either to prevent or remove the contamination of indoor air with microbial allergens. The discussion below addresses these issues briefly; additional information is presented in Chapter 7.
To prevent the contamination of indoor air by microbial aerosols, the penetration of outdoor aerosols must be reduced, and growth in indoor reservoirs must be eliminated. Keeping indoor environments physically separated from outdoors (by keeping doors and windows closed) and using mechanical ventilation and air conditioning are effective ways to control penetration. Water must not be allowed to accumulate, particularly, in ventilation systems; in addition, airborne water vapor must be kept to a minimum. Relative humidity should be maintained below 60 percent to prevent absorption of water by hygroscopic materials and to avoid condensation on cool surfaces. The effectiveness and utility of biocides (used either on surfaces or incorporated into fabrics or paint) have not been clearly established (see Chapter 7).
Vacuum cleaning removes some fungus spores from carpeting, but it probably also reintroduces them into the air, either through the action of the beater in the cleaner or through the bag. High-efficiency particulate arresting (HEPA) filters or release of air and dirt directly to the outdoors, as is the case in central vacuum systems, will reduce such contamination. Wet cleaning of carpeting probably removes some microorganisms (Wassenaar, 1988a), but unless drying is rapid, the added water may spur the growth of those that remain. Water reservoirs associated with portable humidifiers can be cleaned each day and treated with a biocide every third day to maintain relatively low bacterial levels in the reservoir. However, the biocides must be removed before the humidifier is operated.
Biocides are usually used in commercial systems (sumps of machining fluid, spray humidification systems, etc.). For example, aldehydes and quaternary amine compounds have been used to control fungal growth (Kapyla, 1985). These kinds of compounds are clearly irritating, may also be sensitizing, and can also enter the air conveyance system. Reducing the amount of biocide in order to minimize risk to the occupants, however, can result in concentrations that are too weak to prevent the growth of all organisms. Biocide usage can also result in changes in the kinds of organisms in reservoirs rather than in a significant decrease in the total number of organisms. The multiple risks of exposure to fungi and biocides must be carefully balanced.