Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
environments. This process depends on the inertial properties of the
particles, such as size, density and velocity, and on the physical
parameters of the impactor, such as inlet-nozzle dimensions and
airflow paths. Because of differences in characteristics, samplers
differ in cut-off size ( d 50 ) (e.g., the particle size above which
50% or more of the particles are collected). As most impactors have
very sharp cut-off characteristics, almost all particles larger than
the d 50 are collected and d 50 is therefore assumed to be the size
above which all particles larger than that size are collected
(Nevalainen et al., 1992). No sampler collects all particles with
equal efficiency, and it is therefore not surprising that different
quantitative and qualitative results are obtained using different
sampling devices for culturable fungi (Verhoeff et al., 1990). The
choice of the collection (culture) medium also affects the kinds and
levels of fungi recovered (Verhoeff et al., 1990). No single
collection medium will enable the entire range of viable fungi in the
air to be isolated. Media which are generally accepted for
aerobiological studies include malt extract agar (MEA), V8 juice agar
and dichloran 18% glycerol agar (DG18) (EC, 1993; ACGIH, 1995). MEA
and V8 agar are broad spectrum media, whereas DG18 is intended to be a
selective medium for xerophilic fungi, but many of the common fungal
species in air can also be isolated (Verhoeff et al., 1990).
Few published data are available on the validity (accuracy and
precision) of the measurement of fungi in air as estimate of exposure.
All commonly used cultural air samplers use short sampling periods,
typically 30 seconds to several minutes (Table 32). The
reproducibility of parallel duplicate samples and sequential duplicate
samples is only moderate, both in terms of CFU/m 3 and in terms of
species isolated (Verhoeff et al., 1990). More importantly, repeated
sampling within weeks has demonstrated that variation in time within
homes is much higher than the variation between homes (Verhoeff et
al., 1992). This means that a single air sample has only a low
predictive value for exposure over time. Furthermore, the use of
cultures for quantifying fungal particle concentrations in air samples
will give an underestimate of the actual particle concentrations, and
may cause significant fungal contamination to be missed altogether.
The culturable fungal particles may comprise only a few percent of the
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Table 32. Overview of sampling techniques for airborne fungal particles a
Method with examples Sampling rate and time Rem
Non-viable, non-volumetric
- settling surface, adhesive-coated undefined, minutes to days sem
lar
Non-viable, volumetric
- rotating tape/slide impactors
Burkard trap 10 litre/min, 7 days cut
- rotating arm impactors
Rotorod sampler 47 litre/min, intermittent cut
- filter methods
cassette filters 1-4 litre/min, hours via
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