Superfund and Mining Megasites Lessons from the Coeur d'Alene River Basin (2005) / Chapter Skim
Currently Skimming:
6 Human Health Risk Assessment: Lead Exposure and Uptake--Use of the IEUBK Model
6 Human Health Risk Assessment: Lead Exposure and Uptake--Use of the IEUBK Model
Pages 223-283
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 223... ...
A primary human exposure pathway to lead is through soil and dust, which children are assumed to incidentally or deliberately ingest. Empirical evidence for this assumption comes from reports of excess amounts of soil tracer elements, especially silicon and aluminum, in the feces of children (Wong et al.
|
From page 224... ...
Children exposed to lead who develop elevated blood lead levels (BLLs) may have already been irreversibly damaged by the time they have been identified in screening programs.
|
From page 225... ...
As a result, the toxicity and risk characterization steps of a typical risk assessment, as described in the previous chapter, are combined in lead risk assessment into a prediction of BLLs arising from associated lead exposures. Whether risk is deemed acceptable or unacceptable is assessed by comparing the predicted BLLs with target BLLs established by the Centers for Disease Control and Prevention (CDC 1991)
|
From page 226... ...
Biokinetic and pharmacokinetic models relate exposure dose to the lead concentration in various target tissues; they represent the mathematics of the time course of absorption, distribution, metabolism, and excretion (ADME) of the substance being followed.
|
From page 227... ...
. Predictive blood lead models generally distinguish between the intake of lead during exposure and its uptake by the body.
|
From page 228... ...
(EPA 1986) and National Primary Drinking Water Regulations.
|
From page 229... ...
When concentrations of lead in environmental media are specified, the model calculates a point estimate of a child's blood lead values over the age range of 0-84 months. The IEUBK model is defined operationally by EPA's computer program(s)
|
From page 230... ...
230 SUPERFUND AND MINING MEGASITES FIGURE 6-2 Compartments and functional arrangement of the IEUBK components for prediction of children's blood lead values.
|
From page 231... ...
For example, the intake of lead from soil is the product of the soil lead concentration (milligrams [mg] of lead per kilogram [kg]
|
From page 232... ...
Soil lead concentration = dust lead concentration = 200 µg lead per gram of soil/dust. Soil = 45% of total ingestion, dust = 55% of total ingestion.
|
From page 233... ...
LEAD EXPOSURE AND UPTAKE 233 FIGURE 6-3 Mathematical treatment of the lead absorption in the IEUBK model.
|
From page 234... ...
are then integrated over age to obtain the masses of lead in each compartment at any age. Lead concentrations (or mass fractions)
|
From page 235... ...
, from Table 6-2, t age, I total lead intake rate (mass per unit time) into the plasma-ECF compartment (from the gut and lung)
|
From page 236... ...
the default exposure values related to ingestion rates and to bioavailability of lead, and (3) extension of a deterministic, point value for blood lead concentration to a probability distribution function for a population.
|
From page 237... ...
Nevertheless, the documentation should accurately reflect the programming. Uncertainty in Key Default Parameters Soil/dust ingestion rates and lead bioavailability are two key variables the user may specify in making blood lead value predictions with the IEUBK model.
|
From page 238... ...
Contention arises in part because EPA's blood lead target of protecting 95% of such a population at a BLL of 10 µg/dL means that the outcome, either in predicted 95th percentile blood lead or in estimated soil lead cleanup level, is very sensitive to the value of the GSD. EPA materials (EPA 2002)
|
From page 239... ...
From all appearances, there has been little to no follow up on the work or recommendations regarding the develop ment of a fully probabilistic blood lead prediction model. BLLs as a function of the variability in all exposure and environmental parameters and would obviate the need for such an ad hoc approach as tacking on a GSD at the end of the calculation in the current version of the model.
|
From page 240... ...
of the basin homes sampled have environmental lead concentrations high enough that children in the 1- to 2-year age group would have lead exposures expected to produce BLLs greater than 10 µg/dL. As employed in the ATSDR Health Consultation (2000a)
|
From page 241... ...
Column 3 entries were computations based on the Bunker Hill Superfund site box model conditions for the IEUBK model detailed above. The results indicate that the original computations (column 1)
|
From page 242... ...
f 7.93 (1.5) g 37% < 10.0 µg/dL 73% < 10.0 µg/dL Abbreviations: BLLs, blood lead levels; GM, geometric mean; GSD, geometric standard deviation; RI/FS, remedial investigation/feasibility study.
|
From page 243... ...
Calculation of the soil lead cleanup level requires two items, one mathematical and the other involving policy. The IEUBK model provides the mathematical relationship between environmental lead levels and BLLs that form the basis for the soil lead cleanup level.
|
From page 244... ...
. The individual GSD is also used to estimate the percent of BLLs greater than 10 µg/dL in an 8EPA states that the GSD is not intended to address variability "in blood lead concentrations where different individuals are exposed to substantially different media concentrations of lead" (EPA 1994a)
|
From page 245... ...
