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OCR for page 327
10
IMPACT OF VENTILATION AND AIR
CLEANING ON AS th ma
Indoor exposures to pollutants associated with the incidence
or symptoms of asthma are affected by many aspects of building
design, maintenance, and operation. Building features modify the
indoor sources of pollutants, the rates of pollutant entry from out-
doors, and the rates of pollutant removal from indoors. Building
ventilation and air cleaning are the two primary processes used
intentionally within buildings to remove pollutants from the in-
door air and maintain acceptable indoor environmental condi-
tions. This chapter provides an overview of the relationship of
building ventilation and particle air cleaning to exposures to in-
door-generated pollutants that are associated with asthma. The
findings from experimental assessments of the effects of air clean-
ing on allergy and asthma symptoms are also summarized. Be-
cause the association of asthma with pollutants from outdoor air
is not a primary focus of this report, even though the exposures
may occur primarily indoors, the dependence of these exposures
on ventilation and air cleaning is not addressed in this chapter.
THEORETICAL BACKGROUND
This section provides a very brief overview of theoretical con-
siderations that are necessary to understand the influence of
building ventilation and air cleaning on indoor pollutant concen
327
OCR for page 328
328
CLEARING THE AIR
"rations. Emphasis is placed on indoor particles because the in-
door-generated pollutants most clearly associated with asthma
are particles. Appendix A provides a more detailed technical dis-
cussion of this topic along with the equations and parameter val-
ues used for the theoretical predictions later in this chapter.
From conservation of mass, the steady-state indoor air con-
centration~ of a pollutant that is emitted indoors and absent from
outdoor air equals the indoor pollutant generation rate divided
by the sum of all pollutant removal rates. In the present context,
the most important pollutant removal processes are (1) ventila-
tion (i.e., the flow of indoor air containing pollutants to outdoors);
(2) pollutant depositional losses on indoor surfaces; and (3) air
cleaning (i.e., intentional removal of pollutants from indoor air
by air filters and other types of air cleaners). The influence of
changes in ventilation or air-cleaning rates on the indoor pollut-
ant concentration depends on the magnitude of the other two pol-
lutant removal processes.
Many of the indoor-generated pollutants important for
asthma are particles with diameters ranging from a fraction of a
micrometer (1 ,um equals one-millionth of a meter) to approxi-
mately 20 ,um. Table 10-1 provides information of the sizes (aero-
dynamic diameters)2 of these particles. The available data are lim-
ited and sometimes contradictory. Many of the bioaerosols
associated with asthma, particularly dust mite allergens, whole
pollens, cockroach allergen, and many fungal spores are large par-
ticles greater than a few micrometers in diameter. There are con-
tradictions among available data on the size of particles with cat
allergen; however, a significant fraction of airborne cat allergen
appears to be associated with particles smaller than a few mi-
crometers. Environmental tobacco smoke is composed almost en-
tirely of submicron-size particles (i.e., particles smaller than 1 rim).
Droplet nuclei from coughs and sneezes, which often contain vi-
rus, are included in Table 10-1 because viral infections are strongly
iFor this discussion, we have assumed perfect mixing of the indoor air. See
Appendix A for more information.
2Except that the sources of the data for pollens and fungal spores do not indi-
cate whether the sizes are physical or aerodynamic diameters.
OCR for page 329
329
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OCR for page 330
330
linked to exacerbation of asthma, at least in children Johnston et
al., 1995~. There is evidence that rates of building ventilation and
occupant density modify the rates of respiratory illness experi-
enced by building occupants (Fisk, 1999; Fisk and Rosenfeld,
1997), presumably by changing exposures to infectious droplet
nuclei. Data on the size distribution of droplet nuclei are ex-
tremely limited and the methods employed to obtain the data may
have resulted in an undercounting of the larger particles. The
available data indicate that most of these particles are submicron
in size but most of the particle volume is associated with particles
larger than 1 ,um. It is not clear whether the number concentration
or volume concentration of infectious droplet nuclei is more rel-
evant for disease transmission.
