Clearing the Air Asthma and Indoor Air Exposures (2000) / Chapter Skim
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10 Impact of Ventilation and Air Cleaning on Asthma
10 Impact of Ventilation and Air Cleaning on Asthma
Pages 327-393
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From page 327... ...
Building ventilation and air cleaning are the two primary processes used intentionally within buildings to remove pollutants from the indoor air and maintain acceptable indoor environmental conditions. This chapter provides an overview of the relationship of building ventilation and particle air cleaning to exposures to indoor-generated pollutants that are associated with asthma.
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From page 328... ...
air cleaning (i.e., intentional removal of pollutants from indoor air by air filters and other types of air cleaners)
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From page 329... ...
329 Cal In ~5 Cal .
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From page 330... ...
The magnitude of two of the particle removal processesdeposition on surfaces and air cleaning can vary dramatically with particle size. Particles deposit on indoor surfaces when indoor air motion, gravitational settling, electrostatic forces, and other phenomena cause them to collide with indoor surfaces.
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From page 331... ...
Increasing the rate of ventilation generally leads to overall improvements in indoor air quality; however, the indoor concentrations of some pollutants from outdoors, such as outdoor particles and ozone, can increase with the ventilation rate. Indoor humidity can increase or decrease with ventilation rate.
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From page 332... ...
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]
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From page 333... ...
and air conditioning was installed in 70°/O of these dwellings. The average conditioned floor area of mobile homes was 87 m2 (940 square feet)
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From page 334... ...
When operated, these fans draw outdoor air into the building. Window and door opening by occupants, predominantly during mild weather, also has a large influence on residential ventilation rates.
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From page 335... ...
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.
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From page 336... ...
Much like singlefamily dwellings, these buildings are ventilated primarily by uncontrolled infiltration and natural ventilation windows that can be opened. Intermittently operated bathroom and kitchen exhaust fans cause temporary increases in ventilation rates.
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From page 337... ...
Outside air enters either from unintentional leaks and vents at windows or via ventilation systems that supply air continuously to each apartment. 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.
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From page 338... ...
Larger buildings, as well as many smaller buildings, typically use HVAC systems that thermally condition air in mechanical rooms or in equipment located at the rooftop. The conditioned air, usually a mixture of outdoor air and recirculated indoor air known as the supply airstream, is supplied throughout the building or building section.
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From page 339... ...
In some smaller buildings, the supply air fan stops when there is no demand for heating or cooling. The supply airstream usually contains more recirculated indoor air than outdoor air.
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From page 340... ...
Measured minimum ventilation rates in commercial buildings vary a great deal among buildings and often deviate substantially from code requirements (Dols and Persily, 1994; Fisk and Faulkner, 1992; Lagus Applied Technologies, 1995; Persily and Grot, 1985; Teijonsalo et al., 1996; Turk et al., 1989~. Outside air drawn into HVAC systems and mixed with recirculated air is distributed to the various rooms of a building with a complex duct system.
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From page 341... ...
However, the ventilation rates measured when the HVAC systems provided the minimum amount of outside air were less than the corresponding minimum rate specified in the current ASHRAE standards in a majority of 6Some buildings were constructed when the applicable ventilation standard specified lower minimum ventilation rates than ASHRAE Standard 62-1999 (ASHRAE, 1999)
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From page 342... ...
Therefore, occu pants' exposures to indoor air pollutants depend on the operating schedule for the HVAC system. Even less is known about the proportion of commercial building ventilation that results from air infiltration; however, the existing data suggest that infiltration is appreciable, particularly in the smaller buildings (Lagus Applied Technologies, 1995; Persily, 1999~.
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From page 343... ...
Sites of reported microbiological contamination include the outside air louvers, the mixing box where outside air mixes with recirculated air, filters, cooling coils,
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From page 344... ...
Min. vent.a 1.4 0.8 2.2 2.4 0.7 0.4 aWith the ventilation system providing the minimum rate of supply of outside air, a normal condition during cold weather and also when outside temperatures exceed the indoor temperature.
