many fungi that can invade living plant tissue (plant pathogens such as the rusts, smuts, and powdery and downy mildews). Fungal growth and spore dissemination depend on available substrates, season, climate, and human activity such as agriculture.

Stored organic material such as compost, silage, hay, or grain supports the growth of fungi that favor low oxygen levels, dryness, and heat (e.g., Aspergillus fumigatus, A. flavus, Penicillium spp.; Campbell et al., 1989). Their spores become airborne when the moldy stored material is handled.

Spores can penetrate interior environments from outdoors either with ventilation air, or on the surfaces of people, animals, or objects (Pasanen et al., 1989; Su et al., 1992). Air conditioning and mechanical ventilation and filtration, however, allow doors and windows to remain closed, thus preventing entry of most outdoor fungal aerosols (Hirsch et al., 1978; Pan et al., 1992).

Indoors, fungi grow in wet environments such as basements, window sills, and shower stall surfaces (Kapyla, 1985; Verhoeff et al., 1990b). Species of Aspergillus and Penicillium often are dominant indoors (Brunekreef et al., 1990). Some fungi (e.g., Aspergillus versicolor, A. flavus, Wallemia sebi) prefer environments that are relatively dry and will grow at a water activity level of 0.65 (i.e., on substrates that contain 65 percent water) (Kendrick, 1985). The humidity at which hygroscopic materials in dust (including human skin scales) absorb enough water from the air to support the growth of these xerophilic ("dry-loving") fungi is unknown, but it is probably close to from 60 to 70 percent. Other residential substrates that have been attractive to fungi include urea formaldehyde foam insulation, wicker baskets used as plant containers, and carpeting installed in bathrooms (Bisset, 1987; Kozak et al., 1980b).

Fungal spores and other effluents become airborne indoors when disturbed by air movement and normal human activities (O'Rourke et al., 1990). Contaminated air conditioners and humidifiers can actively spray spores, fragments, and dissolved allergens into the air (Baur et al., 1988; Burge et al., 1980; Kumar et al., 1990). Humidifiers that cause water to evaporate into the air are less likely to produce measurable aerosols (Burge et al., 1980), although exposure to such aerosols probably occurs during cleaning.

NATURE OF THE AEROSOLS

The composition of aerosols of fungus-derived particles depends on the abundance and strength of sources, as well as on dissemination factors, mixing, dilution, and particle removal. Natural aerosols are almost always composed of mixed species. In agricultural situations and in indoor environments with actively disseminating reservoirs, aerosols may be monospecific (i.e., containing particles derived from a single fungus species), increasing



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