The magnitude of two of the particle removal processes-
deposition on surfaces and air cleaning can vary dramatically
with particle size. Particles deposit on indoor surfaces when in-
door air motion, gravitational settling, electrostatic forces, and
other phenomena cause them to collide with indoor surfaces. For
particles, larger than a few micrometers in diameter, depositional
losses are dominated by rates of gravitational settling. A 20-,um
particle falls a distance of 1 m in about 80 seconds so it remains
suspended indoors for only a short period. The deposition losses
of such large particles tend to overwhelm normal rates of particle
removal by ventilation or air cleaning. In contrast, a 0.2-,um par-
ticle falls a distance of 1 m in about five days. The rate of deposi-
tional removal of 0.2-,um particle from the indoor air, which is
controlled by the indoor air motion, indoor surface roughness,
and other factors, is almost a factor of 100 lower than the rate of
depositional removal of a 20-,um particle.
Some gaseous pollutants such as nitrogen dioxide and ozone
are also removed from indoor air at a significant rate by deposi-
tion (often called sorption) on or reaction with indoor surfaces.
Rates of depositional removal depend on the chemical nature of
the pollutant, the intensity of indoor air motion, and other fac-
tors. Gravitational settling is unimportant for gaseous pollutants.
Particles deposited on indoor surfaces can be resuspended in
indoor air when the surfaces are disturbed by human activities
(e.g., walking, vacuuming) or by high air velocities (e.g., air exit-
ing a fan). Theory (Hinds, 1982) and limited empirical data
OCR for page 331
IMPACT OF VENTILATION AND AIR CLEANING ON ASTHMA 331
(Thatcher and Layton, 1995) indicate that resuspension occurs
predominantly for particles larger than approximately 2 ,um.
Based on our current knowledge of the behavior of particles, ex-
posures to some of the larger particles associated with asthma
may be substantially influenced by the localized resuspension of
particles that results from occupant activities.
BUILDING VENTILATION
Background
In this document, the term "ventilation" refers to the flow of
outside air indoors, which is always accompanied by an equal
flow of indoor air outdoors. Ventilation removes and dilutes in-
door airborne pollutants, brings outdoor air pollutants into build-
ings, and often removes or supplies heat and water vapor. Venti-
lation is also needed to maintain oxygen concentrations inside
buildings, although the quantity of ventilation needed to supply
oxygen is very small relative to other ventilation requirements.
Increasing the rate of ventilation generally leads to overall
improvements in indoor air quality; however, the indoor concen-
trations of some pollutants from outdoors, such as outdoor par-
ticles and ozone, can increase with the ventilation rate. Indoor
humidity can increase or decrease with ventilation rate. When it
is cold and dry outdoors, increased ventilation usually reduces
the indoor humidity.
While increased ventilation rates are usually considered ben-
eficial for health and for improving perceived air quality (e.g.,
odors), ventilation air must often be heated (and sometimes hu-
midified) or cooled and dehumidified. Consequently, the ventila-
tion rates selected for buildings must strike a balance between the
benefits of energy savings with reduced ventilation and the
known or suspected benefits to health with increased ventilation.
Several metrics are used to specify the rates of building venti-
lation. Generally, these metrics are flow rates of outside air nor-
malized by the number of occupants, floor area, or indoor vol-
ume. Corresponding units of ventilation rates include the
following: liters per second per person (L so per person); liters
OCR for page 332
332
CLEARING THE AIR
per second per square meter of floor area (L so per square meter);
and air changes per hour (h-~.
