Reisman et al. (1990) found that use of a portable HEPA-filter air cleaner was without effect in reducing allergic respiratory symptoms, although patients subjectively believed that use of air cleaners was beneficial. Measurements made with a direct reading particle counter showed that the air cleaner used in these studies did lower the concentration of particles equal to or greater than 0.3 µm over placebo controls. However, no attempt was made to identify the kinds of particles excluded by the air cleaners, which could include small aeroallergens (e.g., some fungi) and non-specific irritants (e.g., environmental tobacco smoke).
A laminar flow air cleaner with HEPA filter when attached to beds of mite-allergic asthmatics was beneficial in reducing patient symptoms. This device is useful in producing a small zone of high-quality air in the breathing zone of the resting patient but, because of its small capacity will likely have little beneficial effect in the entire patient room or residence.
The inability of portable HEPA units to remove Fel d I emitted from carpets (de Blay et al., 1991a) is analogous to the failure of HEPA filter units in hospitals to remove small-diameter spores such as those of Penicillium and Aspergillus species emitted from strong fungal reservoirs in rooms housing immunocompromised patients. For example, Streifel and colleagues (1987), found that rotting wood in a medication room beneath a sink was the source of thermophilic Penicillium species found in a corridor in bone marrow transplant ward that was supplied with air that was HEPA filtered. Penicillium spores are approximately 2 to 4 µm in size. A baseline thermophilic fungi concentration of 812 colony-forming units/m3 was present in the medication room (when doors to the rotted cabinet were open), even though the room contained a portable HEPA filter unit operating at 20 air changes per hour. This demonstrates that unexpected emissions of small-diameter (2- to 5-µm) allergens from indoor reservoirs can overwhelm even the best possible ventilation systems.
Important considerations in the potential use of air cleaners for removal of aeroallergens include the volume of room or building air that passes through the filter and the particle size of the air contaminant to be removed. If the airflow rate through the device is low and the emission rate of allergen is high, then the beneficial effect of the air cleaner is likely to be nonsignificant.
Dilution is seldom effective in controlling existing contamination, because strong emission sources of allergens (reservoirs) overwhelm the dilution capacity of highly filtered (high-quality) outdoor air (see example given by Streifel et al.  above).