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TEM: Total Exposure Model
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Sponsor: United States Environmental Protection Agency (USEPA) and the United States Air Force |
Project History:IntroductionThe Total Exposure Model (TEM) is a computer model that has been developed with support from the USEPA and the US Air Force. The model is a descendent of a model named MAVRIQ (Model for the Analysis of Volatiles and Indoor Air Quality, Wilkes et. al., 1992, Wilkes et. al., 1996). TEM (Wilkes, 1998) is designed to predict the exposure and dose to an individual resulting from use of a contaminated water supply by modeling the fundamental physical and chemical processes that occur during interaction between the contaminated media (in this case water and air) and the exposed individual. The dermal, inhalation, and ingestion calculations are separately maintained to allow a comparison of their respective effects. The integrated model combines emission and fate and transport submodels to estimate resultant human exposure to an individual or population group. The model also estimates body burden (dose to the individual's blood supply). The model also contains an integrated simplified physiologically based pharmacokinetic (PBPK) model and is compatible with a comprehensive PBPK model (ERDEM, Exposure Related Dose Estimating Model). The feasibility of applying TEM to predict exposure and uptake to chloroform based on field data has been demonstrated. (Wilkes and Nuckols, 2000, Lynberg et. al., 2000).
Earlier versions of the model were developed primarily as research tools, but the current version has been developed into a user-friendly tool meant to assist in both population based and individual based exposure assessments. TEM uses Monte Carlo techniques to merge activity pattern data with stochastic or deterministic multiroute exposure models (ingestion, inhalation, and dermal contact) to estimate distributions of exposure of the population to drinking water constituents.
Platform/Model DesignTEM is a stand-alone program developed in C/C++ that runs under Windows 95 or later. Model results are stored in an Access database. The model outputs airborne concentrations, personal concentrations, exposures, absorbed dose, and individual target organ concentrations, which can be viewed using built-in charting features. More detailed information about the model inputs, sampled activity patterns, simulated water uses, and a variety of other simulation information is contained in the Access database. In addition, the results can be exported into charting and analysis programs such as Microsoft Excel.
PopulationTEM is developed to function either as an individual case study model or for estimating population-based exposures. When applied to an individual case study, the user specifies the exact scenario of activities being studied. The model can also be applied in a population mode with user-selected populations and user-defined population characteristics. TEM combines stochastic representation of exposure-related behavior with deterministic calculations of emissions, air concentrations, exposures, and doses. When applied in a population mode, the user specifies the characteristics of each population group as made available by the activity pattern survey. For example, NHAPS provides characteristics such as age, gender, employment, and location in the US. Subsequently, TEM can sample activity patterns from NHAPS meeting the desired characteristics and use the sampled activity patterns as a basis for predicting the distribution of exposures to the specified population. Other input behavioral variables, such as source use behavior may be represented stochastically as a Poisson process for event occurrence and as a lognormal distribution for event duration.
The model implements a number of algorithms to allow stochastic representation of population behavior. The model allows the user to define airflow patterns that respond to user behavior (e.g., when a user takes a shower and the bathroom door is closed, a set of airflows specific to that user behavior may be automatically invoked by the model).
Duration of time addressed by the modelThe model simulates the user defined time period for an individual case study and a 24 hour period for a population-based exposure assessment.
Sources of contamination evaluatedTEM is designed to represent emissions from a domestic water supply during normal household water uses. As such, TEM is ideally suited for evaluating exposure and dose to any waterborne contaminant. TEM contains a variety of emission models specifically developed to address the release of contaminants from water use appliances in the home. The models are fundamental mass-transfer models developed based on laboratory experiments and the two-film mass transfer theory originally published by Whitman (1923). Specific water use emission models are provided in TEM to represent releases from showers, baths, clothes washers, dishwashers, faucets, and toilets. In addition, a generic plug flow model and a completely mixed flow models (CMFM) are provided.
In addition to the emissions specific to waterborne contaminants, TEM also contains general emission models for release of contaminants to the air, including constant, decaying exponential, and burst release. A reversible sink model is also provided.
TEM evaluates exposure and dose to waterborne contaminants that are ingested. Algorithms are provided for stochastically representing ingestion of contaminated drinking water in accordance with user input values for quantity and frequency of consumption. These parameters are also being analyzed based on information contained in the CSFII data (EPA, 2000), and these data will be incorporated into the model.
InhalationThe primary route of exposure for volatile waterborne constituents is the inhalation route. TEM evaluates this exposure by modeling the release of the contaminant during water uses by the aforementioned emission models, the fate and transport of the contaminants, the activities and locations of the occupants, and the physiological and chemical processes leading to uptake into the body. Calculations are preformed utilizing an equilibrium lung model to estimate the mass transferred from the inhaled air into the bloodstream.
DermalThe dermal exposure and uptake is also estimated based on skin diffusion mass transfer models. The membrane model (Cleek and Bunge, 1993, Wilkes, 1998) is used for estimating the mass transfer across the skin. A simpler, steady state model is also provided (Wilkes, 1998).
TEM solves the system of contaminant mass-balance ordinary differential equations (ODEs) by the fourth-order Runge Kuttaa method (also known as the Kutta-Simpson formula) for temporal integration. Contaminant emissions, sinks, pressure, and airflow relationships are described using the appropriate equations within the system ODEs.