Municipalities typically adopt one of the several building de-
sign codes used in the United States. These codes, or state energy
codes, include building design provisions intended to maintain
ventilation rates above a minimum rate that varies with the build-
ing type. The American Society of Heating, Refrigerating, and Air
Conditioning Engineers (ASHRAE) publishes a minimum venti-
lation standard that is the basis for the ventilation specifications
in many codes. The current version of the ASHRAE standard is
Standard 62-1999 Ventilation for Acceptable Indoor Air Quality
(ASHRAE, 1999~. Standard 62-1999 lists 0.35 ho as a minimum
ventilation rate in residences,3 10 L so per person (20 cubic feet
per minute [cfm] per person) as a minimum ventilation rate in
offices, and 8 L so per person (15 cfm per person) as a minimum
ventilation rate in schools. Due to a paucity of scientific data on
the relationship of building ventilation rates with the health and
well-being of occupants (Seppanen et al., 1999), the minimum
ventilation rates in the ASHRAE standard are based substantially
on professional judgment and on studies performed in laborato-
ries with conditions quite different from those encountered in real
buildings.
Building design codes and ASHRAE's minimum ventilation
standard do not ensure that all buildings maintain the specified
minimum ventilation rates. In most states and municipalities,
there are no legal requirements to actually maintain ventilation
rates at or above the levels in building design codes. Additionally,
building ventilation rates are difficult to measure accurately, in-
frequently measured, and as discussed later, poorly controlled.
Ventilation systems, although intended to remove indoor pol-
Jutants, can also become sources of pollutants. Portions of venti-
ration systems, particularly components that become wet, can be-
come colonized by microorganisms and produce bioaerosols that
3Standard 62-1999 states that the 0.35 h-i of ventilation is normally satisfied by
infiltration and natural ventilation but includes no technical specifications for the
building to ensure that this ventilation rate is met continuously or on average.
Standard 62-1999 also specifies installed mechanical exhaust capacities of 50
L s-i (100 cfm) per kitchen and 25 L s-i (50 cfm) per bathroom.
OCR for page 333
IMPACT OF VENTILATION AND AIR CLEANING ON ASTHMA 333
are transported by the airflow to the occupied space. In addition,
particles, fibers, and odorous and potentially irritating volatile
organic compounds (VOCs) may be emitted from synthetic mate-
rials, including fibrous insulation materials, from residual oils
used in component production, from deposited dusts, and from
microorganisms. Ventilation also affects the indoor humidity
which in turn influences the growth or survival of microorgan-
isms within buildings.
Heating, ventilating, and air conditioning (HVAC) is the more
general process of thermally conditioning and ventilating build-
ings. In commercial buildings, these functions are usually inte-
grated. The HVAC process employed in commercial buildings is
reviewed here because HVAC features may influence exposure to
pollutants that are known or thought to be associated with
asthma.
Methods and Rates of Ventilation in
U.S. Single-Family Residences
Diamond (1999) has summarized many of the basic physical
characteristics of the U.S. residential building stock. In 1997, de-
tached single-family units and row houses constituted 73°/O of the
U.S. housing stock, 6% of the housing stock was mobile homes,
and the remainder was apartments. The average heated floor
space in all U.S. housing stock was 181 m2 (1,950 square feet) and
air conditioning was installed in 70°/O of these dwellings. The av-
erage conditioned floor area of mobile homes was 87 m2 (940
square feet) and 70°/O of mobile homes had air conditioners. Four-
teen percent of all housing units used humidifiers, and nine per-
cent had dehumidifiers.
When windows are closed, the ventilation of single-family
residences in the United States is almost exclusively an uncon-
trolled process. In air infiltration (or infiltration and exfiltration),
air leaks through unintentional cracks and holes in the building
envelope. The infiltration rate is driven by small pressure differ-
ences across the building envelope that are typically less than a
few pascals in magnitude. These pressure differences arise due to
the differences between the indoor and outdoor air temperatures,
resulting in different indoor and outdoor air densities, and also as
OCR for page 334
334
CLEARING THE AIR
a consequence of wind. Unintentional air leakage in the ductwork
of forced-air heating and air-conditioning systems located in at-
tics and craw! spaces, also causes large increases in air infiltra-
tion. Even if the ducts do not leak, forced-air systems can pressur-
ize or Repressurize specific rooms relative to the outdoor pressure,
forcing air leakage through the building envelope.
