Moisture can also enter buildings for a number of primarily climatic reasons. Wind-driven rain, especially in coastal regions, can penetrate the building envelope and saturate construction materials, especially if roof and window flashings are inadequate. Moisture that enters walls and roofs must be removed by drainage to the outside or by indoor ventilation air. Wind-driven snow can also enter HVAC system outdoor air inlets, especially those that are located at grade level or flush with horizontal roof surfaces.
Carpeting, which is widely used in homes and offices, can provide niches for both the accumulation (e.g., Fel d I and nonviable spores) and production (e.g., viable mites and xerophilic fungi) of allergens. Carpeting typically consists of fibers made into tufted yarn and looped through a backing by means of an automated manufacturing process. The fibers are anchored to the backing with latex, which itself may be an allergen (see Chapter 3), and mixed with a filler such as crushed marble. A secondary backing is added to give the carpet body and to promote dimensional stability. Carpets are installed over a padding, traditionally felted jute, but urethane paddings are also widely used. The installed carpet system therefore consists of three distinct layers: the fiber, the backing, and the padding.
A carpet has some similarity to an air filter, in that dirt particles can become mechanically trapped and are not easily dislodged during cleaning. Particle adhesion is increased if the carpet is moist and if the particle and the carpet fiber are both wettable. Oil, greasy dirt, and sticky substances such as detergent residues have a strong tendency to adhere to the carpet fiber. Sticky substances such as detergent residues can also bind dust to fiber.
Carpets have been characterized when wetted as "cultivation media" for microorganisms (Gravesen et al., 1983). Indeed, the concentration of fungi and bacteria in the air above carpet has been found to be consistently higher than that over noncarpeted floors (Anderson et al., 1982; Gravesen et al., 1983). The log count of bacteria per square inch of new wool carpet reaches a steady state of about 5 within 2 weeks after installation (Anderson et al., 1982). By contrast, the log count of bacteria varied between 2 and 3 for bare floors in the same facility. In addition to microorganisms, carpeting can function as a reservoir for pollen and pollen fragments that are tracked into the indoor environment from outdoor sources.
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 concrete