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From page 345... ...
vent. = minimum ventilation; SD = standard deviation; and Yearly avg.
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From page 346... ...
If the average ventilation rate is 10 L so per person, Emmerich and Persily (1998) have estimated that 46% of the total heating and cooling load in the stock of office buildings is attributable to ventilation.
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From page 347... ...
Data from surveys with ventilation rate and pollutant concentration measurements have emphasized measurements of pollutants that are generated indoors and also present in outdoor air reducing the influence of ventilation rates on indoor concentrations. None of the large cross-sectional surveys have monitored concentrations of indoor-generated particles associated with asthma, such as environmental tobacco smoke (ETS)
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From page 348... ...
(1997) increased ventilation rates in two classrooms from about 0.7 to 2-3 ho and indoor particle concentrations decreased by approximately 50°/O.
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From page 349... ...
(1990) increased ventilation rates in an office building from 0.84 ho to 1.08 ho and indoor concentrations of formaldehyde, nicotine, respirable particles, and carbon monoxide did not change significantly.
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From page 350... ...
and also roughly equivalent to the minimum ventilation rates in schools and commercial buildings (Table 10-2~. The influence of ventilation rate on pollutant concentrations depends on the magnitude of pollutant removal by air cleaning.
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From page 351... ...
For indoor-generated particles, the predicted effects of ventilation rate depend highly on particle size because the depositional losses of particles increase dramatically with particle size. Practical changes in ventilation rates have a substantial predicted impact on indoor concentrations of small particles, such as ETS particles and the portion of airborne cat allergens associated with particles smaller than a couple micrometers.
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From page 352... ...
If there is some form of air cleaning within the building, ventilation rates will change indoor pollutant concentrations by a smaller amount. The previous discussion has focused on indoor-generated air pollutants.
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From page 353... ...
In warm humid climates, air-conditioning systems must remove moisture from the indoor air to maintain acceptable indoor relative humidities. Increasing the ventilation rate in warm humid climates will increase indoor humidity ratios and relative humidities unless the rate of water removal by the airconditioning system increases accordingly.
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From page 354... ...
mecha ~0.26 ventila Harving et al., 1994 Aarhus, Denmark 32 families moved to houses0.4 h-4 i (experimental study with (mild) with mechanical ventilation;before control group)
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From page 355... ...
For the control group, the median ventilation rate was unchanged at 0.29 ha. Not assessed Not assessed Indoor RHs during autumn and winter were about 10% lower with increased ventilation, but still generally above 50%.
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From page 356... ...
Based on these predictions, the indoor concentrations of some pollutants increase dramatically as ventilation rates become unusually low (e.g., <0.25 hub. The energy costs associated with avoiding these particularly low ventilation rates are modest; however, current methods and technologies of ventilation do not consistently prevent these low ventilation rates.
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From page 357... ...
The impact of ventilation rates on concentrations of gaseous pollutants associated with asthma will generally be modest. Without a strong indoor source of nitrogen dioxide, outdoor NO2 concentrations are often higher than indoor concentrations, and increases in ventilation rates will increase the concentrations indoors.
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From page 358... ...
The limited data available suggest that using mechanical ventilation systems to increase ventilation rates in residences can result in significantly lower dust mite levels only in cold climates.
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From page 359... ...
Because indoor pollutant source strengths can vary temporally, experiments should be repeated several times. To assess how changes in ventilation rates affect indoor humidities and, in turn, the proliferation of dust mites and molds in buildings will require either long-term experiments lasting a year or more or large cross-sectional studies with control for confounding factors.
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From page 360... ...
Although particle air cleaning reduces indoor particle concentrations, microorganisms can grow on some air-cleaning equipment such as filter media; thus, air cleaners are also a potential source of indoor pollutants. Particle Air-Cleaning Technologies By far, the most common method of air cleaning is to circulate
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From page 361... ...
