s
and 1930s) were doctors seeking to alleviate acute hay fever and pollen asthma in their patients.
They often created their filters and fan units themselves, or depended on local engineers to do
that for them. They were imaginative about the design and quick to incorporate improvements.
For example, one of the first air cleaners diagrammed already took advantage of the idea of
pleating the filter to increase the surface area within a limited space. Almost concurrently with
the development of the electrostatic precipitator for industrial use, doctors were adapting it for
use with their patients in hospital wards and in the patients’ bedrooms. Within 3 years of
developing a prototype for use in a hospital room, one air cleaner began to be marketed
The early articles written by these doctors often included tests of the air cleaners, generally by
exposing glass slides and counting pollen grains, either artificially generated or naturally
occurring outside the room with the air cleaner. These tests often showed excellent results, since
even the simplest filters are in fact highly effective at trapping large particles such as pollen
grains. The physicians could also note very satisfactory results in many of their patients, since
the acute hay fever or pollen allergy is often quickly relieved by removal from the source.
However, the physicians also reported more stubborn cases, usually in persons with chronic non-
seasonal allergy. Often 20% or so of patients would show little or no improvement.
What we would in these days consider good study design was not done in these early years.
None of the physicians considered sham approaches (non-working air cleaners) to guard against
the placebo effect. None used a control population. No statistical tests of significance were
employed. However, there is little doubt that these early studies (about 10 articles between 1924
and 1936) were a powerful argument for the effect of air pollution on health, and the benefits of
After the ESP, the next advance in filter design was the HEPA filter, which was developed in the
1940s to protect radiation workers from exposure to inhalable radioactive particles. No new
principle was involved in the development of the HEPA filter—only a recognition that
increasing the twists and turns that air was forced to go through as it passed through the filter
would eventually succeed in trapping nearly all particles in the air. This was accomplished by
vastly increasing the density of fibers in the matrix and adding many layers of material. The
drawback to this approach is the much greater pressure drop that resulted. Since the amount of
power is proportional to the amount of pressure drop to overcome, the power requirements for
the HEPA filter have often been the reason it is not as popular as it could be.
A number of articles were published dealing with the effects on radiation exposure and dose of
these new filters. For the first time, it became important to deal with ultrafine particles, because
the “unattached fraction” of the radioactive particles consists of clusters of particles smaller than
a few nanometers (nm). When the air cleaner reduces particle levels, it changes the balance
between the unattached and attached fractions. Since the unattached fraction can be breathed
deeper into the lungs, it has a higher ultimate dose. Thus the air cleaner can reduce total
exposure but does not reduce total dose as much. Since most measurement techniques were
unable to deal with ultrafine particles, new methods and equipment needed to be developed, as
outlined by Hopke and others in a series of articles.
A new wave of articles dealing with more difficult allergies than hay fever or pollen began in the
1980s. Allergies to cats, dogs, dust mites, and cockroaches were recognized at this time, and
asthma began increase in prevalence and severity. The authors of these articles recognized the
a) guarding against the placebo effect,
b) running single-blind or double-blind studies to preserve objectivity,
c) using a control group with no treatment or else a crossover design in which all persons receive
treatment or no treatment first (randomly selected) and then the other of the two choices (thus
each person is his own control), and
d) treating results statistically with tests to determine significance.
This new wave of articles began in about 1980 and has continued to date. As it became clear that
asthma was far more complicated and resistant to treatment than the hay fever and pollen
allergies of earlier years, it was recognized that a multifactorial strategy would need to be
developed. This strategy included such things as instruction on better cleaning techniques,
HEPA vacuum cleaners, HEPA air cleaners, impermeable bedding, and pest removal techniques.
The largest and most complete of these studies eventually resulted in a clear indication of
benefits outweighing the cost, even though the cost was high (>$1000 per household).
There has also been a focus on the possible benefits of negative ions, at first according to a belief
on the part of some that negative ions had a beneficial effect on mood. This idea has not been
generally accepted, although there are still proponents. A second approach has been to consider
the effect of negative ions on particle removal. Several studies using quite powerful ion
generators have showed reasonable removal capabilities. However, these powerful ion
generators are not acceptable for residential use, partly because of undesirable effects on static
electricity in the home and partly because of generating ozone at an unacceptable rate.
