Mechanical solutions Modifying environmental conditions

Public Health Significance of Urban Pests 105

3.3.3.3. Mechanical solutions

One method for which success has been claimed is mechanical ventilation with heat reco- very MVH R, whereby the heat from the outgoing warm stale air such as from bathrooms and kitchens is recovered and transferred to the cold incoming fresh air. Higher ventilation rates can thus be achieved for less energy loss, thus encouraging hou- seholders to raise their ventilation standards. Several studies have found that when hou- seholders use MVHR installations correctly – that is, on 24 hours a day at a high enough volume setting to achieve at least 0.5 air changes an hour – both mite-population and allergen levels have fallen as a result of improved ventilation H arving et al., 1991; McIntyre, 1992; Warner et al., 2000. For this method to work, however, both outgoing and incoming air need to be ducted, with an electric fan in each direction, and the dwelling needs to be relatively airtight, to maximize the proportion of ventilated air that passes through the system. This last requi- rement makes the method more relevant to new housing than to older housing, where airtightness is more difficult to achieve. The amount of ductwork required is another obstacle for use in refurbished houses. Other possible problems relate to the correct posi- tioning of inlet and outlet air grilles and to the need for regular cleaning of input air duct- work, as well as filters. Although electric fans are becoming quieter, the energy consump- tion and noise of two fans is also likely to attract the attention of householders. In one study, it was found that the system was turned off overnight in 50 of the dwellings with MVHR, thereby significantly reducing its effectiveness and the standard of ventilation McIntyre, 1992. MVHR is thus a somewhat complex high tech solution. Even if properly installed, it puts a considerable onus on householders to use it correctly and to keep it well maintained. Moreover, the energy apparently saved is offset by the energy used and carbon dioxide emissions produced by the two electric fans. T he system thus makes most sense in regions with very cold winters and hydroelectric power, such as northern Scandinavia. Besides whole-house MVHR systems, small single-room versions are available. In these systems, the two fans, filters and grilles are all combined in one compact unit that can be installed in an external wall. With virtually no ductwork, cleaning and maintaining, these systems are less problematic. However, Htut and colleagues 1996 found that the unit could only be operated 24 hours a day at the lowest setting without producing unaccep- table noise at night. At this setting, the unit did reduce humidity and mite numbers in an occupied bedroom compared with a control, but not sufficiently to effect a permanent reduction. Positive pressure ventilation PPV is another solution. With PPV, a large quantity of air is sucked in, using a fan and ductwork, to put the whole dwelling under positive pres- sure. Unlike MVHR, the stale air simply leaks out of the dwelling through cracks or tric- kle vents in individual rooms, making strict airtightness less of a requirement. On the other hand, although PPV is a less complicated and more feasible option in refurbish- ment projects, there are similar concerns: maintaining clean ducts and filters and choo- sing to leave the system running for 24 hours a day, as required. House dust mites 104 control by modifying environmental conditions, where this is feasible.

3.3.3. Constructional changes to improve conditions