Lead Human Exposure and Health Risk Assessments for Selected ...

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
3.5 UNCERTAINTY CHARACTERIZATION
The characterization of uncertainty associated with exposure assessment included
performance evaluation which is discussed in this section. In addition, uncertainty in exposure
assessment is also considered as part of characterizing uncertainty in the risk estimates and is
reflected in the sensitivity analysis, inclusions of multiple modeling approaches and the
qualitative discussion of key sources of uncertainty which are discussed in Section 4.3.
Performance evaluation for the exposure assessment focused on evaluation of projections
of Pb in exposure media (i.e., ambient air, outdoor soil, and indoor dust) (discussed in Section
3.5.1) and projections of Pb in blood (covered in Section 3.5.2). Those case studies (or case
study elements) for which media concentrations were estimated using empirical data as the basis
are not considered here; only those estimates based directly on modeling were included.
Comparing model output to monitoring data is often considered the most desirable form
of performance evaluation. In addition to monitoring data, or in the absence of such data,
outputs from other models and expert opinion about how outputs should look can be used as
comparison benchmarks in performance evaluation. The performance evaluation activities
reported here have focused on the comparison of model outputs to measurements. Favorable
comparisons, including a lack of systematic trends in either over- or underestimation of modeled
results relative to empirical data, provide confidence in the modeling estimates.
3.5.1 Performance Evaluation Related to Exposure Media Modeling
This section discusses performance evaluation conducted for modeling of Pb
concentrations in exposure media including ambient air, outdoor soil and indoor dust.
3.5.1.1 Evaluation of Modeled Ambient Air Pb Concentrations
Performance evaluation for ambient air Pb predictions focused on the two point source
case studies, in which air dispersion modeling was used.
For the primary Pb smelter case study current NAAQS scenario, the ISCST3-Prime
model was used with input files (e.g., source characterization, meteorological data) used in
developing the 2007 proposed revision to the SIP for that location (Appendix D). The
submission to EPA for the proposed SIP revision included a model performance evaluation,
focused on the “actual value” modeling scenario (MDNR 2007a). The actual value modeling
included three separate comparisons based on relating model predictions to measured Pb
concentrations at five monitor sites in the study area. These comparisons included:
• Day-to-day evaluation of modeling output compared to monitor values: The review
of the model performance evaluation conducted by the state of Missouri concluded
that all sites demonstrated a pattern of overall accuracy for directional prediction (i.e.,
July 2007 3-42 Draft – Do Not Quote or Cite