Developing Individual Human Exposure Estimates for Individual DBPs

Objectives

Introduction

Table of Contents for Final Report

Report Citation

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Objectives

The goal of this project was to implement a comprehensive exposure model to estimate population-based exposures and doses to various DBPs. The DBPs of concern in this project are listed in the following table. The populations of concern in this project were the following: (a) women of reproductive age (ages 15-45); (b) men of similar age (ages 15-45); and (c) children (age 6). To begin the process of estimating the exposure of these populations to the given DBPs, we chose the Total Exposure Model (TEM) as our modeling tool and identified, collected, and summarized all the model parameters necessary to set up the modeling study. The end result of this project was a final report that presents and discusses these various model parameters needed for running TEM, specifically those related to chemical volatilization, human activity patterns, ingestion, building characteristics, and chemical concentration in the water supply. Furthermore, to assess the population doses associated with the resultant exposures, the PBPK model ERDEM was adjoined with TEM. The report also presents and discusses the model parameters necessary for ERDEM.  The table of contents and the introduction for the final report are below.

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Introduction

Disinfection of drinking water is widely recognized for its significant role in reducing illness due to waterborne pathogens that are responsible for numerous diseases.  Although disinfection is necessary for the elimination of these pathogenic organisms, it can also lead to the generation of a variety of chemicals, known as disinfection byproducts (DBPs), which are formed as a result of reactions of the disinfectant with organic matter in the water. In the U.S., where the primary form of disinfection is chlorination, public drinking water contains low levels of many DBPs and is a potential source of exposure to these compounds.  The potential for exposure is significant by ingestion, but has also been shown to be significant through inhalation and through contact with the skin. The importance of each route varies with chemical characteristics, use patterns, physiological characteristics, and a variety of other factors (Wilkes et al., 1996; Olin, 1999).  For example, exposure to a volatile chemical, such as chloroform, occurs most significantly during large household water uses, such as showering, bathing, and clothes washing activities.  Although all three primary routes can be significant, typically inhalation dominates the exposure for these volatile compounds.  For the less volatile compounds, ingestion and dermal contact play more significant roles in exposure and uptake.

In the early 1970s, advances in gas chromatography and mass spectrometry led to improvements in the detection of various DBPs in drinking water. In 1974, Rook (1974) and Bellar et al. (1974) showed that trihalomethanes (THMs) result from the chlorination process. Subsequently, a significant amount of research identified THM formation pathways as complicated reactions involving aqueous halogen species and natural aquatic humic substances, particularly humic and fulvic acids (Glaze et al., 1979; Peters et al. 1980; Urano et al., 1983). In addition, more recent research has identified the formation of haloacetic acids (HAAs), haloacetonitriles (HANs), and a variety of other DBPs and verified their existence in water supplies (Krasner et al., 1989, Westrick et al., 1984, Miller et al., 1990, Richardson, 1998).

U.S. EPA reported median concentrations of dichloroacetic acid in United States drinking water ranging from 6.4 to 17 mg/L, and a similar range (5.5 to 15 mg/L) for trichloroacetic acid. In areas where naturally-occurring bromine ion is present in surface water, significant amounts of bromo- and chlorobromo acetic acids can form (Ireland et al., 1988).  In addition to HAA, several haloacetonitriles (HAN) (dichloroacetonitrile, trichloroacetonitrile, dibromoacetonitrile, bromochloroacetonitrile) can form in chlorinated drinking water. In addition to THM, HAA, and HAN, two haloketones (1,1-dichloropropanone and 1,1,1-trichloropropanone), chloropicrin, and trichloroacetaldehyde monohydrate (chloral hydrate) have all received some attention as potential DBPs. Alternative forms of disinfection can also produce DBPs. For example, ozonation has been shown to lead to the formation of aldehydes and ketones (Miltner et al., 1992). A study involving the ozonation of humic substances revealed the formation of mutagenic compounds, primarly glyoxal and glyoxylic acids (Matsuda et al., 1992).

In 1979, the U.S. EPA issued the National Interim Primary Drinking Water Regulations, which established a maximum contaminant level (MCL) of 100 mg/L for total trihalomethanes (TTHM) in drinking water. In 1986, Congress passed amendments to the Safe Drinking Water Act (SDWA), an action that required the U.S. EPA to establish regulations for a wide range of drinking water contaminants. In 1988, the U.S. EPA published the Drinking Water Priority List (DWPL), and revised the list in 1991. The DWPL includes THMs, as well as several of the other DBPs described above.

Exposure to DBPs originating in the drinking water is a very complex problem, influenced by a multitude of factors, including chemical properties of the contaminant, physical characteristics of the indoor environment, behavior of the individual relative to the contaminant, and behavioral and physiological characteristics of the exposed population.  Previous modeling studies have demonstrated the considerable impact human behavior has on an individual’s exposure to waterborne contaminants (Wilkes et al., 1996; Wilkes, 1999), demonstrating that differences in behavior can produce exposures varying across more than an order of magnitude. Mathematical exposure and uptake models represent a realistic, cost-effective means for estimating human exposure.  Mathematical models within a probabilistic framework allow a close examination of the factors that lead to exposures and provide a basis for addressing higher risk populations.  However, in the case of exposure to waterborne contaminants, previous modeling studies (Wilkes et al., 1996; Wilkes, 1999) have shown that a strictly probabilistic framework would fail to capture the effect of an individual’s activities on his or her exposure. The ideal model would therefore combine a probabilistic representation of human behavior related to water use and exposure with a deterministic calculation of the concentrations in the contact media leading to the exposure (i.e. in the water and air). Such modeling frameworks also offer the ability to evaluate the impacts of parameter uncertainty and variability, such that results may be incorporated into meaningful and useful sets of outcomes.

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Table of Contents

To view the table of contents for this report in a PDF format, click on the following link.  The document will be viewed in an Acrobat Reader.

    Developing Individual Human Exposure Estimates for Individual DBPs:  Table of Contents

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Citation

This report is both a stand alone report and an appendix an EPA NCEA report. The EPA NCEA report citations is as follows:

"The Feasibility of Performing Cumulative Risk Assessments for Mixtures of Disinfection By-Products in Drinking Water," Teuschler, LK, Rice, GE, and Lipscomb, JC, National Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati OH, EPA/600/R-03/051, June 2003.

The report citation for this project is as follows:

"Developing Individual Human Exposure Estimates for Individual DBPs," Wilkes, CR, Mason AD, Niang, LL, Rector, HE, Power, FW, Tsang, AM, Stephen, PM, Harrison, LS, Blancato, JN, and Hern, SC. March 2002.

These reports have been peer reviewed and are in the process of being finalized.  It will be publicly released in the near future. For a copy of the project report contact Charles R. Wilkes at c.wilkes@wilkestech.com

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Last modified on November 2006