(1994) and data from manufacturers, Figure 10-2 provides examples of particle removal efficiency versus particle size for filters with a range of efficiency ratings, using an ASHRAE rating method (ASHRAE, 1992~.
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From page 362... ...
For indoor applications, the efficiency of a HEPA filter is effectively 100% at all particle sizes if all air that flows through the air cleaner actually passes through the filter. In addition to fibrous filters, a wide range of electronic air cleaners are available.
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From page 363... ...
Air drawn from the occupied indoor space flows though a duct system, through the air cleaner, and then back to the occupied space. With this type of installation, often called "in-duct" air cleaning, the rate of air cleaning is limited by the rate of airflow through the HVAC system, and air cleaning occurs only when the HVAC fan operates.
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From page 364... ...
are assumed, the annual incremental fan energy cost would be $130. The rates of airflow through these air cleaners correspond to the rates of airflow through the residential heating and air-conditioning system, with about four indoor air volumes per hour being typical.
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From page 365... ...
Although a very large range of products is available, many of the heavily marketed products incorporate HEPA filters and a multispeed fan, with airflow rates at maximum fan speed ranging from 50 to 200 L so-. This range of flow rates corresponds to 8 to 32 room air volumes per hour in a small 22-m3 bedroom and to 0.4 to i6Electronic air cleaners may also be used in commercial buildings, but they are much less common than filters.
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From page 366... ...
The annualized costs per unit airflow of other commercially available products could be considerably higher or lower. Higher-capacity supplemental air cleaning units, sometimes mounted at ceiling level, are also readily available.
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From page 367... ...
. Predicted and Measured Influence of Particle Air Cleaning on Indoor Concentrations of Indoor-Generated Particles of Various Sizes Measurements Figure 10-3 presents the results of experimental studies of air cleaners that specify both the rate of airflow through the air cleaner and the reduction in indoor pollutant concentration.
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From page 368... ...
(1991) on the effects of air cleaning on airborne cat allergen concentrations serve as an example of changes in allergen sources.
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From page 369... ...
Several observations based on these predictions follow. First, to obtain a substantial reduction in indoor concentrations of 10,um particles, the rate of airflow through the air cleaner per unit of indoor air volume must be high.
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From page 370... ...
; bedroom 0.6 assuming 450-m3 house Warburton et al., 1994 Homes HEPA in bedroom 2-4 in bedroom Nonsignifi decrease gram-pc bacteria Studies Without Information on Air Cleaning Rate van der Heide, 1997 Homes (smokers Portable unit with Not specified in 15 of 45) coarse filter, cyclone, high efficiency filter Nonsignifi decrease .
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From page 371... ...
with asthma 12 atopic Placebo air adult cleaners, asthmatics with foam filtered smoking allergic asthmatic patients No significant changes in symptom scores, spirometry, or bronchial hyperreactivity. Nonsignificant trend toward higher peak flow with air cleaner use Placebo air Significant improvement in airway cleaners with hyperresponsiveness and coarse filters eosinophils, but only with both and cyclones air cleaners and impermeable mattress covers.
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From page 372... ...
Antonicelli et al., 1991 Homes HERA in bedroom Not specified No signifier in dust r in bedro samples Warneretal., 1993 Homes ionizer Not applicable Airborne r decrees' (p <.OC Studies with Fresh Air Delivery from or near the Headboard of the Bed Verall et al., 1988 Homes HEPA-filtered air Not applicable from bed's headboard Zwemer and Karibo, Homes 1 973 Villaveces et al., 1977 Homes Studies with Air Conditioners Trasoff and Hospital rooms Blumstein, 1936 Not asses' HEPA-filtered air Not applicable from bed's headboard HEPA-filtered air Not applicable directed over pillow Air conditioner 8 with filters Not asses' Not asses' Not asses' aPlacebo air cleaner may have removed some larger allergen particles.
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From page 373... ...
