1.0 Introduction

1.0 INTRODUCTION

    The exposure pathway and health impact assessment component of the Multimedia Environmental Pollutant Assessment System (MEPAS) provides an estimate of health impacts to selected individuals and populations from exposure to pollutants. The exposure pathway analysis starts with pollutant concentration in a transport medium and estimates the average daily dose^(a) to exposed individuals from contact with the transport medium or a secondary medium contaminated by the transport medium. The average daily dose is then used to estimate a measure of health impact appropriate to the type of pollutant considered.

    Each exposure pathway analysis in MEPAS involves definition of a transport medium (or medium of measurement), an exposure route for transfer of pollutant from the transport medium to man, exposure conditions for the individual receiving the pollutant, and conversion of the average daily dose to a measure of health impact. These steps are illustrated in Figure 1.1 for all transport and exposure pathways except exposure to measured direct radiation fields. The direct radiation exposure pathway does not involve a specific pollutant, but rather is used to estimate health impacts from exposure to a measured radiation field. With this exception, the steps for the measured direct radiation pathway parallel those indicated in Figure 1.1

    The pollutant concentration in the transport medium is the starting point for the exposure and health impact analysis. This concentration is generally represented within MEPAS as a 70-year averaged value. When the exposure duration is less than 70 years, the concentration represents the average for the exposure duration considered for a given exposure scenario.

(a) In the remainder of this section on exposure pathway and human health impact assessment models, the term average daily dose will be used to represent the average daily intake rate of chemicals (mg/kg/d), the average radiation dose rate from intake of radionuclides (rem/d), and the average external radiation dose rate (rem/d).

FIGURE 1.1 General Components of an Exposure/Health Impact Analysis

Such an average value can be provided by the user for the measured soil concentration and measured food concentration transport pathways. For other transport pathways, the calculated 70-year average value is used as an approximation for individual and population exposures.

    The transport medium may or may not be the medium of exposure. For example, the groundwater transport pathway generates estimates of pollutant concentration at the well. In this case, the well water is also the medium of exposure, although some modifications to the concentration are possible during transfer through the treatment plant and distribution system to the individuals exposed during domestic water uses. When the well water is used for irrigation of agriculture crops, the exposure medium is not the well water, but the foods produced. For agricultural pathways, models are use to estimate the transfer of pollutants from the irrigation water to the food consumed by humans. For each transport and exposure pathway, the processes affecting the concentration and transfer to the exposure medium are defined in the following sections.

    The average daily dose of a pollutant for an exposure pathway involves consideration of the rate of intake (ingestion, dermal absorption, inhalation, or external radiation dose), the frequency of exposure, the exposure duration, the averaging time, and the body weight of the exposed individual or an average member of the population. The general method for converting the medium concentration to average daily dose is described in Section 2.0. Detailed applications to specific pathways are described in subsections of Section 2.0.

    Health impact models are used to estimate the health impacts from exposure to the pollutant of concern. Models are defined for noncarcinogenic chemicals, carcinogenic chemicals, and radionuclides and radiation dose in Section 3.0. When populations are exposed to carcinogenic pollutants, including radiation, estimates of health impacts to an exposed population may also be made by multiplication of the effect to an average member of the population by the number of people in the exposed population.

    The models for the exposure pathway and health impact assessments require definition of several parameters. A summary of the notation used for the parameters in the exposure pathway and health impact models is given in Section 4.0. The default values used in MEPAS for these parameters are presented and discussed in Section 5.0.

    Appendix A provides auxiliary equations used to calculate intermediate summary intake factors. Appendix B describes the algorithms used to evaluate radioactive chain decay.