Indoor Allergens Assessing and Controlling Adverse Health Effects (1993) / Chapter Skim
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7 Engineering Control Strategies
7 Engineering Control Strategies
Pages 206-232
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From page 206... ...
This chapter characterizes the existing building stock in the United States and describes relevant engineering principles and practices that can be employed to prevent or minimize occupant exposures to indoor allergens. There are approximately 4 million commercial (i.e., nonindustrial and nonresidential)
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From page 207... ...
, inadequate specification and installation of drain pans and drain lines for removal of water condensed from cooling coils and humidifiers (60-63 percent incidence) , inadequate specification and installation of duct work to prevent microbial contamination (38~5 percent incidence)
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From page 208... ...
Two basic strategies for controlling occupant exposure to allergens can be identified: source control, which can eliminate occupant exposure, and exposure control, which can minimize but not eliminate occupant exposures by methods of dilution or air cleaning. A simple, one-compartment model of a control system for a uniformly mixed occupied space, shown in Figure 7-1, illustrates the interrelationship that exists among the variables that affect air quality (Woods, 1991; Woods and Rask, 19881.
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From page 209... ...
Problem of Occurrence Category Physical Cause Woods Robertson Design System problems Inadequate outdoor air 75 64 Inadequate air distribution to occupied spaces (supply and return device) 75 46 Equipment problems Inadequate filtration of supply air 65 57 Inadequate drain lines and drain pans 60 63 Contaminated duct work or duct linings 45 38 Malfunctioning humidifiers 20 16 Operations Inappropriate control strategies 90 NA*
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From page 210... ...
. Ambient relative humidity is often considered the major controlling factor for indoor allergens.
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From page 211... ...
The operative temperature and relative humidity/dew point ranges in ASHRAE Standard 55-1992 are specified for the occupied zone, which is defined as the space approximately 18 to 72 inches above the floor. For thermal acceptability, this standard allows no more than a 5° F vertical difference in air temperature within the occupied zone, a 9° F vertical radiant asymmetry, and an 18° F horizontal radiant asymmetry.
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From page 212... ...
In buildings (including residences) in cold climates, the higher water vapor pressure generally occurs in the occupied spaces and the vapor retarder is located on the side of the insulation facing the occupied spaces.
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From page 213... ...
Carpet moisture is a special problem when it penetrates into the carpet backing and the padding because carpet backing is essentially a porous barrier that permits downward flow by gravity, but essentially blocks the passage of water vapor upward. Therefore, if the padding is wetted by cleaning, flooding, or water vapor migration upward through a cracked con -- D - ~
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From page 214... ...
Carpet that has been chronically flooded can function as a significant fungal reservoir long after drying has occurred higher concentrations of microorganisms are found in the occupied spaces of flooded versus nonflooded floors (Kozak et al., 1980b; Morey, 19841. The dust and debris obtained from the backing of chronically flooded carpet may be heavily contaminated by fungal spores, most of which are nonviable (Kozak et al., 1980b)
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From page 215... ...
When outdoor air is provided to HVAC systems, it is usually mixed in a compartment or plenum with return air from the occupied spaces before it is filtered or thermally treated. This mixed-air plenum can collect debris such as leaves and feathers if bird or leaf screens on the upstream outdoor air
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From page 216... ...
Dirt, debris, and fungi can be expected to accumulate in AHUs and main and branch supply air ducts, especially in HVAC systems with inefficient filters, where filters do not fit properly in filter frames, or in poorly designed filter banks where significant volumes of air can bypass the filter bank. One study in Kuopio, Finland, found that pollen from outdoor air made up 9 percent of the weight of supply air duct dust (Laatikainen et al., 1991~.
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From page 217... ...
This occurs when the dew point temperature of the airstream is greater than the surface temperature of the cooling coil. Consequently, the relative humidity in the air supply plenum downstream of the coils will approach 100 percent.
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From page 218... ...
These devices are usually installed in the supply air plenum or duct work, downstream from the heating coils. In some residential installations, however, they may be found in the return air.
