Building Design for Source Control

Building design as it relates to controlling indoor allergens is discussed below in terms of moisture, carpeting, and other substrates and reservoirs.

MOISTURE

As described in previous chapters, control of relative humidity or water vapor pressure in the occupied space and in the HVAC systems is important for allergen control in both residences and commercial buildings. It is therefore important to determine and define the indoor air, temperature, and relative humidity conditions that will provide for occupant comfort and also suppress the growth of allergen-producing microorganisms and mites.

The development of allergen-containing reservoirs depends on available water in the microenvironment of the allergen-producing organism. The amount of water in these environments depends on the relationship between the amount of airborne water vapor and the temperature in the environment (which controls condensation), the ability of substrates in the environment to absorb water, and the presence of liquid water sources (e.g., flooding, water reservoirs). The primary sources of water vapor are outdoor air that is used for ventilation or that infiltrates into the building, bathing, washing, and cooking processes, and evaporation of perspiration (latent heat dissipation) from building occupants. Secondary indoor sources of water (vapor or liquid) include condensation on surfaces that are colder than the dew point temperature (e.g., on walls and floors) and materials that can adsorb moisture (e.g., carpets, wood products, mattresses, clothes).

Ambient relative humidity is often considered the major controlling factor for indoor allergens. Performance criteria for acceptable temperature and humidity ranges are specified in ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard 55-1992, primarily for human comfort (ASHRAE, 1993). This 1992 standard considers an acceptable range of relative humidity to be from less than 30 percent (depending on operative temperature) to an upper limit of 60 percent which was selected for control of microbial and arthropod growth and prevention of conditions that lead to condensation. The lower limit of dew point temperature (the temperature at which moist air becomes saturated—100 percent relative humidity—with water vapor when cooled at constant pressure) for comfort is 35° F (ASHRAE, 1993). The operative temperatures (i.e., simple averages of dry-bulb and mean radiant temperatures) that define the comfort zone (ASHRAE defines acceptable conditions as those that satisfy 80 percent or more occupants) range from 68º F to 78.5º F and vary according to seasonal considerations. This temperature range is influenced



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