Finally, there was also a period when ozone generators were sold in the belief that ozone acted to
kill bacteria and viruses as well as having beneficial effect on mood. However, the adverse
health effects of ozone have been well documented, and there is general recognition that ozone
generators are not desirable. California has banned the sale of appliances that can increase the
level of ozone in homes to 50 ppb.
There is no question that some air cleaners can perform a valuable service in lowering particle
levels in homes. Fortunately, there are three important sources of objective information that
homeowners can use to select suitable air cleaners—the AHAM and ConsumerReports.org
Websites described above, and the newest ASHRAE ratings of filters (the MERV scale described
above). However, there is one important gap in our knowledge concerning whole-house air
cleaners: the quality of the installation process. There is evidence that faulty installation can
sharply reduce the effectiveness of a whole-house air cleaner. Therefore selection of an installer
must be done with great care.
Appendix A. Measuring efficiency and effectiveness
The theory of filtration effectiveness has recently been presented by Nazaroff (2000). The
following discussion follows that presentation closely.
We first define a single-pass efficiency as the fraction of particles removed when air passes
through a filter. Assuming we measure a particle concentration immediately upstream and
downstream of the filter, the single-pass efficiency is:
If the total air flow per unit time through the filter is F, then the rate of delivery of “clean air”
(Clean Air Delivery Rate, or CADR) is
The CADR is a crucial ingredient in determining the effectiveness of portable air cleaners.
However, it is not the only ingredient. There are other considerations in determining the total
effectiveness E of air cleaners, by which we mean the ability to reduce the average concentration
in a room from an initial equilibrium concentration C0 to a final equilibrium concentration Cf:
The effectiveness ranges from 0 to 1, with 1 standing for perfect cleaning of the air.
The general equation governing indoor air concentrations as a function of outdoor air levels and
indoor sources is called the mass balance equation. Although the equation can be complicated if
all processes are considered (Nazaroff and Cass, 1989), for our purposes we can consider only
five major processes: infiltration (particles entering from outdoors), exfiltration (particles leaving
the house), deposition (particles depositing on surfaces), filtration (particles removed by an air
cleaner), and generation (particles created by indoor sources). The differential form of the mass
balance equation can then be written as the sum of these 5 processes:
in out in in in
dm V PQC QC kVC CADRC S
≡ = −− − +
m = mass of all airborne particles in the volume V
V = volume of space considered (e.g., a room or an entire residence)
Cin = concentration indoors
P = penetration factor (a dimensionless quantity between 0 and 1 describing the fraction of
outdoor airborne particles able to penetrate the building envelope)
Q = the air flow rate entering the building
Cout = concentration outdoors
k = deposition rate
S = source generation rate
It should be noted that this equation rests on the well-mixed assumption: that is, perfect
instantaneous mixing of the particles throughout the entire volume such that the concentration
Cin is identical at every point. Although this seems to be a stringent assumption, and is not true
when there are rapid changes of the source generation term, it is surprisingly valid for many
Another aspect of this equation is that parameters such as the penetration coefficient, deposition
rate, and generation rate depend strongly on particle size. Therefore the equation applies to
particles of any size, but the parameter values must be changed for each size category
Since the outdoor concentration, the air flow rate, and the source generation rate are generally
unknown functions of time, the differential equation cannot be solved for the general case.
However, we can consider several widely applicable cases with simple solutions.
In this case, the outdoor concentration is constant over time, as are the airflow entering the
building, the source generation rate, the penetration factor, and the deposition rate. The resulting
indoor concentration C0 before turning on the air cleaner is
C PQ S
After turning on the air cleaner (and waiting for a sufficient length of time for the unit to pass
enough air through it to fully clean the air), the final indoor air concentration Cf is
fC PQ S
CQ kV CADR
The effectiveness of the air cleaner is then given (after some tedious algebra) by
ECADR Q kV
From this equation, one can see that the effectiveness is greater when CADR increases. Also, if
one were to tighten up the house (i.e., reduce the airflow Q), the effectiveness is increased. It is
interesting to see that the source term S has no effect on the effectiveness of the air cleaner.
