Development of Predictors for Intermedia Transfer Factors for Toxic Air Pollutants

This study developed an integrated system of tools for the prediction of intermedia transfer factors for toxic air contaminants, in support of the State's multi-pathway exposure assessment methodology, which is used to assess the public health risk posed by emissions from a facility. It was necessary to predict the intermedia transfer factors because literature data for most of these substances is lacking, and obtaining experimental data for hundreds of substances is not feasible. The study was conducted by the University of California, Los Angeles.
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As a result of legislation in 1992, the number of substances identified as toxic air contaminants was increased by an order of magnitude. In addition, the management of air toxics has been shifting toward integrated multimedia approaches, because pollutants that are emitted to the environment as the result of a variety of human-related activities move across environmental boundaries and are therefore found in most media (air, soil, water, etc.). The development of rational risk reduction strategies designed to minimize the adverse effects of air toxics emissions requires an understanding of the sources of pollutants and their pathways to receptors of concern.The evaluation of exposure of human and other ecological receptors to air toxics requires identification of the various chemical transport pathways between environmental media. Estimates for the rates of contaminant intake by the receptors via the various exposure pathways can be obtained using a variety of methods, all of which require knowledge of the multimedia distribution of the air toxics of interest. Field monitoring of the multimedia distribution of the numerous air toxics could only be achieved at great expense. Thus, the ARB, in cooperation with other agencies, has proposed various models for determining the multimedia human exposure to air toxics and subsequent pollutant intake. These multimedia models require a range of chemical-specific and region-specific input parameters that pertain to the estimation of chemical flux across environmental phase boundaries (such as air/water or water/biota) and accumulation in the various media. These are called intermedia transfer factors, or ITFs.Because of the large number of chemicals for which exposure analysis and risk assessment may be needed, the ARB funded this project to develop estimation methods that are based on available theory, experimental data, and appropriate correlations. This project provides flexible and user-friendly software with integrated ITF estimator and a physicochemical property database, using algorithms that require minimum user input of model parameters. The software tool developed can be used in conjunction with the exposure and risk assessment models currently utilized by Cal/EPA.
The researchers reviewed an exhaustive list of references to determine the extent of information available on physical and chemical properties for toxic air pollutants. Where available they determined the most reliable values of input parameters for use in Cal/EPA's Health Risk Assessment model. However, reliable data were not available for most of the necessary parameters for most of the pollutants. For these pollutants, the researchers developed methods to estimate various physical and chemical parameters, based on chemical similarities with chemicals for which reliable data were available. In a similar way, the researchers reviewed and developed algorithms to predict intermedia transfer factors for these chemicals.
The intermedia transport factor predictor (ITFP) software consists of three main components. The first, called CHEMBASE, is a chemical property database that contains, for selected air toxics, environmentally relevant physicochemical properties, compiled and critically evaluated from various literature sources. The second component is a chemical property predictor (CPP), which contains various methods for estimating a selection of physicochemical parameters for different chemical classes. It is based in part on chemical-class-specific correlations developed for this software. The third component is the intermedia transport parameter predictor (ITPP), which is capable of estimating parameters such as dry deposition velocity, wet scavenging coefficients for chemicals in particle-bound and gaseous forms, chemical-specific water/air mass transfer coefficients, and half-lives for chemical volatilization from soil. The ITPP includes both literature-based algorithms and new algorithms developed specifically for the ITPP. The ITPP is modular in its construction and can be expanded to include additional intermedia transport parameters and chemical property estimation modules.The draft final report describes the three above components in detail, and includes extensive references. Appendices include chemical property predictor correlations developed for this contract, parameter estimation for the bioaccumulation factor model, and discussions of the particle size distribution function, surface area of atmospheric particles, and volume distribution of particles.
The California Health and Safety Code requires Cal/EPA to conduct multi-pathway exposure assessments of toxic air pollutants according to specified methodologies. The information developed in this study will support Cal/EPA's assessments of these pollutants. Algorithms have been provided that allow the estimation of region-specific and chemical-specific intermedia transfer factors, which can be used according to the State's published guidelines. The draft final report includes a flexible and user-friendly software package with an integrated predictor of intermedia transfer factors and a physicochemical properties database. The software has been useful to Cal/EPA staff, and it is helpful for air toxics fate and transport estimation work.
The ARB has also funded these related studies (ARB contract number in parentheses): Development of Intermedia Transfer Factors for Toxic Air Pollutants (A032-170); Updating the Multi-Pathway Exposure Assessment Computer Program (92-318).
This research was conducted under contract with the University of California, Los Angeles (ARB Contract No. 92-344). Comments or questions can be directed to the contract manager, Ralph Propper by mail, FAX (916) 322-4357, phone (916) 323-1535, or e-mail rpropper@arb.ca.gov. For an index of Research Notes, call (916) 445-0753 or FAX (916) 322-4357.
Copies of the research report upon which this Note is based can be ordered from:
National Technical Information Service5285 Port Royal RdSpringfield VA 22161Request NTIS No. (Pending)
Title: Development of Predictive Capabilities for Intermedia Transfer Factors for Toxic Air Pollutants
Authors: Yoram Cohen and Arthur M. Winer