Recontamination of indoor spaces can occur if the fundamental reasons for the initial contamination were not adequately addressed. For example, cleaning of air conveyance systems and replacement of gypsum board in a building contaminated by xerophilic fungi will be ineffective if the elevated indoor moisture that led to the growth of fungi is not addressed. 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.


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.


As indicated by the list of general control strategies that follows, ambient relative humdity is often considered to be the major controlling factor for indoor allergens.

  1. Control ambient relative humidity. For example:

    • Use air conditioning to remove moisture that has entered indoor air. Natural ventilation does not necessarily remove moisture from indoor air.

    • Use dehumidifiers to remove moisture from air in occupied spaces that are not adequately air conditioned.

    • Exhaust strong indoor moisture emissions (e.g., steam from simmering foods, bathroom moisture, clothes dryer emissions) directly outdoors.

  1. Prevent condensation. For example:

    • Install vapor retarders in the building envelope.

    • Install insulation in the building envelope so as to prevent condensation in wall cavities and on wall or ceiling surfaces.

  1. Use nonporous floors, walls, or ceilings whenever possible or cover existing fleecy surfaces with impervious sheeting.

  2. Design buildings and systems to minimize the potential for flooding. If floods occur, then ensure comprehensive cleanup.

  3. Avoid the creation of moist microenvironments. For example, carpet should not be installed on floors that are likely to be flooded (near bathtubs) or on concrete floors in basements.

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