U.S. homes often have intermittently-operated exhaust fans
in bathrooms and kitchens. When operated, these fans draw out-
door air into the building. Window and door opening by occu-
pants, predominantly during mild weather, also has a large influ-
ence on residential ventilation rates.
A very small portion of single-family dwellings in the United
States have mechanical ventilation systems (i.e., fans operating
continuously or intermittently to provide ventilation). Mechani-
cal ventilation is most common in the State of Washington be-
cause the state energy code now requires mechanical ventilation.
The technologies used to mechanically ventilate residences are
described in Roberson et al. (1998~.
Ventilation rates in residences vary considerably over time.
The lowest ventilation rates occur during mild weather with win-
dows and doors closed. When weather is more severe, windows
remain closed but ventilation rates are higher due to increased
indoor-to-outdoor temperature differences and increased use of
forced-air heating and air conditioning. The highest ventilation
rates generally occur when windows or doors are open.
Present data on ventilation rates in U.S. single-family resi-
dences are limited and possibly not representative of the building
stock. One source of information is measurements of the airtight-
ness of building envelopes with windows and doors closed. Ven-
tilation rates are predicted with semiempirical models, using mea-
sured values of building airtightness4 combined with climate data
and indicators of a building's shielding from wind, as mode] in
4Airtightness is used here as a general term understandable to a broad audi-
ence. The actual measured parameter is the effective leakage area (ELA) at a ref-
erence pressure, usually 25 or 50 Pa across the building envelope. The ELA is the
area of an orifice that would leak air at the same rate as all the leakage paths in the
building envelope. The ELA is usually normalized with building floor area and
height to produce a normalized leakage.
OCR for page 335
IMPACT OF VENTILATION AND AIR CLEANING ON ASTHMA 335
puts. When annual average ventilation rates are desired, the pre-
dictions may also include terms to account for natural ventilation
via windows; however, the current knowledge of window use
and effects on ventilation is cursory. The second source of infor-
mation on residential ventilation rates is measurements made us-
ing a tracer-gas procedure. Although considered more accurate
than predictions based on airtightness, the measured data are
more sparse than airtightness data. Therefore, we presently have
only crude estimates of residential ventilation rates.
Based on airtightness and climate data for about 12,000
houses, Sherman and Matson (1997) estimate that the arithmetic
average effective ventilation rate of houses in the United States is
1.1 hot. This average reflects ventilation rates when windows are
closed and also the higher ventilation rates that occur with open
windows during mild weather. Airtightness normalized by house
size is highly variable (Sherman and Dickerhoff, 1994), with a
standard deviation that is approximately 50% of the mean. The
mean of the airtightness data from individual states varies among
states by more than a factor of three. In the available data, there is
no trend in airtightness with severity of climate. The available
data indicated that houses constructed after 1980 are more air-
tight (by ~50%) than older houses (Sherman and Dickerhoff,
1994~; however, there was no trend evident in airtightness with
age for houses constructed after 1980.
A set of 2,844 measurements of residential ventilation rates in
U.S. houses was analyzed by Murry and Burmaster (1995~. The
measured data from 66 research projects are not from a represen-
tative sample of residences; however, this analysis is probably the
best available information on the distribution of ventilation rates
in U.S. houses. When considering all climate zones and seasons,
the arithmetic and geometric mean ventilation rates were 0.76 ho
and 0.53 hot, with a geometric standard deviation of 2.3. There are
large variations in ventilation rates with season and climate zone.
The winter and summer arithmetic means, for all climate zones,
are 0.55 and 1.50 hot. Approximately one-third of the measure-
ments in the winter season are less than the 0.35 hot, the rate in
the current ASHRAE ventilation standard. In the coldest climate
zone, approximately 55% of the measured ventilation rates, from
all seasons, are less than 0.35 hot.
