Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
of house dust mite allergens in air and dust (Oliver et al., 1995).
Allergenic responses to dust mite allergens may be induced by
short-duration high-concentration exposure events. Therefore, the
clinical importance of integrated air samples may be more relevant in
predicting prevalence of atopy to mites rather than predictive of
acute health effects.
At present no reliable information is available that will support
adoption of a standardized method for air sampling of house dust mite
allergens. According to an international workshop held in 1987
(Platts-Mills & De Weck, 1989) airborne sampling has not been shown to
be better than dust sampling to measure the level of mite infestation
in homes or schools. This was confirmed by a second international
workshop in 1990 (Platts-Mills et al., 1992). It was also stated that
there are few or no data showing a relationship between airborne
measurements and sensitization to house dust mites or symptoms. In
contrast, a relationship is apparent between the concentrations of
mite allergens in settled house dust and sensitization or symptoms.
Therefore, air sampling was not recommended (Platts-Mills et al.,
1992).
9.2.2 Dust sampling for house dust mites
Dust sampling for measurement of the level of mite infestation is
accepted and recommended as the best-validated "index of exposure" to
house dust mite allergens. The approach assumes that the quantity of
allergens released into the air is a function of what is present in
settled dust, or, conversely, that the measurement of allergen in
settled dust is related to both the long-term dose a person receives
and to the short-term airborne levels experienced during events that
raise dust.
Standardized sampling procedures to measure house dust mites and
their allergens in house dust have been proposed (Platts-Mills & De
Weck, 1989; Platts-Mills et al., 1992; EC, 1993; Dreborg et al.,
1995). Sampling sites should be consistent throughout the study and
preferably include the upper mattress surface and the floors of the
living room and bedroom. Sampling can be conducted with vacuum
cleaners equipped with a special attachment to collect dust on a paper
filter. Vacuuming 1 m 2 of surface in 2 min is a commonly used
sampling method. Depending on experiences with the amount of dust
recovered in specific situations, investigators may have to modify the
sampling procedures. Samples can also be obtained from upholstered
furniture, soft toys and clothing. Alternative techniques for
collecting dust samples include shaking blankets in a plastic bag and
scraping flat surfaces higher than floor level with a piece of firm
card. However, these techniques are less effective than collection by
vacuum cleaner and not standardized. The dust samples may be sieved
before analysis to obtain a sample of fine dust that can be weighed
accurately. Unfortunately, dust samples may still vary in density
after sieving. An alternate method for sampling airborne mite
allergens is to collect settling dust on large Petri dishes over a
period of 14 days (Tovey et al., 1992; Oliver et al., 1995). Brown
(1994) developed a variation on the integrated settling method. A
100-cm 2 piece of sticky tape is placed in contact with the surface
for 24 h. Under low-power magnification (36×), the trapped mites are
counted. Using an empirically derived collection efficiency of 30%,
the number of live mites per area is estimated. However, this does not
reflect the true extent of exposure to mite allergens (see section
9.2.3.1).
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