. The 1998 OSWER directive also makes clear that EPA views blood lead data alone as insufficient for performing a risk assessment, stating "that predictive tools should be used to evaluate the risk of lead exposure, and that cleanup actions should be designed to address both current and potential future risk." The insufficiency of blood lead observations alone is linked to the policy of protecting individual children, because blood lead information without accompanying environmental lead levels cannot adequately assess the exposure potential that exists, and information about today's blood lead concentrations is insufficient to address what BLLs might occur for other current and future
|
From page 246... ...
There is almost never a situation in which model predictions are more accurate than a representative set of observations. EPA should clarify that the IEUBK model is preferred because it does two things that blood lead information alone does not do: it mathematically describes the relationship between environmental lead levels and BLLs, and, because of that description, it allows the calculation of a soil lead cleanup level that will be sufficiently protective.
|
From page 247... ...
EPA's guidance for the sampling and analysis of lead-contaminated soils recommends that the maximum sieve size for such soil is 250 µm (a No. 60 sieve)
|
From page 248... ...
. Epidemiological studies investigating the relationships between blood lead and environmental/socioeconomic parameters have shown that children's contact with lead-bearing household dust (represented by lead loading on floor surfaces, rather than by lead concentration in the dust)
|
From page 249... ...
on entryway sampling mats explained 46% of the variance in log-transformed blood lead concentrations in children 9 years old and younger. Although this analysis did not control for such confounding factors as lead paint, the results are similar to those of the HHRA (TerraGraphics et al.
|
From page 250... ...
Although this value is greater than the expected air concentrations in the basin, the overall contribution of this pathway to absorbed blood leads is just a few percent of the lead intake (EPA 2001b)
|
From page 251... ...
Figure 6-5 presents the TSP levels and associated concentrations of lead in ambient particles for the years 1982 to 2001. The most significant features of the graph are the dramatic decline in the lead concentrations in suspended particulate matter and the gradual reduction in TSP.
|
From page 252... ...
(2003a) the major yard remediation work did not begin within the box until 1998; consequently, the substantial declines observed in particulate lead levels before that time depicted in Figures 6-4 and 6-5 undoubtedly are associated with weathering of soil lead.
|
From page 253... ...
Although IEUBK guidance recommends averaging the lead concentrations in the first-draw and purged samples, the HHRA used only the purged values for the batch-mode runs of the IEUBK model. No rationale was given for that decision; however, the consequences are expected to be minor given the relatively small contribution that drinking water provides to overall lead intake.
|
From page 254... ...
To estimate dietary intakes for the incremental exposure scenario -- designed to represent exposures associated with a limited subset of the population -- information is required on both the concentrations of lead in selected foods and related intakes. Residual lead in Coeur d'Alene River basin soils and surface waters can produce elevated dietary exposures to lead for children in households that rely on homegrown produce or locally caught fish for a portion of their regular diets.
|
From page 255... ...
The SEM also highlighted the apparent role of communitywide soil lead concentrations in the exposure dynamic. A reduction in lead uptake was indicated by the SEM analysis, and the box model implemented this by reducing the bioavailability values used by the model; default soil/ dust ingestion rates were maintained.
|
From page 256... ...
The default model overpredicts in the upper basin and underpredicts,
|
From page 257... ...
8.0 Lead 6.0 Blood Mean 4.0 Geometric 2.0 0.0 Basin Kingston Gulches Osburn Silverton Wallace Mullan Lower Side Burke/Ninemile FIGURE 6-6b Geometric mean blood lead (µg/dL) by study area for children aged 1-5 as observed and predicted using IEUBK default and box models in batch mode.
|
From page 258... ...
258 SUPERFUND AND MINING MEGASITES a Default Model vs Observed % 10 µg/dL 50 40 30 g/dL) µ 20 Difference 10 (% 10 Absolute 0 -10 -20 Basin Kingston Gulches Osburn Silverton Wallace Mullan Lower Side Burke/Ninemile FIGURE 6-7a Absolute differences between batch mode IEUBK default model prediction and observed fraction exceeding 10 µg/dL by study area for children 1-5.
|
From page 259... ...
between batch mode IEUBK default model prediction and observed geometric mean blood lead (µg/dL) by study area for children 1-5.
|
From page 260... ...
Neighborhood soils therefore may be a poor surrogate in the lower basin, leading to box model underprediction. As described in the OU-3 HHRA, follow-up studies of children with high levels of lead in their blood in the lower basin suggest strongly that riverbank material may be an important source of lead exposure (TerraGraphics et al.
|
From page 261... ...
Good agreement between observation and model predictions is one indication that the observed BLLs are typical of the environmental conditions. Because the batch mode predictions of BLLs based on environmental lead levels for the paired data set are reasonably good, the next step in the HHRA is to apply the batch mode calculation to all residences and yards in the community for which environmental lead concentrations are available.
|
From page 262... ...
· The IEUBK box model configuration provides appropriate soil cleanup lev els for the Coeur d'Alene River basin OU-3 as a whole. · Adjusting some of the IEUBK model default values to box model conditions provided a better fit between observed and predicted blood lead values for some but not all geographic subregions of OU-3.
|
From page 263... ...