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336
CLEARING THE AIR
Methods, Patterns, and Rates of Ventilation in
U.S. Multifamily Apartment Buildings
In 1997, 21% of U.S. housing units were apartments. The aver-
age conditioned floor area of apartments was 85 m2 (920 square
feet) and air conditioning was installed in 65% of apartments (Dia-
mond, 1999~. Published information on the methods and rates of
ventilation in multifamily apartment buildings are extremely
sparse. Based on the limited information available,5 older low-
rise (i.e., less than Three stories) apartment buildings usually
have no mechanical supply of ventilation air. Much like single-
family dwellings, these buildings are ventilated primarily by un-
controlled infiltration and natural ventilation windows that can
be opened. Intermittently operated bathroom and kitchen exhaust
fans cause temporary increases in ventilation rates. Leakage in
the ductwork of forced-air heating and air-conditioning systems
and pressurization or Repressurization of individual rooms can
drive infiltration and exfiltration in apartments, just as it does in
single-family houses. Newer low-rise apartment buildings are
ventilated similarly to older low-rise buildings; however, a larger
portion of these buildings have continuous mechanical exhaust
ventilation from the bathrooms and/or kitchens of each apart-
ment.
Older apartment buildings with more than approximately
three stories typically have no mechanical air supply or some
mechanical supply to the interior hallways. The air supply sys-
tem, when present, is frequently not functional (Shapiro-Baruch,
1993~. Apartments within these buildings sometimes have a sys-
tem for continuous exhaust ventilation from bathrooms and kitch-
ens, although it is not always operational. Some portion of these
older high-rise buildings have a vertical ventilation shaft that
functions much like a chimney and passively draws air from the
apartments.
In new apartment buildings with more than three stories,
5The information in this section is based primarily on case studies, on two gen-
eral guidance documents (Diamond et al., 1999; Liddament, 1996) and on discus-
sions with Dr. Rick Diamond of Lawrence Berkeley National Laboratory. who
conducts research on energy use and ventilation in apartment buildings.
OCR for page 337
IMPACT OF VENTILATION AND AIR CLEANING ON ASTHMA 337
exhaust ventilation is usually drawn continuously from the
kitchen and bathrooms of each apartment. The exhaust fans may
serve groups of apartments or individual apartments. Outside air
enters either from unintentional leaks and vents at windows or
via ventilation systems that supply air continuously to each apart-
ment. When a mechanical air supply is present, often this air is
supplied to a single room of each apartment from a duct system
in the building's interior hallway.
The airflow in heated multistory apartment buildings with-
out mechanical ventilation often occurs in an upward direction
from lower-level to upper-level apartments (e.g., Diamond et al.,
1986; Modera et al., 1986~. Coo] outdoor air leaks into the lower
apartments; flows upward, picking up moisture and pollutants;
and exfiltrates through the walls and ceilings of upper-level apart-
ments. Due to this airflow pattern, the lower-level apartments
tend to have more fresh air supply, lower humidity, and more
cold drafts. Humidity and pollutant levels are often increased in
upper-level apartments because a portion of the air entering these
apartments comes from lower levels of the building. As moist air
exfiltrates out of the upper-level apartments, water vapor may
condense within cold walls and ceilings. Possibly, the higher pol-
lutant levels and humidity in upper-level apartments could con-
tribute to asthma symptoms.
This same upward-flow phenomenon occurs to a variable de-
gree in all heated multistory buildings. When the building is air
conditioned (i.e., cooled), the airflow direction can reverse; how-
ever, the downward airflow in air-conditioned buildings will be
less pronounced because the indoor-to-outdoor temperature dif-
ferences are typically much smaller during air conditioning than
during heating of buildings. By reducing the openings between
floors, the vertical airflow between floors can be reduced. Me-
chanical ventilation can also reduce or overwhelm the upward
buoyancy-driven airflow.