To help familiarize ourselves with the practical meaning of this equation, we can consider an
Suppose first of all that no indoor source is active. Then the indoor-outdoor ratio before the air
cleaner is turned on is given by
C Q kV
To remove the size of the room or house from this equation, divide top and bottom of the
equation by the house volume V:
where a is the air exchange rate: Q/V. The air exchange rate is the number of house volumes of
air entering the house per unit time. Usually the air exchange rate is measured in air changes per
hour (ach) or inverse hours (h-1). Several thousand measurements of air change rate have been
made in US homes, and studies show that the rates may vary from about 0.1 h-1 for a very tight
house to about 2 h-1 for a house with windows wide open, with a typical value in the
neighborhood of 0.5-0.75 h-1. Recent measurements in Canadian homes suggest rather tight
homes in general, with typical values around 0.2 h-1 (Wheeler et al., unpublished)
The indoor-outdoor ratio in equation  is called the infiltration factor. It is a crucial descriptor
of the penetration of outdoor air particles into a home. If the air exchange rate is the average rate
over a year for the home, then the infiltration factor tells us what percent of the outdoor air
particles, on average, will penetrate the home. A recent study of 37 North Carolina homes
showed that annual average infiltration factors in this one small area ranged from about 0.27 to
about 0.81. That is, the amount of outdoor air particles entering those homes varied by a factor
of 3. Since epidemiological studies relating outdoor air concentrations to health effects assume
identical exposures for a given outdoor air concentration, we can see that actual exposures to
outdoor air will be widely different for persons living in a tight vs. loose home, and thus there
will be considerable misclassification of subjects in these studies.
Several studies of human exposure and indoor air quality have shown that a typical infiltration
factor for particle mass is on the order of 0.5-0.6—that is, about half of outdoor air particles
remain airborne after penetrating the home envelope. By insulating the house, adding storm
windows, etc., this fraction can be reduced, but not eliminated. (If the air exchange rate were
reduced to zero, we would suffocate!)
In fact, many studies have shown that a certain minimum air exchange rate is required for health.
This rate has been set at about 0.35 h-1 in ASHRAE Standard 62 (ASHRAE 2004). Since new
homes are often designed to be energy efficient, having tight construction with low air change
rates on the order of 0.1 h-1, there may be a need for additional ventilation to assure adequate
fresh air for breathing.
The other parameters entering into the infiltration factor are the penetration coefficient P and the
deposition rate k. The penetration coefficient is particularly difficult to measure, and actual
values in existing homes are not known well. One large study of 178 California homes indicated
that the penetration factor for inhalable particles (< 10 μm in diameter) was unity (Őzkaynak et
al., 1996). This is a somewhat surprising result at first, since one imagines that most particles
will bang up against the house walls. However, these particles are extremely small, invisible in
fact, and follow the streamlines of the air as it enters the house through windows, door openings,
electrical and plumbing entries, and cracks. To a 10-μm particle, a crack the width of a human
hair (100 μm) looks like a vast open area that the particle can easily traverse. Smaller particles
have even easier access, until we reach the smallest (ultrafine) sizes, where Brownian motion
comes into play, and the particles are tossed about by molecular collisions and can get thrown
into the sides of the crack before they get through it.
The deposition rate k is only slightly easier to measure than the penetration coefficient, and is
also not well known in a practical sense. The same California study (Ozkaynak 1996) resulted in
an estimate for k of 0.39 (+ 0.16) h-1 for fine particles (<2.5 μm) and 0.65 (+ 0.28) h-1 for
inhalable particles (<10 μm).
Putting these estimates together, (say, P = 1, a = 0.5, k = 0.4) we can see that a typical infiltration
factor for fine particles might be (1)(0.5)/(0.5+0.4) = 5/9 = 0.55. This was in fact close to the
infiltration factors of 0.55 to 0.61 found for three PM10 studies (Ott et al., 2000).
