Science and Judgment in Risk Assessment (1994) / Chapter Skim
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3 Exposure Assessment
3 Exposure Assessment
Pages 43-55
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From page 43... ...
Section 112 of the Clean Air Act Amendments of 1990 applies to major sources that either singly or in combination emit defined quantities of one or more of the 189 hazardous air pollutants. The sources to which the act applies emit pollutants both continuously and episodically, and the pollutants can move from air to water, soil, or food.
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From page 44... ...
Exposure assessment involves numerous techniques to identify a pollutant, pollutant sources, environmental media of exposure, transport through each medium, chemical and physical transformations, routes of entry to the body, intensity and frequency of contact, and spatial and temporal concentration patterns of the pollutant. Mathematical models that can be used to describe the relationships among emissions, exposures, and doses are shown in Appendix C
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From page 45... ...
assumed to be living near the source and constantly exposed for 70 years to the highest estimated air-pollutant concentration. EPA does not modify exposure estimates by including mobility of the population, shielding due to indoor locations, or additional exposures from indoor or other community sources.
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From page 46... ...
~; rather, the new exposure guidelines discuss some of the issues and procedures that should be considered as part of the choice of the methods and criteria. The HEEE is "a plausible estimate of exposure of the individual exposure of those persons at the upper end of an exposure distribution." High end is stated conceptually as "above the 90th percentile of the population distribution, but not higher than the individual in the population who has the highest exposure." As is implied by those statements, the new guidelines have adopted the use of individual exposure distributions, and the HEEE is a value in the upper tail of that distribution.
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From page 47... ...
From the individual exposures, it is possible to develop population exposure (and risk) distributions and include uncertainty estimation, and personal-activity patterns.
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From page 48... ...
. Emission factors are calculated on the basis of average measured emissions at several facilities in a given industry (Compilation of Air Pollutant Emission Factors, commonly known as AP-42 [EPA, 1985b]
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From page 49... ...
There are often substantial variations in the spatial distribution of radiation within a microenvironment, so individual dosimeters have been thought to provide the best estimates of individual exposure. Individual monitoring and extensive microenvironmental measurements are not generally practical for assessing exposures of the general population, but because of cost and the unwillingness of individuals to participate in exposure assessments, new instruments, including passive dosimeters for airborne chemicals, are likely to permit a similar strategy.
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From page 50... ...
. Exposure models can be used to estimate population exposures from small numbers of representative measurements.
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From page 51... ...
Gaussian-plume models are derived from atmospheric diffusion theory assuming stationary, homogeneous turbulence or, alternatively, by solution of the atmospheric-diffusion equation assuming simplified forms of the effective diffusivity (Seinfeld, 1986~. Within the limits of the simplifications involved in their derivation, they can describe the individual processes that affect pollutant concentrations, such as diffusion, bulk transport by the wind, and deposition.
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From page 52... ...
Stochastic models can easily incorporate real physical phenomena, such as buoyancy, droplet evaporation, variations in the dispersity of released particles, and dry deposition. Stochastic modeling is typically implemented as a numerical Monte Carlo model in which the movement of a large number of air parcels is tracked in a Lagrangian reference frame.
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From page 53... ...
It is the exposure-prediction models that combine microenvironmental concentration estimates with information on time-activity patterns of people to estimate individual exposures or the distribution of individual exposures in a typical population. Activity patterns and microenvironmental concentrations can both be measured or modeled.
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From page 54... ...
However, there will be problems in existing models if occupancy times and concentrations of other contaminants correlate, as they might for irritating toxicants, such as formaldehyde. Current exposure models use a variety of crude assumptions about the constancy of concentrations in microenvironments, the human activity patterns that determine the amount of time people spend in each microenvironment, and how representative the sampled population is to the total population that might be exposed to a contaminant.
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From page 55... ...
Short-Term Exposure Modeling The typical steady-state airborne-concentration models are not able to provide estimates below 1-hour averages and have difficulty in modeling concentrations that vary widely over time and that can lead to short-term high exposures. If an exposure model is to estimate the effects of peak exposures on sensitive populations, the concentration model must provide reliable estimates for the time scales needed.
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