In addition to the buoyancy-driven upward airflow in apart-
ments, other unintentional flows of air between adjacent apart-
ments are reported commonly from case studies. These flows oc-
cur through unintentional openings in walls and floors, and may
be driven by mechanical ventilation systems, buoyancy, and
wind.
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IMPACT OF VENTILATION AND AIR CLEANING ON ASTHMA 383
to save energy, some modern air conditioners will stop the recir-
culation and cooling of indoor air and instead provide ventilation
(i.e., supply outdoor air to the building) when the outdoor air is
relatively coo! and suitable for cooling the house.
Summary and Discussion of Limitations of Assessment
Many of the indoor pollutants associated with asthma are air-
borne particles; thus, particle air cleaning has been considered a
potentially beneficial technology for the prevention of asthma or
asthma symptoms. Technologies for particle air cleaning are well
developed. Air filters with a moderate to high efficiency for par-
ticles larger than approximately 2 ,um are used routinely in the
heating and air-conditioning systems of buildings.
The magnitude of the reduction in indoor-generated particle
concentrations accomplished with particle air cleaning depends
on the air cleaner's particle removal rate relative to the particle
removal rate by all other processes including ventilation and par-
ticle deposition on surfaces. The rate of particle removal by an air
cleaner varies with particle size and is proportional to the flow
rate of air through the air cleaner multiplied by the air cleaner's
size-dependent particle removal efficiency. The two primary air
cleaning options for reducing indoor particle concentrations are
to replace the existing filters in heating and air-conditioning sys-
tems with higher-efficiency filters and to operate supplemental
air cleaners with integral fans in the occupied space.
In field studies, enhanced air cleaning has been associated
with reductions in airborne particle concentrations that range
from negligible to more than 90°/0. For the airborne particles asso-
ciated with asthma, the published data are very limited. Simple
mode] predictions indicate that substantial reductions in indoor
concentrations of 10-,um particles can be obtained only when the
rate of airflow through the air cleaner per unit of indoor air vol-
ume is large, for example, 10 room volumes per hour or more.
The predicted effectiveness of air cleaning diminishes rapidly
with increases in particle size above 10 ,um because gravitational
settling rates increase with particle size. Thus, air cleaning does
not appear to be an attractive option for reducing exposures to
dust mite allergen, which predominantly involves particles larger
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384
CLEARING THE AIR
than 10 ,um. However, based on predictions it is feasible to reduce
concentrations of particles smaller than 2 ,um, such as ETS par
i]
ticles, droplet nuclei, and smaller particles with cat allergen, by
70°/O or more using air cleaners with a moderate to high efficiency
rating and a flow rate of several indoor air volumes per hour.
Both the available experimental data and mode] predictions
ndicate that HEPA filters, which are more expensive and often
require larger and noisier fans, are not likely to be superior to
lower-efficiency filters in reducing concentrations of many of the
bioaerosols associated with asthma. Even for submicron-size ETS
particles, available data indicate that HEPA filters are not neces-
sarily the preferred option. Thus, the very common recommenda-
tion that HEPA filtration, in contrast to lower-efficiency air clean-
ing, be used by allergic and asthmatic individuals when they
choose to employ air cleaning, is not supported by either experi-
ments or theoretical predictions. Unfortunately, the limited per-
formance data available for many non-HEPA residential air clean-
ers make it difficult to provide alternate recommendations.
The influence of air cleaner use on asthma and allergy out-
comes has been evaluated in numerous experimental studies;
however, most of these studies have important limitations. Over-
all, the data suggest that air cleaners are helpful in some situa-
tions in reducing allergy or asthma symptoms, particularly sea-
sonal symptoms, but it is clear that air cleaning, as applied in the
studies, is not consistently and highly effective in reducing symp-
toms. The available data provide no information regarding the
effects of air cleaning on the development of asthma or the devel-
opment of sensitization to allergens.