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From page 219... ...
provides advice with regard to humidification of the air. Section 4.6.6 of this standard recommends that where humidification is required, a type of device "which does not involve the risk of microorganisms being released into the air shall be chosen." AIR SUPPLY PLENUM AND DUCT WORK After passing through the heat exchanger and supply fan (the location of the fan may be upstream or downstream of the heat exchanger)
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From page 220... ...
PERIPHERAE UNITS In some commercial buildings, a separate system may be installed to heat and cool perimeter zones that are more affected by outdoor climatic conditions than interior zones that are not in contact with the building envelope. Fan coil and induction units often mounted in sheet metal enclosures along exterior walls, are two common types of peripheral units that are used .
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From page 221... ...
POROUS INSULATION As mentioned previously in this chapter, porous insulation is often installed on the inside surfaces of HVAC system components such as AHU plenums and the sheet metal of fan coil units, induction units, unit ventilators, and return air ducts. The fiberglass insulation often used in HVAC system components when new, can accumulate dirt and debris.
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From page 222... ...
. Although the dry-bulb or operative temperature in the occupied spaces may continue to be adequate for comfort, insufficient latent heat may be removed from the airstream and the indoor air in the building will become too humid.
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From page 223... ...
in carpet suggests that vacuum cleaners with high-efficiency particulate arresting (HEPA) air filters may be necessary to prevent reaerosolization of fine particles in occupied spaces.
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From page 224... ...
Similar devices are apparently not available for vacuum cleaning devices used for carpet mainte nance. Renovation and repair work, such as replacing windows or repairing furniture in residences, generally results in increased concentrations of fungi in occupied spaces (Hunter et al., 19881.
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From page 225... ...
The mere presence of microorganisms in an air supply duct, however, is not an adequate basis for the initiation of duct cleaning. The presence of sufficient dust and debris to restrict airflow or to result in the dissemination of particulates through diffusers into occupied spaces is a valid reason for cleaning air supply ducts.
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From page 226... ...
Important considerations in the potential use of air cleaners for removal of aeroallergens include the volume of room or building air that passes through the filter and the particle size of the air contaminant to be removed. If the airflow rate through the device is low and the emission rate of allergen is high, then the beneficial effect of the air cleaner is likely to be .
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From page 227... ...
Replacement of carpet containing Der p I with new carpet will not prevent recontamination unless the carpet removal is associated with other actions such as lowering the relative humidity in the indoor air and in the new carpet. SUMMARY OF PRACTICAL CONTROL STRATEGIES The following summary of engineering principles and practices that can be employed to prevent or minimize occupant exposures to indoor allergens is organized into three categories: general, residential buildings, and commercial buildings.
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From page 228... ...
For allergen control it is especially important to use the air-conditioning system to keep the relative humidity below 70 percent (preferably below 60 percent) in occupied spaces, including basements and in parts of the building where that air mixes with the occupied space.
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From page 229... ...
may be required to maintain thermal environmental conditions specified in ASHRAE Standard 55-1992 (ASHRAE, 1993~. Commercial Buildings A number of principles and practices can be employed for controlling indoor allergens in commercial buildings, as outlined below: 1.
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From page 230... ...
Little information is available, however, on the effectiveness of this cleaning method in removing the various types of particles, including specific allergens that may adhere to pile fibers, carpet backing, and other furnishings. In addition, the physical cleaning process itself may be sufficient to disperse fine allergenic particles.
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From page 231... ...
As described throughout this report, ambient relative humidity is often considered to be a major controlling factor for indoor allergens. Control of relative humidity, or water vapor pressure in occupied space and in the HVAC system is an important part of allergen control in both residential and commercial buildings.
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From page 232... ...
232 INDOOR ALLERGENS strategies for source and exposure control are needed to address the problems associated with normal degradation of the HVAC performance that occurs as a building ages. Research Agenda Item: Determine the relative efficacy of currently recommended environmental control strategies and develop costeffective strategies for controlling aeroallergens throughout the lifetimes of residences and other buildings.
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