slab floor, it may not dry out as long as the carpet above the padding is left in place.

Chronic flooding of carpet will result in the amplification of indoor microorganisms. 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, 1984). 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).

OTHER SUBSTRATES AND RESERVOIRS

Many substrates found in indoor environments can support microbiological growth. Materials found in buildings, such as wood, cardboard, and paper, all contain carbon sources adequate to support growth. The dirt and debris present in HVAC systems, including internal insulation, also provide adequate substrates for growth. However, water must be present in the substrate if growth is to occur.

Some unusual substrates in indoor environments have been shown to support the growth of indoor organisms. For example, the occurrence of Aspergillus infections in cancer patients in a new hospital was associated with the growth of fungi on cellulosic fireproofing materials present above the ceiling in patient rooms (Aisner et al., 1976). Also, casein-based self-leveling compounds used in concrete flooring in the late 1970s in Sweden provided an unusual substrate that supported the putrefactive fermentation of various microorganisms including Clostridium species (Bornehag, 1991; Karlsson et al., 1984).

In addition to providing nutrient materials in situ, many existing building or construction materials contain greatly elevated fungal and bacterial concentrations compared to new materials (i.e., as they left the factory). For example, old chipboard may contain up to 50,000 times more bacteria and fungi than new chipboard (Strom et al., 1990). Plastic flooring and sheeting contain variable degrees of microbiological contamination possibly associated with the presence of additives such as plasticizers, oils, and resins that can act as carbon and nitrogen sources. When some natural building materials such as cork are used, they may be heavily contaminated by microorganisms such as Streptomyces (Strom et al., 1990) and Aureobasidium species.

HVAC System Design for Source Control

The primary purpose of HVAC systems is to provide for the thermal and air quality requirements of the occupants. Well-designed and maintained



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