Rather, it uses the model as applied in step 1 and calculates the highest soil lead concentration that is still consistent with a BLL that, combined with the blood lead GSD, will produce no more than a 5% probability of being above 10 µg/dL. The Coeur d'Alene HHRA takes a somewhat broader, although nearly equivalent, approach, selecting a possible soil lead cleanup level, rerunning the step 2 batch mode run, and considering the predicted blood lead exceedance rate for the residences with soil lead levels within 200 mg/kg of the possible soil lead cleanup level.
|
From page 264... ...
One study was conducted on soil from a residence within the Bunker Hill box (but not the basin) in human volunteers using a stable isotope approach (Maddaloni et al.
|
From page 265... ...
When the batch mode approach is used, as it generally is, and when EPA's individual target for blood lead protection is used, as it typically is, then the blood lead data need not be representative of the community but rather must be representative of the exposures that arise for the observed environmental lead levels. This concept is not articulated in any EPA guidance documents, and clarification is needed; the usefulness of nonrepresentative epidemiological blood lead data may be counterintuitive for scientists and community members alike.
|
From page 266... ...
EPA makes no decisions based on the predicted or actual average blood lead in the community. So for EPA's purposes, the question is, are the data representative of the BLLs that typically would arise in this community in children who live in houses with the observed environmental conditions (soil and dust lead levels)
|
From page 267... ...
If a child is entered in the data set twice by virtue of having a repeat blood lead measurement, then the same environmental lead levels will be used in multiple predictions for this child, and one of the predictions will be closer to observation than the others. Thus comparison or calibration of the IEUBK to site-specific conditions relies on the children sampled being representative of the relationship between blood lead and environmental lead (not on their BLLs being representative of the community)
|
From page 268... ...
The geometric mean values range from 0.48 mg/m2/day (in the lower basin/ Caltaldo) to 4.28 mg/m2/day (for Burke/Ninemile)
|
From page 269... ...
Unfortunately, house dust is associated with many indoor surfaces, including nonfloor horizontal surfaces such as sofas, chairs, tables, beds, and the concentrations of lead in the associated dust loadings will vary, as will ingestion exposures related to hand-to-mouth contacts with those surfaces. In essence, the IEUBK exposure module is really an oversimplification of the transport and fate processes that control indoor lead, and it is time that more mechanistically based approaches are adopted so that the exposure component of the IEUBK model is commensurate with the lead biokinetic module.
|
From page 270... ...
, "the committee will strive to provide guidance to facilitate scientifically based and timely decision making for this site in the future." As such, the conclusions and recommendations herein are intended to guide future decision making and not to elicit a reconsideration of the ROD for the Coeur d'Alene River basin. Conclusion 1 Multicompartment predictive blood lead models are powerful tools for pediatric lead-exposure risk assessments, for exploring lead risk management options, and for crafting remediation strategies.
|
From page 271... ...
Most importantly, site-specific bioavailability would have improved the application of the model, and better characterization of the physicochemical properties of the exposure materials would have enhanced the credibility of the results. Conducting IEUBK model evaluations using solely default parameters, without their justification, has little utility because risk assessments should not be based on default parameters.
|
From page 272... ...
If the EPA had used their bulk analyses for the crustal elements iron and manganese as the committee did, a better justification would have evolved for the structure of the box model extension to the rest of the Coeur d'Alene River basin. Recommendation EPA should require that IEUBK model use for determining cleanup levels be supported by site-specific measures of bioavailability and that particle-size-range lead concentration determinations be undertaken.
|
From page 273... ...
This could substantially improve their application as regulatory instruments. Conclusion 5 The committee finds that EPA guidance concerning specific use of the IEUBK model and additional use of blood lead studies is incomplete.
|
From page 274... ...
· Definitive guidelines for the conduct of blood lead studies should be established. The focus should be on the coherence of the joint data set covering the full range of lead exposure risks and the collection of blood lead data associated with that range of exposure.
|
From page 275... ...
1998. The impact of low technology lead hazard reduction activities among children with mildly elevated blood lead levels.
|
From page 276... ...
1994. Assessing the relationship between environ mental lead concentrations and adult blood lead levels.
|
From page 277... ...
1992. Predicting blood lead concentrations from environmen tal concentrations.
|
From page 278... ...
1989. Review of the National Ambient Air Quality Standard for Lead: Exposure Analysis Methodology and Validation.
|
From page 279... ...
Office of Ground Water and Drinking Water, U.S. Environmental Protection Agency [online]
|
From page 280... ...
1998. The contribution of lead-contaminated house dust and residential soil to children's blood lead levels.
|
From page 281... ...
1998. Predicting blood lead concentrations from lead in environmental me dia.
|
From page 282... ...
2003a. Assessing reme dial effectiveness through the blood lead: Soil/dust relationship at the Bunker Hill Superfund Site in the Silver Valley of Idaho.
|
From page 283... ...
1977. The Silver Valley lead study: The relationship between childhood blood lead levels and environmental exposure.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.