Conclusion Regarding Air Cleaning and Asthma
There is limited or suggestive evidence that particle air clean-
ing is associated with a reduction in the exacerbation of asthma
symptoms. There is insufficient evidence to determine whether
or not the use of particle air cleaners is associated with decreased
asthma development. Theoretical and limited empirical data sug-
gest that air cleaners are most likely to be effective in reducing the
indoor concentrations of particles smaller than approximately 2
,um. Much of the airborne allergen appears to be within larger
OCR for page 385
IMPACT OF VENTILATION AND AIR CLEANING ON ASTHMA 385
particles. Relevant particles smaller than 2 ,um include environ-
mental tobacco smoke particles, significant portions of airborne
cat, grass, and birch allergen, and virus-containing droplet nuclei
from coughs and sneezes.
Research Needs Related to Air Cleaning and Asthma
The results of existing experimental studies are inadequate to
draw firm conclusions regarding the benefits of air cleaning for
asthmatic and allergic individuals. Many of the existing studies
have important limitations, such as small study size, lack of blind-
ing, a small or undefined rate of air cleaning, placebo air cleaners
that may significantly remove the larger particles associated with
asthma, and no exposure assessment or inadequate assessment.
Additional research to assess the benefits of air cleaning is clearly
warranted, but future studies must overcome as many of these
limitations as possible. Because air cleaning is most promising for
reducing indoor concentrations of particles smaller than a couple
of micrometers, future research should emphasize these agents.
Sensitization to allergens a critical step in the development
of allergic asthma often occurs early in life. No information is
available to indicate whether air cleaning of spaces occupied early
in life can reduce the rate of allergic sensitization. Research is
needed to address this issue.
As described in Appendix A, particles larger than a few mi-
crometers have a complex and inadequately understood behav-
ior in the indoor environment, including rapid rates of gravita-
tional settling, resuspension from surfaces, and possibly
incomplete mixing with the indoor air. Consequently, the influ-
ence of air cleaning systems on exposures to particles in this size
range is not well understood and the associated benefits from air-
cleaning cannot be predicted with a high degree of confidence. A
combination of aerosol science and air-cleaning research is needed
to fill this gap in our knowledge.
The limited data on the size distribution of many of the
bioaerosols and allergens associated with asthma limit our un-
derstanding of the benefits of air cleaning. Additional data are
needed particularly for pet allergens and pollens.
As stated earlier, HEPA filter units have been widely recom
OCR for page 386
386
CLEARING THE AIR
mended for allergy and asthma patients who desire to use air
cleaners. Air cleaner manufacturers have responded by aggres-
sively marketing air cleaners with HEPA filters and offering few
other products. However, experimental data and theoretical pre-
dictions indicate that air cleaners with a lower efficiency rating
are likely to be equally effective in reducing the concentrations of
most, and perhaps all, of the indoor-generated particles associ-
ated with allergies and asthma. These lower-efficiency air clean-
ers could have a lower product cost, less powerful or noisy fans,
higher rates of airflow and particle removal, and reduced energy
consumption. The scientific and medical community should de-
velop revised recommendations regarding the selection of air
cleaners by allergic and asthmatic individuals, and air cleaner
manufacturers should respond by providing new air-cleaning
products.
REFERENCES
Aberg N. Sundell J. Eriksson B. Hesselmar B. Aberg B. 1996. Prevalence of allergic
diseases in schoolchildren in relation to family history, upper respiratory
infections, and residential characteristics. Allergy 51~4~:232-237.
American Lung Association. 1997. Residential air cleaning devices: types,
effectiveness, and health impact. American Lung Association, Washington,
DC.
American Thoracic Society. 1997. American Thoracic Society Workshop,
Achieving Healthy Indoor Air. American Journal of Respiratory and Critical
Care Medicine 156(Suppl 3~:534-564.
Antonicelli L, Bilo MB, Pucci S. Schou C, Bonifazi F. 1991. Efficacy of an air
cleaning device equipped with a high efficiency particulate air filter in house
dust mite allergy. Allergy 46~8~:594-600.
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Representative terms from entire chapter:
air cleaning
