contaminated by microorganisms should be discarded. Disinfection of contaminated surfaces is not sufficient because dead microbial particulates are antigenic and still capable of reacting with the immune system.
Protocols for cleaning air conveyance systems are being developed by the National Air Duct Cleaners Association (NADCA, 1991). Most duct-cleaning procedures involve the physical removal of dust and debris using vacuum systems with HEPA filters. The mere presence of microorganisms in an air supply duct, however, is not an adequate basis for the initiation of duct cleaning. The presence of sufficient dust and debris to restrict airflow or to result in the dissemination of particulates through diffusers into occupied spaces is a valid reason for cleaning air supply ducts.
In occupied buildings, duct-cleaning procedures that require the use of biocides in place of physical removal of dust or that require encapsulation of dust and debris that may contain microorganisms are of questionable value. Biocides, (e.g., acaricides), that are used to directly lower viable mite populations have been variously effective (Lau-Schadendorf et al., 1991; Lundblad, 1991; Tovey et al., 1992). Polyphenolic materials added to acaricide formulations are thought to denature or modify the antigenic characteristics of mite allergens so that the IgE response is not elicited (Green et al., 1989). The beneficial effect of chemicals used to control mite and mite antigens must be balanced against the effect of these agents on nontarget populations (for example, the irritating effect of benzyl benzoate on humans). More information can be found in a recent forum on the duct-cleaning industry (IAQU, 1991).
As for portable air filtration devices, their beneficial effects over and above those associated with the central air conditioning are thought to be minimal (H. S. Nelson et al., 1988). Additionally, the beneficial effect of an air cleaner in removing aeroallergen is small when considered against the large amount of some allergens (such as those from mites and cats) that exist in surface reservoirs. Luczynska and colleagues (1990), for example, found that the beneficial effect of a portable air cleaner with a HEPA filter in lowering the air concentration of Fel d I occurred only when surface reservoirs were undisturbed and only when the air exchange rate was unusually high (20 air changes per hour).
The use of portable HEPA-filtered air cleaners does not lower mite allergen levels in settled dusts (Antonicelli et al., 1991). Since mite allergens are predominantly large particles (greater than 10 µm) that settle rapidly from the air after disturbance of surface reservoirs, it would not be expected that air cleaning itself will significantly lower allergen contents in surface reservoirs.