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
35
36
concentration conversion algorithm may contribute to overestimate of dust Pb. It is
not know to what extent these two biases cancel out each other. Overall comparison
of indoor dust Pb concentrations generated for the three case studies against available
empirical data suggest that: (a) for the general urban case study, estimates fall within
the range of measured values from a national-scale study, adding confidence to the
estimates, (b) central tendency estimates for the primary Pb smelter also fall within
the range for the national-scale dataset referenced above for the general urban case
study and high-end estimates seem to fit with available data near smelters, and (c)
comparison of estimates for the secondary Pb smelter against empirical data suggest
that these estimates may be biased low.
• Evaluation of candidate blood Pb models: A number of performance evaluations
were completed on the two candidate blood Pb models considered for this analysis
(IEUBK and Leggett). The results of these performance evaluations, which included
application of both models in replicating national-scale child blood Pb levels
(NHANES IV results) and blood Pb levels for an urban child cohort, suggested that
the Leggett model consistently over-predicted blood Pb levels by a factor of 3 to 6,
while IEUBK estimates were usually within a factor of 2. These findings resulted in
our selecting the IEUBK model as primary blood Pb model for this assessment, with
the Leggett model being reserved for application in the sensitivity analysis. Note also
that the empirical Lanphear Pb model was considered for use in the analysis but not
selected because the child cohort to which it applies (16 month olds) does not match
either of the blood Pb metrics used in the analysis (i.e., concurrent or lifetime
average).
• Outdoor air Pb-to-blood Pb ratios: Three sets of outdoor air Pb-to-blood Pb ratios
were derived. These related outdoor ambient air Pb to blood Pb resulting (1)
inhalation pathway only, (2) all recent air pathways (inhalation plus ingestion of
indoor dust Pb predicted to be associated with ambient air Pb levels, with ambient air
potentially included resuspended, previously deposited Pb), and (3) all recent and past
air pathways (see Section 2.4.3). All ratios were derived prior to application of the
GSD reflecting inter-individual variability in blood Pb levels and therefore reflect
central tendency blood Pb levels and not high-end population percentiles. The
modeled ratios were compared to both empirical data and statistically derived ratios
based on a pooled analysis (Section 3.5.2.2). With the exception of the primary Pb
smelter case study recent air ratio using 95 th percentile air concentration and the
general urban case study recent air ratios for the hybrid dust model, the ratios for
recent air contribution to concurrent blood Pb level (Table 3-24) generated for the
July 2007 4-38 Draft – Do Not Quote or Cite