Modeling residential exposure

Zartarian, V. G., et al. (2000). A modeling framework for estimating children s residential exposure and dose to chlorpyrifos via dermal residue contact and nondietaiy ingestion. Environmental Health Perspectives 108(6) 505-514. 

There is a transition away from nsing a deterministic approach in which high end or upper bound point estimates and defanlt valnes are nsed towards nsing a probabilistic approach in distribntional models which incorporate complex data sets to build realistic estimates of exposure. While probabilistic dietary exposure assessments can now be carried out routinely for many pesticides, available occupational and residential exposure data sets are typically insufficiently robust. Work on developing newer exposure databases (e.g. ARTF, ORETF, AHETF and EUROPOEM II) and distributional use pattern data would facilitate this transition. The topic of probabilistic exposure assessment is covered in Chapter 8. 

Modeling Residential Re-entry Dermal Exposures 157 Integrated Physical-Stochastic Dermal Model 158 Macro-Based Dermal Exposure Methodology 159 Scenario 1 159... 

Figure 4.2 Factors involving models for residential exposure assessment...

The models and methods used for purposes of estimating potential residential exposure (and absorbed dose) continue to be refined and validated as new monitoring studies become available. The goal is to simulate actual exposure conditions as closely as possible. The following sections present an example of a simplistic screening-level exposure assessment calculation for a consumer product, followed by a discussion of how more refined, probability-based or uncertainty analysis methods can be used. Screening-level methods typically include conservative bias in the form of default assumptions that are used in the absence of directly relevant and robust exposure monitoring data and other information. These methods can be used to predict potential exposure. However, it... 

Residential exposures to pesticides and other chemicals are estimated by means of either monitoring and/or predictive modeling but, unfortunately, little or no guidance is available for those attempting such estimates. Key areas requiring attention include the following ... 

McKone, T.E. (1993). Understanding and Modeling Multipathway Exposures inthe Home, Reference House Workshop II Residential Exposure Assessment for the 90s, 1993 Annual Conference, Society for Risk Analysis, Savannah, GA, USA. 

INTRODUCTION 210 STATUS OF RESIDENTIAL MODELS 211 EXPOSURE PHASES IN RESIDENTIAL EXPOSURE 212 Mixing and Loading Phase 212 Application Phase 212 Post-Apphcation Phase 213 MODEL CONCEPTS FRAMEWORKS 214 Mass-Balanced Air Qnality Model 214 Fngacity Model 215 Flnid Dynamics Model 216 MODEL CONCEPTS SOURCES AND SINKS 216 Sonrce Evaporation of Pesticides 216 Vapor-Pressnre-Driven Evaporation 216 Chinn Evaporation 217... 

Because data on non-professional exposure is scarce but exposure assessments are necessary, models have become a main tool in assessing residential exposure. It appears to be easier to obtain good quality data on exposure factors (room sizes, typical amounts used, etc.), which can be used as parameters in models, than to obtain direct exposure measurements. Critics of the modeling process say that the information generated through models is suspect because of the inherent simplifications involved. This criticism may be valid if risk assessors who utilize models do not make the appropriate selections that are needed in their application. They need to ask the following which model should be used which data should be fed into the model why do these two (three) models produce different results, and are the differences significant for risk assessment which data are necessary... 

Compared with the occupational exposnre of applicators and workers following pesticide application in the field, post-application residential exposure to pesticides used in and around the home is lower in level, but encompasses a wider variety of scenarios, such as age distribution, activity patterns and product use. Typically, few data are available on residential exposure, while a large body of data does exist for occupational exposures. Residential exposure assessment and modeling may benefit from the new data requirements under the United States Food Qnality Protection Act of 1996 (Lewis et al, 1994 Hill et al., 1995 Lu and Fenske, 1998 USEPA, 1990 Whitmore et al., 1994). In occupational exposnre assessment, a database approach is favored, while in residential exposure assessment a mechanistic and statistical modeling approach is dominant. 

Many simulation models can be used to estimate residential exposures, but the following models are considered to be typical and specialized residential simulation models currently in use. 

Exposure databases are considered as a compilation of empirical exposure data which can be subset for relevant characteristics. Models are mathematical algorithms that transform known inputs or conditions into an exposure estimate (i.e. output). Both have utility in occupational and residential exposure assessment. 

More sophisticated probabilistic models are used by EPA to comply with the aggregate and cumulative risk provisions of the FQPA. These models consider rolling windows of exposure, toxicological equivalence factors for pesticides that have common toxicological mechanisms, and include methods to incorporate exposure from drinking water and residential pesticide use into the pesticide exposure estimates. 

Deterministic sensitivity analysis performed during modelling of population exposures to ambient fine particulate matter by using high (H), medium (M) and low (L) values during the analysis of the impact of uncertainties associated with key inputs and parameters on model predictions (e.g. time spent outdoors [H = 95%, M = 80%, L = 50%], residential building infiltration fractions [H = 0.7, M = 0.5, L = 0.2], deposition rates [H = 0.4, M = 0.3, L = 0.1]). 

Because actual exposure measurements are often unavailable, exposure models may be used. For example, chemical emission and air dispersion models are used in air quality studies to predict the air concentrations for down-wind residents. Residential wells located down-gradient from a site may not currently show signs of contamination, but they may become contaminated in the future as chemicals in the groundwater migrate to the well site. In these situations, groundwater transport models may estimate the period of time that chemicals of potential concern will take to reach the wells. 

The Residential-SHEDS algorithms are presented in Zartarian et al. (2000). Eor each specified exposure scenario, the model randomly selects an individual... 

The application of pesticides is widespread in agriculture and elsewhere, and the concomitant risks depend on their toxicity, and duration and frequency, as well as the level of exposure (Henderson et al., 1993 Krieger and Ross, 1993). Exposure may be incidental or almost continuous. This is true not only for workers (occupational exposure), but also for the general public and people who may be considered as bystanders, who are not involved in the actual occupational activities with pesticides, but are close enough to get exposed. In this present chapter, only operator exposure will be discussed because agricultural re-entry modelling is discussed in Chapter 2 and residential post-application exposure modelling in Chapter 6 of this book. 

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