Improved maintenance and the avoidance of excess moisture Improved insulation and ventilation standards

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

In advance of such modelling studies, it is still worth discussing, in general terms, chan- ges in building construction that improve conditions.

3.3.3.1. Improved maintenance and the avoidance of excess moisture

According to Eldridge 1976: Defects in buildings and building materials are often said to be caused by the weather, espe- cially when severe or unusual conditions have been experienced. However, careful diagnosis will demonstrate that faulty design, the wrong choice of materials or faults on site are usually the root cause, the weather only providing the appropriate conditions for the failure to occur. New building requirements and ways of using buildings can also give rise to unforeseen consequences. Even though regularly updated guides for diagnosing and avoiding buil- ding defects are published in most high- and middle-income countries, and the impor- tance of avoiding excess moisture is generally well recognized, many obstacles to making progress persist. Elaborating on this point, the Committee on Damp Indoor Spaces and Health 2004 called attention to the lack of sufficient information on which to base quan- titative recommendations, as well as to institutional, social and economic factors that tend to hinder the widespread adoption of technical measures and practices that could improve the situation.

3.3.3.2. Improved insulation and ventilation standards

As suggested earlier, providing affordable heating and improved insulation standards is likely to have the beneficial effect of lowering bedroom relative humidity levels and thus reducing mite populations, provided that ventilation standards are maintained. This pro- viso needs to be stressed, and it is vital that insulation standards are improved in conjunc- tion with measures that enable and encourage householders to achieve good ventilation. Householders can do a great deal to modify hygrothermal conditions, to reduce mite population growth, both by controlling moisture production and by being aware of the need to ventilate adequately in winter. For example, the spread of moisture vapour to other rooms can be restricted by keeping kitchen and bathroom doors closed, as well as by drying clothes only in rooms that can be closed and well ventilated. The use of thres- holds and automatic door closers would undoubtedly also be beneficial. Above all, hou- seholders need to be provided with ways of ventilating their home that are both effective and easy to use. Some windows, for example, do not allow sufficient flexibility or range of opening positions, so that it is difficult to achieve the desired level of ventilation or to change it easily in response to varying external conditions. Trickle vents, such as those in window frames, can often improve the situation, by allowing more precise control of incoming air. Public Health Significance of Urban Pests 107 HDMs. However, the mites were soaked in the solutions for a considerable period of time, which does not necessarily reflect real life conditions, where mites may only be exposed to the disinfectants for a short period of time or else come into contact with them after the water has evaporated.

3.3.4.2. Washing