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sons that are not yet understood. Colloff 1991b reported that mites in climates where conditions are nearly ideal all year round appear to be more susceptible to the effects of
minor hygrothermal fluctuations than mites accustomed to temperate climates. It is also possible that they suffer more from competition and predation in such constantly humid
climes.
The association between mite concentration and the variation in hygrothermal environ- ment within dwellings is thus primarily relevant to geographical areas that experience
several months a year with either cold or dry or both outdoor conditions. Fortunately, this still accounts for a high proportion of those affected worldwide by mite-related ill-
ness.
3.2.3. Impact of building construction
Although seasonal culling of mites is possible in many parts of the world, whether or not it is achieved in practice depends on several other factors, including those that relate to
building construction. In addition to climate, two aspects affect the hygrothermal envi- ronment within a dwelling:
1. the integrity of the building envelope and its ability to keep out excessive moisture 2. the ventilation and heating system it provides.
3.2.3.1. The building envelope
The presence of excessive moisture is almost certainly the cause of more building defects than any other single factor and, in the worst cases, can adversely affect the health of its
occupants Eldridge, 1976. The Committee on Damp Indoor Spaces and Health 2004 summarized the extent to which this is recognized as a significant public health problem.
The various ways in which the building envelope can fail to keep out moisture are des- cribed by Singh 1994, who provided many illustrations of how excess moisture can get
into or arise within and accumulate within a building; in many cases, this occurs because of simple failures of maintenance, such as broken roof tiles, damaged water pipes, spilla-
ges and overflowing cisterns. If not dealt with adequately, it is clear that the resulting increase in humidity is likely to lead to higher HDM proliferation. Other examples relate
to common mistakes in design or construction, such as the excessive use of impermeable membranes that do not allow moisture trapped within the building envelope to evapo-
rate outwards.
A particular problem of the building envelope is how its design responds to moisture from the ground. Several studies have found that relative humidity and HDM prolife-
ration are higher in dwellings where the ground floor consists of a concrete slab in direct contact with the ground Wickman et al., 1994, for example. With this type of cons-
truction, it is often difficult to achieve the comprehensive seal required to prevent mois- ture penetration and rising damp. At the same time, it can be sufficiently impermeable
to trap indoor moisture – for example, from leaks, accidental spillage or condensation. If the floor covering is absorbent – a carpet, for example – it can act as a reservoir, lea-
ding to long-term dampness and high relative humidities. If the concrete slab is not ade-
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94
thermostat controlling the heating is set: the colder the outside air, the more the ventila- tion and the warmer the indoor temperature, the lower the resultant relative humidity.
Once relative humidity falls below about 50, HDM populations start to dwindle Arlian et al., 1998, 2001.
Unlike winter, the higher outdoor temperatures of summer limit the ability to reduce mite populations in this way. As a result, marked seasonal variations are typically obser-
ved in HDM populations, with peaks in late summer and autumn, when indoor condi- tions are most favourable for proliferation, and troughs in late winter and spring, when
conditions are least favourable Voorhorst, Spieksma Varekamp, 1969; van Bronswijk, 1981; Arlian et al., 1983; Platts-Mills et al., 1987. However, the precise timing of these
peaks and troughs tends to vary with climatic conditions from year to year and also tends to differ from region to region Colloff, 1991b. Voorhorst, Spieksma Varekamp 1969
showed that while this seasonal variation is easiest to see in damp homes, it can also be detected in dry homes, but here the peaks are orders of magnitude lower. Although a
few mites may survive to take advantage of the favourable conditions of summer or autumn, if the conditions of winter and spring are dry enough too few of them will sur-
vive to cause medical problems.
Several acarologists – such as van Bronswijk 1981 and Arlian and colleagues 2001 – have suggested that HDM populations might be controlled by exploiting the natural sea-
sonal culling effect and modifying the hygrothermal environment within homes during the critical winter months. Korsgaard 1979, 1983b, in particular, suggested that, if car-
ried out regularly, this might lead to permanent reductions in mite populations and even eradication. He was also clear about the key to such reductions being adequate winter
ventilation. Thus successful culling can only be achieved if both a winters are cold or dry enough and b the standard of ventilation in winter is sufficiently high. In other words,
even in cold winter regions, the natural culling of mites can be overridden by inadequate ventilation, allowing mite populations to survive and even prosper. This helps to explain
the variation in the mite numbers found in homes within cold winter regions.
In winter, continental interiors are typically drier than coastal areas, making the seaso- nal culling of mites easier to achieve. The critical factor is the amount of moisture contai-
ned in the air brought in from outside to ventilate a dwelling. This can be low, either because the outside air temperature is low or because the air is dry due to other geogra-
phical factors. In cold winter regions, whether coastal or not, the laws of physics limit the amount of moisture in the air during this period. In regions with mild or warm winters,
however, air sufficiently dry to allow seasonal culling is more likely to occur in conti- nental interiors than by the coast, which is burdened by the additional moisture in the
sea air.
The natural seasonal culling of mites is most difficult to achieve in humid tropical and subtropical regions, where there is no part of the year when the outside air is either cold
or dry enough. This helps to explain the high mite concentrations and asthma prevalence in such cities as Sydney, Singapore and Caracas Colloff, 1991b. On the other hand,
Arlian 1989 reported considerable variation in mite density in humid regions, for rea-
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takes a HDM to develop from an egg to an adult increases rapidly as room temperature falls below 23°C, thereby significantly slowing population growth even when room rela-
tive humidity is high van Bronswijk, 1981. Raising room temperature thus tends to shorten egg-to-adult development time and to favour mite population growth. On the
other hand, raising room temperature, assuming the moisture content of the room air stays constant, has the simultaneous effect, by the laws of physics, of lowering room rela-
tive humidity, which is unfavourable for HDM population growth. The two effects thus tend to cancel each other. Modelling studies suggest that the favourable effect on mite
growth of the rise in room temperature that results from improved insulation and hea- ting systems tends to be outweighed by the unfavourable effect of the fall in relative
humidity Pretlove et al., 2005. This is to be welcomed, since it means that modifying the hygrothermal environment without sacrificing the health benefits of providing affor-
dable warmth can potentially control mite populations.
The key is the provision of adequate ventilation. Although this necessarily involves some loss of energy, this can be lessened in some cases by technological means see section 3.3.2.
However, even without such active interventions, studies have shown that ventilation heat loss can be relatively modest Marsh, 1996. Adequate ventilation is thus not neces-
sarily incompatible with energy efficiency.
3.2.4. Occupant behaviour
