A primary function of mechanical ventilation systems is to provide comfort to the occupants. Providing comfort means controlling the moisture content of the air. Controlling the moisture content of the air also controls allergen content as a corollary benefit. Appropriate filtration can provide additional allergen control as a part of cleaning the air. For residences with forced air heating and cooling systems, a number of principles and practices can be employed for controlling allergens as outlined below:
Use forced-air heating and cooling systems to maintain thermal environmental conditions at all times in the occupied spaces as recommended in ASHRAE Standard 55-1992 (ASHRAE, 1993). 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.
Include in the central forced air system filters with at least a moderate atmospheric dust spot efficiency (30 to 50 percent) to remove aeroallergens from the ventilation airstream. Care is needed to ensure that the fan capacity of the system is sufficient to overcome the additional airflow resistance (pressure drop) imposed by these higher-efficiency filters.
Ensure that outdoor air is provided at least at the minimum rates (0.35 air changes per hour, but not less than 15 cubic feet per minute per person) recommended by ASHRAE Standard 62-1989, Table 2.3. This minimum outdoor air ventilation rate is also recommended for naturally ventilated residences.
Consider design options where outdoor air ventilation requirements are met through provision of outdoor air directly into the HVAC system. This would allow outdoor aeroallergens to be removed by HVAC system filters. Additionally, overall pressurization in the conditioned space could be made slightly positive compared with that in the atmosphere so that aeroallergens from outdoor sources do not infiltrate through loose construction or through cracks in window or door frames.
Ensure that the forced-air ventilation system itself does not become a source of allergens. For example, accumulation of water in ventilation systems should be prevented. Ventilation systems should be kept clean by regular maintenance because dirt and debris can accumulate in poorly maintained systems, and the dirt and debris can contain allergens or function as substrates for microbial growth. Access panels into plenums and the air conveyance system is essential to allow for regular maintenance. Internal surfaces in forced-air systems should be smooth and not provide substrates for fungal growth.