Human Exposure Assessment for Airborne Pollutants Advances and Opportunities (1991) / Chapter Skim
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6 Models
6 Models
Pages 169-206
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From page 169... ...
Models are useful tools for quantifying the relationship between air-pollutant exposure and important variables, as well as for estimating exposures In situations where measurements are unavailable. Exposure models may obviate extensive environmental or personal measurement programs by providing estimates of population exposures that are based on small numbers of representative measurements.
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From page 170... ...
For epidemiological studies, the modeler must understand the study design sufficiently to recognize the trade-offs between levels of uncertainty in exposure estimates and the ultimate risk evaluation that also depends on the level of uncertainty in the health-effects data. Given an exposure limit, the analyst needs to determine whether any particular exposure scenario constitutes a significant fraction of that limit.
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From page 171... ...
. Although concentration models are not truly exposure models, their output can be used to estimate exposures when combined with information on human time-activity patterns (see Figure 6.1~.
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From page 172... ...
Models to describe and predict their concentrations and, ultimately, human exposures must, therefore, incorporate the rates and products of the chemical reactions. The development of faster, larger, and less costly computers has greatly enhanced our ability to model complex phenomena like the turbulent flow of air in the outdoor and indoor environments.
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From page 173... ...
This autocorrelation can be modeled approximately and the motion of a large number of individual parcels can be calculated. ~7 IMPORTANT MODEL CHARACTERISTICS Limited information is available regarding the accuracy of most contaminant concentration models and less is known about exposure models because most models have not been adequately validated.
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From page 174... ...
The section concludes with a discussion of recent advances In outdoor and indoor concentration models. Outdoor Models~ontaminant Source Emissions Emission models based on the properties of the chemicals, design parameters of the emission sources, the physics of mixtures, and the ambient weather conditions can provide an alternative to source monitoring (Owens et al., 1964; MacKay and Matsugu, 1973; Reinhardt, 1977; Tung et al., 1985~.
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From page 175... ...
The development of empirical models for emission rate estimations has focused mainly on issues related to fugitive emissions. The rate of fugitive emissions at any process point (valve, pump, etc.)
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From page 176... ...
For example, the petroleum refining industry commonly involves hightemperature processing of chemicals in large equipment, but the chemical industry commonly uses ambient temperatures and small equipment and has substantially lower emission rates. Losses from large open ponds and pits are more difficult to quantify and have caused difficulty in validation of emission models.
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From page 177... ...
In developing control strategies for contaminants regulated by the NAAQS, EPA developed models that combined source emission rates with atmospheric dispersion to predict the concentrations of the contaminants at a receptor site and to test the effectiveness of control strategies. Prediction of the concentration of ozone, a contaminant regulated by the NAAQS, requires modeling of the photochemical transformation of its precursors, i.e., volatile organic compounds and NOX, as well as their transport.
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From page 178... ...
over conventional Gaussian plume models. Most of the studies to calibrate and validate plume dispersion models have: involved the release of inert tracer gases from near the ground in nonbuoyant plumes-conditions very different from real stack plumes.
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From page 179... ...
The dense and neutral-buoyancy models use mixing factors to represent the surface under a plume. For example, the factors used for rural terrain are equivalent to flat, low-friction surfaces, which cause a minimum of plume turbulence.
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From page 180... ...
formation and transport is in its rudimentary stages. Previously, most particulate sulfate models dealt with the transformation of SO2 to the SO4= ion, but did not follow the transformations to the ammonium salts, partly because of lack of information on the location, emission rates, and transport of ammonia and partly because of lack of information on the concentrations of particulate NH4NO3 and gaseous and particulate HNO3.
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From page 181... ...
Receptor Models Receptor models use data on contaminants at a specific site to identify the sources of contaminants. They are not predictive but can be used to validate predictive dispersion models, as in the Portland Aerosol Characterization
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From page 182... ...
In general, receptor modeling uses measured constituents of ambient samples as tracers to infer the contributions of different sources to the ambient air on the basis of a mass balance and expected differences In the properties of particles emitted from different sources (Miller et al., 19723. For example, assume that the airborne lead measured at a site is the sum of lead from several sources of different types, such as automobiles (auto)
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From page 183... ...
have reported that carbon and nitrogen thermograms might provide a rapid and inexpensive method for distinguishing sources of indoor particulate matter with receptor models Generally, indoor source emissions have not been sufficiently well characterized for this application of receptor models.
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From page 184... ...
Current interest in this area is high and important work is under way. Attempts have been made to estimate airborne contaminants in workroom air by using the physicochemical properties of the substance combined with information on site variables, such as ambient air temperature, temperature of the substance, production rates, surface area, and ventilation.
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From page 185... ...
When this equation is applied to indoor industrial environments, the outdoor air is assumed to be contam~nantfree. In many cases the mixing factor is assumed to be unity.
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From page 186... ...
. Although diffusion models have been widely used in describing the ambient concentrations from source emissions, this approach was not used in the indoor environment until Roach presented a simple indoor diffusion model.