DatabasesTEM uses databases to provide information necessary for modeling population based exposure behavior. The databases currently linked to TEM or databases used to implement TEM include the following:
Ingestion: The 1994-96 USDA’s Continuing Survey of Food Intake by Individuals (CSFII) is the most recent and comprehensive consumption database available. This database contains total, direct and indirect tap water intake (including beverages) by age, sex, etc. CSFII was conducted over the three-year period between January 1994 and January 1997. A nationally representative total of 15,303 persons in the United States were interviewed on two non-consecutive days with questions about what drinks and foods they consumed in the previous 24 hours. The dietary recall information was collected by an interviewer who came to the participants homes and provided instructions and standard measuring cups and spoons to assist in recalling consumption quantities. The EPA report, Estimated Per Capita Water Ingestion in the United States (Jacobs et al., 2000), explains the details of the study and presents the results
Human activity patterns: National Human Activity Pattern Survey - NHAPS is the largest and most current (1992-1994) human activity pattern data set available. Data for 9386 respondents in the 48 contiguous United States were collected via minute by minute 24-hour diaries. Detailed data were collected for a maximum of 82 possible locations and a maximum of 92 activities. In addition, demographic data was collected for each respondent (gender, age, education, week day/weekend, race, census region, race etc.). Advantages of the NHAPS data set are that it is representative of the U.S. population and it has been adjusted to be balanced geographically, seasonally, and for day/time. Also, it is representative for all ages, gender, and race. This data set has many of the attributes for constructing stochastic activity pattern models (location, duration, frequency) and it is very data rich in water-related activities including taking baths, taking showers, amount of time in the bathroom after bathing or showering, washing of hands, using dishwasher, washing dishes by hand, swimming, etc.
Water use behavior: The Residential End Uses of Water Study (REUWS) database contains water use data obtained from 1,188 volunteer households throughout North America (Mayer et. al., 1998). The REUWS study was funded by the American Water Works Association Research Foundation (AWWARF). During the period from May 1996 through March 1998, approximately 100 single-family detached homes in each of 12 different municipalities (located in California, Colorado, Oregon, Washington, Florida, Arizona, and Ontario) were outfitted with a data-logging device (Meter Master 100 EL, manufactured by Brainard Co., Burlington, NJ) attached to their household water meter (on only magnetic driven water meters). The data logger recorded the water flows at 10-second intervals for a total of four weeks (two in warm weather and two in cool weather) at each household. Following the study, the data was retrieved and analyzed by a flow trace analysis software program, called Trace Wizard, developed by Aquacraft, Inc., Boulder, CO, which disaggregated the total flows into individual end uses (i.e. toilet, shower, faucet, dishwasher, clothes washer, etc) (Mayer et.al. 1998). In addition to identifying the type of water use (e.g. shower, faucet, toilet), Trace Wizard identified the event durations, volumes, peakflows, and mode measurements for each water-using event.
The Residential Energy Consumption Survey (RECS) was a nationwide survey conducted in 1997 to obtain household energy use information. The resultant RECS database contains energy usage characteristics of 5,900 residential housing units. The information was acquired through on-site personal interviews with residents; telephone interviews with rental agents of units where energy use was included in the rent; and mail questionnaires to energy suppliers to the units. The database contains information on physical characteristics of the housing units, demographic information of the residents, heating and cooling appliances used, clothes washer and dishwasher use frequency information, fuel types, and energy consumption.
ReferencesCleek, R.L. and A.L. Bunge. 1993. A new method for estimating dermal absorption from chemical exposure. 1. General approach. Pharm. Res. 10:497-506.
EPA, Estimated Per Capita Water Ingestion in the United States, Based on Data Collected by the United States Department of Agriculture’s 1994-96 Continuing Survey of Food Intakes by Individuals, USEPA Report EPA-822-00-008, April 2000.
Jacobs H.L., J.T. Du, H.D. Kahn, and K.A. Stralka. April 2000. Estimated Per Capita Water Ingestion in the United States, Based on Data Collected by the USDA 1994-96 Continuing Survey of Food Intakes by Individuals. EPA/822/00/008. U.S. EPA, Office of Water.
Lynberg M, Nuckols JR, Langlois P, Ashley A, Singer P, Mendola P, Wilkes C, et al. Assessing Exposure to Disinfection Byproducts in Women of Reproductive Age Living in Corpus Christi, Texas, and Cobb County, Georgia: Descriptive Results and Methods. . Environmental Health Perspectives (accepted). 2001.
Mayer, P.W., W.B. DeOreo, E.M. Opitz, J.C. Kiefer, W.Y. Davis, B. Dziegielewski and J.O. Nelson. 1998. Residential End Uses of Water. American Water Works Association Research Foundation.
Whitman, WG, “The Two-Film Theory of Gas Absorption,” Chemical and Metallurgical Engineering, 29(4):146-148, July 23, 1923.
Wilkes CR, Small MJ, Andelman JB, Giardino NJ, and Marshall J. "Inhalation Exposure Model for Volatile Chemicals from Indoor Uses of Water." Atmospheric Environment, 26A(12):2227-2236, August 1992.
Wilkes CR, Small MJ, Davidson CI, and Andelman JB. "Modeling the Effects of Water Usage and Cobehavior on Inhalation Exposures to Contaminants Volatilized from Household Water." Journal of Exposure Analysis and Environmental Epidemiology, 6(4):393-412, 1996.
Wilkes, CR, "Case Study," Chapter in Exposure to Contaminants in Drinking Water: Estimating Uptake Through the Skin and by Inhalation, prepared by ILSI working group, CRC Press, 1998.
Wilkes C and Nuckols JR. Comparing Exposure Classification by Three Alternative Methods: Measured Blood Levels, Questionnaire Results, and Model Predictions (abstract). In: Proceedings of the International Society of Exposure Analysis 2000 Conference. Monterey Peninsula, California October 24-27, 2000.
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