With these values for a and k we can now interpret the fundamental equation  for the
effectiveness of an air cleaner. The average volume of a US home around 2000 was more than
350 cubic meters (m3). For an air change rate of 0.5 h-1, this translates to an air flow rate Q of
175 m3/h. Then the effectiveness of an air cleaner in cleaning an entire home would be given by
ECADR Q kV CADR
++ + +
To achieve an effectiveness of 50%, we can see that the CADR must be 315 m3/h. To get to an
increased effectiveness of, say, 2/3 (67%), the CADR would have to double to 630 m3/h. This
example shows that attempts to increase effectiveness run into a steeply increasing CADR
requirement. In fact, CADRs above 800 m3/h are not generally found in existing portable air
cleaners (The AHAM list cuts off at 450 cfm, or 765 m3/h). Therefore for homes of average size
and larger, achieving an effectiveness better than 50% would require either multiple portable air
cleaners or else an efficient in-duct air cleaner.
Short-term indoor source (smoking, cooking)
The second case we will consider is the common situation of a short-term indoor source, such as
baking a potato. (Both electric and gas stoves emit very large numbers of ultrafine particles
from the stovetop or oven.) In this case, a solution of the differential equation can be found by
assuming a constant rate of production while the source is on, followed by no emission when it is
off. The resulting concentration takes the form of a “shark fin”, with a sharp initial rise followed
by a slowing increase and then a sharp initial decrease that also then slows down as it approaches
zero (Figure A-1). The solution can be plotted for the two cases with the air cleaner off and on.
Then the difference will show the effect of the air cleaner. We can see that during the initial
moments, the air cleaner provides no help at all. This is because the particles need to pass
through the air cleaner before reaching the person, and when the person is close to the source,
this does not happen. However, it can be shown that the air cleaner will reduce the average
particle concentration by exactly the same amount as in the steady-state case.
Figure A-1. Particle concentration during and after an indoor source is on for one hour—effect of an air
cleaner with an effectiveness of 50%. The effectiveness is less than 50% while the source is on, but better than
50% after an hour or so, so that the average effectiveness is 50%.
0 0.5 1 1.5 2 2.5
Concentration (arbitrary units)
No air cleaner
Appendix B. Testing and Ranking Filters
For many years, the standard method of testing filters in the US was Standard 52 (later 52.1)
developed by ASHRAE. This method employs a standardized mixture of particles and a
standardized measure of the effect of the filter. The efficiency of the filter is estimated by
calculating the mass of particles encountering the filter and comparing to the mass collected
downstream. The resulting ratio is transformed into an efficiency rating in percent—for
example, a 65% filter would be expected to collect about 65% of the total mass of particles
impinging on the filter.
Method 52.1 was recognized to have several defects. Since most of the mass of the particles is
contained in the larger (nonrespirable) particles, the test results overestimated the effect of the
filter on respirable particles. Also, the standardized mixture contained about 15% black carbon
(soot), perhaps 5 times as much as is normally found in ambient air. Since black carbon is highly
conductive, testing ESPs with this mixture was impossible since the particles quickly short out
the ESP. Also, the test was unable to estimate accurately the effect of loading the filter over
Therefore ASHRAE contracted with Research Triangle Institute to develop an improved test.
RTI built a testing system and experimented with different mixtures of particles for suitability in
a standard test (Ensor 1988a,b, 1991, 1997; Hanley et al., 1990, 1993a,b, 1994, 1995a,b, 1999,
2000; Hanley 2001, 2002; Hanley and Owen, 2003; Owen et al., 1992 a,b) . One innovation was
to employ monitors capable of measuring multiple particle sizes, to determine efficiency as a
function of particle size. Instead of a single number characterizing a filter (such as a 65%
ASHRAE rating), this would result in multiple efficiencies for different size particles for each
filter tested. A reasonable choice was then to rank the filter by the minimum efficiency observed
for some particle size category. Figure B-1 shows the results of the new test on several filters
with their ASHRAE ratings. As can be seen, all the filters showed a characteristic “U”-shaped
curve, with minimum efficiencies occurring near the 0.3 μm size range, and improved
efficiencies for particles larger and smaller than that. (Smaller particles have increased Brownian
random motions and are more likely to collide with the filter elements; larger particles have more
inertia and cannot follow the air streamlines and therefore collide with the filter elements more
Figure B-1. Size-resolved efficiencies of filters with a range of ASHRAE Dust Spot
Efficiency ratings (Hanley et al., 1994).