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From page 187... ...
The modeler uses the room volume (~ and ventilation rate (Q.) in equation 6.4 to estimate contaminant concentrations in the workroom air.
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From page 188... ...
Nonindustrial Environments Over the last decade, research in air quality in nonindustrial indoor environments has dramatically changed the understanding of human exposures to many airborne contaminants. The Harvard Six Cities Studies showed that exposures to respirable particulate matter and to NO2 were, on average, higher In homes than outdoors (Dockery and Spengler, 1981a,b; Mockery et al., 1981~.
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From page 189... ...
Such models, however, require more input data and much more computation time, which is approximately proportional to the cube of the number of zones modeled. Infiltration and e~ltration of air are key components in modeling contaminant concentrations in indoor air.
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From page 190... ...
developed empirical models for respirable particles and sulfates in indoor air; they combined a basic physical model (the simple mass-balance or box model) with variables that are indicators of suspected sources.
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From page 191... ...
Variability in Emission Rates Variability in contaminant emission rates is important, although often ignored, in modeling of both indoor and outdoor air. Most concentration models use measured, rather than modeled, emission rates.
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From page 192... ...
(1988) used distributions of measured emission rates, rather than a single emission rate, to model indoor air.
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From page 193... ...
Indoor environments have larger surface-to-volume ratios than outdoor environments, so surface deposition and reactions are likely to be more important in indoor environments. Much less is known about chemical reactions in indoor air than in outdoor air; therefore, although the widely used mass-balance model for indoor air explicitly incorporates a term for removal processes other than filtration into outdoor air, removal rates have generally not been
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From page 194... ...
have presented a general model for predicting the concentrations of chemically reactive compounds in indoor air that accounts for the effects of ventilation, filtration, heterogeneous removal, direct emission, and photolytic and thermal chemical reactions. The discrepancy between the calculated formation of HONO from homogeneous reactions in their model and that measured by Pitts et al.
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From page 195... ...
This pressure difference between soil and house interior is due to indoor-outdoor temperature differences, wind, unbalanced mechanical ventilation, and operation of combustion devices that draw indoor air for combustion and vent products outdoors. The structure of the
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From page 196... ...
. McKone has developed a mass-transfer model to estimate human exposures to VOCs due to their transfer from tap water to indoor air.
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From page 197... ...
EXPOSURE-ASSESSMEN] MODELS Current exposure models are based on relatively general assumptions about the distribution of contaminant concentrations in microenv~ronments, the activity patterns that determine how much time people spend in each microenvironment, and the representativeness of a sample to the population that might be exposed to a contaminant.
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From page 198... ...
Some models for predicting exposures make assumptions regarding the independence between contaminant concentrations and time spent and activity in a microenvironment. Such assumptions should be validated for specific applications.
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From page 199... ...
The procedure is repeated until the end of a selected long period. For each time unit, say, 1 minute, in a given microenvironment, the model generates a contaminant concentration according to a microenv~ronment-specific probability distribution: each microenvironment has a specific probability distribution for each contaminant concentration.
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From page 200... ...
within the same microenvironment are stochastically independent and independent of activity patterns. It follows that the microenvironmental concentration is not correlated with activity time in that microenvironment.
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From page 201... ...
But sampling and analysis programs must cover enough time for concentrations to be reasonably estimated for a fuD year, if they are to serve as reliable inputs to exposure models. Very few sampling studies have extended over a long enough period to revead seasonal and year-to-year variations.
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From page 202... ...
Typical steady-state airborne-concentration models are not able to provide estimates for periods shorter than 1 hour and have difficulty In modeling time-varying concentrations, which can lead to high short-term exposures. If an exposure model is to estimate the effects of peak exposures on sensitive populations, the concentration model must provide reliable estimates on biologically relevant time scales.
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From page 203... ...
. Although concentration models are not truly exposure models, their output can be used to estimate exposures when combined with information on human activity patterns.
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From page 204... ...
Such mismatches In the time scale of the measurements with the time scale of the models preclude adequate model development, validation, and application to new biologically relevant exposure situations. Because of the changing nature of sources and source emissions with changes in production and control technology and in the economic conditions, it is necessary to measure periodically the amounts and chemical characteristics of sources of airborne contaminants.
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From page 205... ...
Indoor-Air Chemistry Indoor-air chemistry needs substantial research, including surface reactions on various materials, sorption, deposition, and rates for these processes relative to ventilation or other loss mechanisms. Exposure Models Current exposure models are based on relatively general assumptions about distribution of contaminant concentrations in microenvironments, the activity patterns that determine how much time people spend in each microenv~ronment, and the representativeness of a sample to the population that might be exposed to a contaminant.
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From page 206... ...
Validation Further validation studies are needed for virtually all existing models, including concentration prediction and exposure models. In particular, immediate efforts are needed to validate the NEM model and modify the model to more accurately reflect the actual situations that can result in high population exposures.
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