Environmental Engineering Reference
In-Depth Information
Rainshield
PEM inlets
FIGURE 2.1  Use of an elliptical (aluminum here) body on which to mount a personal exposure (BZ) inlet
used as a ixed location in an outdoor location 2 m above the ground. Location indoors at a 1.5 m elevation to
represent the BZ of a seated adult is recommended.
free stream). Note that larger particles (e.g., 10 μm) are expected to be more impacted by bluff body
effects than are accumulation mode, ine particles (e.g., 2.5 μm).
These data bolstered the notion that ixed-location MEM sampling without a bluff body simulat-
ing the human shape would potentially bias relationships with personal exposure sampling—even
if the contaminant was uniformly distributed. The model also suggested that differences between
personal sampling systems (size, shape, low rate, etc.) could result in between-PEM-type biases.
In order to minimize biases when intercomparing personal and ixed-location monitors, an ellipti-
cally shaped body such as that shown in Figure 2.1 can be used behind the inlet. This approach is
not thought to require a physiologically correct manikin, but simply an approximate body shape to
produce comparable low streamlines around the body. This aluminum bluff body is 40.6 cm tall,
with a 2-to-1 elliptical cross section 40.6 cm wide by 20.3 cm deep.
Only limited empirical data have become available to validate these model projections for either
between-PEM biases or biases between MEM with and without a bluff body shape for nonoccupa-
tional settings. Heist et al. (2003) deined the low streamline vectors around a heated, child-size
manikin in a simulated residential setting and provided data on scaling between body sizes. Data
by Rodes and Wiener (2001) examining the inluence of the presence or absence of a bluff body for
ixed-location sampling in a controlled wind tunnel setting simulating private residences, and utiliz-
ing both ine Arizona Test Dust challenges, found that the measured biases were less than 20% in all
cases typically found in nonoccupational sampling. The data showed that for this aerosol type, the
samplers with a bluff body provided lower concentrations than those without a bluff body. The empiri-
cal data indicated that while the direction of the biases for selected parameters appears to be correctly
predicted by the model, the degree of particle bounce and subsequent capture by the PEM inlet was
much smaller than was predicted by the total aspiration eficiency model of Ingham and Yan (1994).
Further empirical studies are still needed to conirm these indings for the smallest and largest particle
sizes in residential microenvironments, given the potential for adding biases to BZ assessments.
2.2.3  d eFining  P rotocol  w earing  c oMPliance
As noted previously, the burden imposed on a study participant by BZ sampling can result in seri-
ous protocol compliance problems in nonoccupational studies where participation is voluntary.
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