Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Burge (1990, 1995), and Burge & Solomon (1987), Reponen (1994), and
Verhoeff (1994a,b).
9.2 House dust mites
House dust mites are members of the arachnid family having eight
legs and an exoskeleton. They can be up to 300 µm in length and live
off organic debris found in house dust (e.g., skin flakes, hair
follicles and fungi) (Colloff, 1991). Because mites absorb water
vapour they are critically dependent on the absolute humidity.
Survival in the adult stage requires environmental moisture conditions
be sustained not lower than 7-8 g/m 3 (Korsgaard & Iversen, 1991;
Fernandes-Caldas et al., 1994). This is equivalent to a relative
humidity of about 50% at 20°C.
Mite antigen is mainly found in the faecal pellets which may be
10-20 µm in diameter and will not remain suspended for very long.
Feather et al. (1993) identified enzymes derived from the mite gut as
the source of allergens. These enzymes might remain as potent
allergenic material in bedding, mattresses, carpets and furnishings
long after the mite population has diminished, further complicating
exposure determination.
Two different approaches, the sampling of air and of settled
dust, are available to measure the presence of house dust mites and
their allergens as indicators of environmental exposure. The latter is
the most commonly used approach.
9.2.1 Air sampling for house dust mites
Several techniques exist for volumetric sampling of airborne mite
allergens, using cascade impactors or high- and low-volume samplers in
combination with membrane filters (Swanson et al., 1985; Price et al.,
1990; Sakaguchi et al., 1993; Oliver et al., 1995). These techniques
have the advantage that they sample airborne allergens and might
therefore be more representative of the true exposure than assays of
settled dust. The literature is limited, however, on the validity of
air sampling as measure of exposure to house dust mite allergens
(Swanson et al., 1985; Price et al., 1990; Sakaguchi et al., 1993),
and further research is needed.
Mites themselves are not seen in air samples. Furthermore, in
undisturbed rooms amounts of airborne mite allergens are small and
difficult to detect, even after prolonged sampling. Most of the mite
allergens bind to faecal pellets, which become airborne only as a
result of disturbance, and little allergen is associated with
particles that remain airborne for more than a few minutes. Therefore,
practical disadvantages of airborne sampling of mite allergen are the
requirements for long sampling periods (2-24 h) and very sensitive
assays (Thien et al., 1994). Price et al. (1990) used a low-volume air
sampler (2 litre/min) for 3 h to sample suspended dust mite allergen
in homes. They reported that the airborne allergen levels correlated
better with sensitization to mites among children than the levels in
dust. Further, the air and dust antigen levels were not correlated.
Although this is the only study linking atopy to airborne mite
allergens, it does suggest potential limitations of using dust
sampling as a surrogate exposure measure. In a small number of
studies, air sampling and dust sampling were carried out in parallel
(Price et al., 1990; Sakaguchi et al., 1993; Oliver et al., 1995). In
only one study were significant correlations found between the levels
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