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4 Exposure Assessment
Pages 90-107

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From page 90...
... Part I of the Reassessment has a wealth of supporting information and comprises an executive summary and three volumes: Sources of Dioxin-like Compounds in the United States2 ; Properties, Environmental Levels, and Background Exposures; and Site-Specific Assessment Procedures. ASSESSMENT PROCEDURES The comments in this chapter are directed specifically at the use of exposure assessment in the risk assessment provided in Part III of the Reassessment, but the committee consulted the more detailed companion documents in Part I for supporting information.
From page 91...
... . This information includes sources, environmental fate, environmental media and food concentrations, background exposures, and potentially highly exposed populations and particularly sensitive developmental stages.
From page 92...
... 92 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS ment include combustion sources; metals smelting, refining, and processing industries; and chemical manufacturing, biological and photochemical processing, and reservoir sources. PCDDs and PCDFs are formed in most combustion systems -- waste incineration and burning of coal, wood, and petroleum products; other high-temperature sources (such as cement kilns)
From page 93...
... . Because of discrepancies among estimates of TCDD and related compounds in reservoirs relative to known sources, several researchers using a top-down approach concluded that EPA estimates of historical national emissions might underestimate emissions (Rappe 1991; Harrad and Jones 1992; Brzuzy and Hites 1995; Eisenberg et al.
From page 94...
... 94 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS Another apparent purpose of section 4.2 in Part III of the Reassessment is for EPA to make clear that assessment of environmental fate cannot be based on TEQ but must be based on individual congeners, but in section 4.1, EPA presents estimates of environmental releases as TEQ. They elected to present TEQ in place of mass quantities to better facilitate comparisons across sources.
From page 95...
... EXPOSURE ASSESSMENT 95 individual congeners to estimate the persistence of TEQ and the spatial distribution of TEQ. Another issue of interest to the committee is how the reliability of the TEQ estimate becomes more uncertain with time.
From page 96...
... These studies provided intake estimates since 2001 in which the average daily intake for all age groups fall below the WHO tolerable daily intake level of 2 pg TEQ/kg of body weight. However the estimates do not include breast-fed infants.
From page 97...
... Background Exposures The section of the Reassessment that addresses background exposures provides a summary of information on human tissue levels, intake estimates, and variability in intake levels. Tissue Levels The section of the Reassessment addressing tissue levels evaluates data on concentrations of TCDD, other dioxins, and DLCs in human tissues expressed per gram of lipid and the changes in these concentrations that have occurred in recent decades.
From page 98...
... Low levels of occupational exposure are not congruent with their reported inventory of sources. To evaluate the impact of nursing on infants, EPA estimated changes in body burden with a model developed by Lorber and Phillips (2002)
From page 99...
... In preparing its findings, the committee considered a range of potential uses for the Reassessment -- including the following alternatives. Source Characterization Clearly, an important opportunity that EPA overlooks is checking the observed decline in overall environmental concentrations against body burden changes over time.
From page 100...
... Although consideration of individual PCDD, PCDF, and PCB congeners would be informative and useful, doing that for more than 200 congeners would be excessive; summing up mass quantities instead of TEQ contributions would be equally bad, and most other inventories (e.g., in Europe and Japan) were also done in TEQs.
From page 101...
... Estimates of Background Exposures The committee found the text in this section noncontroversial and the conclusions valid. The committee did not find any important errors in this text, but issues arose concerning the interpretation of the background exposure data.
From page 102...
... Source Characterization The magnitude, type, geographic distribution, and time history of TCDD, other dioxins, and DLC sources are essential components for risk characterization. The interpretation of these factors is an important input to decisions about managing both new and historical (reservoir)
From page 103...
... EXPOSURE ASSESSMENT 103 Any errors in interpretation could lead to policies and regulatory actions that are inefficient or ineffective in reducing human exposures to TCDD, other dioxins, and DLCs. EPA exposure characterization excludes basic data quality checks that could provide an opportunity to evaluate key assumptions.
From page 104...
... for confirming or revising this approach. In its consideration of highly exposed subpopulations, EPA found information indicating that breast-feeding might result in higher TCDD-TEQ body burdens of the nursing infant compared with those of non-nursing
From page 105...
... Moreover, because of the potential for causing anxiety among nursing mothers, EPA should expand its discussion about the multiple known benefits of breast-feeding as a footnote to the section describing exposures to nursing infants. Modeling Assumptions In characterizing exposures, EPA relied primarily on measurements combined with assumptions for emissions and relied almost completely on measurements of environmental and tissue levels for estimating exposure and body burdens.
From page 106...
... · Because many users of the Reassessment will be interested in reducing exposures to TCDD TEQ and identifying strategies for achieving reductions in body burden, EPA should add some discussion in the exposure chapter about what factors (such as diet, activities, and location) tend to increase or decrease TEQ intake.
From page 107...
... EXPOSURE ASSESSMENT 107 environment and anticipating that those levels could result in higher contributions of the dioxin-like PCB fraction to total TEQ exposure (e.g., through local fish consumption) , EPA should explicitly characterize the variability of population exposures to PCBs.


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