This test eventually achieved the status of an official standard test (Method 52.2). Since the
particle mixture employed had much smaller black carbon content, it was suitable for testing
ESPs as well as other filters. There was also a new procedure developed of loading filters to
simulate extended use. A new system of ranking filters was also developed, based on the
minimum efficiencies established by Method 52.2. These rankings are called Minimum
Efficiency Rating Values (MERV) and range from MERV-1 to MERV-20 in order of increasing
However, some of the other deficiencies, such as not testing long-term efficiency, or actual real-
world effectiveness, still remain. Research is ongoing to fill some of these gaps. For example,
testing the effectiveness and lifetime of the electric charge on electret filters is being
investigated. To determine the effectiveness of the electric charge itself, as distinct from the
effectiveness due to the mechanical interception provided by the fibers, it is necessary to
somehow remove the electric charge (which is designed to be permanent or at least
semipermanent) from the filter for testing while not affecting the characteristics of the uncharged
fibers. Then it is necessary to see how the electric charge is reduced as a function of time and
the characteristics (composition, electrical distribution, etc.) of the dust loadings.
As another example, it has recently been found that the decline in efficiency over time observed
in electrostatic precipitators is not so much because of the dust deposited on the collector plates,
but rather silicon compounds deposited on the corona wires. Therefore a proper test of the ESPs
would involve exposure to silicon vapor in the test chamber to determine the rate of decline of
efficiency, as well as the proper mode and ease of cleaning the wires. (The source of silicon
vapor in homes is also a subject for further research—apparently several cosmetic preparations
International standard methods of ranking filters also exist. These are based on somewhat
different tests that are standardized for one country (e.g., Britain) or for several allied countries
(Europe). The European rankings run from EU-1 to EU-8—most filters used in European
buildings are EU-7 or EU-8. The European test comparable to ASHRAE Standards 52.1 and
52.2 is known as EN779 (1993).
Increased attention has been paid to protecting buildings from biological or chemical attack.
Because particle behavior in air is governed almost entirely by size, viruses, bacteria, and pollen
will behave very much like nonliving particles of the same size. For example, most pollen grains
are > 5 μm and are easily trapped by even the least efficient filters. A test for effectiveness
against biological aerosols, based on determining the viability of those aerosols penetrating the
filter, has been developed, again at RTI (Foarde et al., 1999a,b). However, no accepted standard
test for biological aerosols has been established.
After the anthrax attack in the US in 2001, the US Post Office sponsored at least one study
testing the effectiveness of the filtration system at three of the processing centers (Martin et al.,
2006). All three had identical high-efficiency filters (MERV-14 followed by a HEPA filter).
However, only two achieved the 99.97% or better efficiency required for a HEPA filter. The
filters were reinstalled with attention paid to blocking bypass leaks, and were brought up to
proper performance in the next test. (One of the three buildings also tested a new filter system at
that time, and the new system also failed the first test but passed the second after repairs.) This
experience underlines the need to consider installation as a serious variable possibly affecting
efficiency even before the filter system comes on line.
With the increasing importance of ultrafine particles, a question arises of how efficient filters
will be for the full range of ultrafines from 1-100 nm. The tests of Hanley et al., (1994) stopped
at 10 nm (see Figure B-1) because that was the limit of the instrumentation available. Some
theoretical and experimental studies have indicated that perhaps the efficiency does not continue
to improve as sizes decrease from 10 nm due to possible new processes (“thermal rebound”)
coming into play (Ichitsubo et al., 1996; Wang and Kasper 1991). However, a recent study by
Kim et al., (2008) shows consistent monotonic increase in efficiency down to the lowest size (3
nm) studied. Therefore it is expected that all ultrafines will be more efficiently filtered than
larger submicron particles by all mechanical filters.
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