No controls, no statistical tests. pacified No significant change 9 patients No No significant improvement in in dust mite allergen allergic to forced expiratory volume or in bedroom dust dust mites symptom score.
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From page 374... ...
no efficiency rating Not asses'
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From page 375... ...
Asthma symptoms and medication use persisted in many patients. However, data from 39 patients were excluded because they left after one night or had infections.
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From page 376... ...
Thus, air cleaning does not appear to be a very attractive option for reducing exposures to dust mite allergens, which predominantly contain particles larger than 10 ,um. The second observation is that increasing the filter efficiency rating from a furnace filter to an ASHRAE 95% efficiency filter, while maintaining a constant rate of airflow through the filter, decreases the predicted indoor concentrations by approximately 40°/0 or less for particles 5 ,um or larger.
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From page 377... ...
efficiency filters. Third, it appears feasible to reduce concentrations of particles smaller than 2 ,um, such as ETS particles, droplet nuclei, and the smaller particles of 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.
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From page 378... ...
Effects of Particle Air Cleaning on Allergy and Asthma Symptoms The influence of air cleaner use on asthma and allergy outcomes has been evaluated in many experimental studies, and was the subject of a 1997 review by the American Lung Association (ALA, 1997~. Building on the review articles of Nelson et al.
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From page 379... ...
, lack of blinding, low or unspecified rates of air cleaning, no information on building ventilation rates, virtually no specification of rel 23The committee is using the perennial versus seasonal categorization of Nelson et al.
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From page 380... ...
Some studies selected subjects who were allergic to dust mites, and as discussed above, air cleaning is unlikely to be highly effective in reducing exposures to the large particles that carry dust mite allergens. A minority of the studies quantified the reduction in airborne allergen levels.
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From page 381... ...
Excluding the two studies using only air conditioning as a form of air cleaning, six of seven studies reported improvements in symptoms, and the seventh study showed a borderline significant improvement (p = 0.07~. However, subjects were blinded in only two of these studies, and most studies were old and without formal statistical tests.
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From page 382... ...
Air Conditioning as ~ Substitute for Air Cleaning Previous reviews of air cleaning and asthma have included studies of air conditioning. Air conditioners can remove some particles from the indoor air because they often contain coarse particle filters and because some particles may be removed from the air along with water in the air conditioner's cooling coil.
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From page 383... ...
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 volume 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.
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From page 384... ...
The influence of air cleaner use on asthma and allergy outcomes has been evaluated in numerous experimental studies; however, most of these studies have important limitations. Overall, the data suggest that air cleaners are helpful in some situations in reducing allergy or asthma symptoms, particularly seasonal symptoms, but it is clear that air cleaning, as applied in the studies, is not consistently and highly effective in reducing symptoms.
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From page 385... ...
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 blinding, 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.
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From page 386... ...
1991. Efficacy of an air cleaning device equipped with a high efficiency particulate air filter in house dust mite allergy.
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From page 387... ...
1936. Air cleaning as an aid in the treatment of hay fever and bronchial asthma.
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From page 388... ...
Lawrence Berkeley National Laboratory Report, LBNL-42123, To be published as a chapter in Indoor Air Quality Handbook. Spengler J
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From page 389... ...
In: Proceedings of the 13th AIVC Conference Ventilation for Energy Efficiency and Optimum Indoor Air Quality. Air Infiltration and Ventilation Centre, Coventry, Great Britain.
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From page 390... ...
1982. Low infiltration housing in Rochester New York: a study of air exchange rates and indoor air quality.
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From page 391... ...
1998. Ventilation strategies for good indoor air quality and energy efficiency.
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From page 392... ...
1987a. Indoor Air Quality and Ventilation Measurements in 38 Pacific Northwest Commercial Buildings, Volume 1: Measurement Results and Interpretation, LBL-22315 i/:, Lawrence Berkeley National Laboratory, Berkeley, CA.
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From page 393... ...
1989. Ozone emissions from electronic air cleaners.
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