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Using Predictive Methods to Assess Exposure and Fate under TSCA

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Information these models can provide

An exposure assessment attempts to answer the following questions for a particular substance or chemical:

  • How much of the chemical will workers be exposed to during the manufacturing, processing, and use of the substance?
  • How much of the chemical will be released to the environment during manufacturing, processing, and use of the substance?
  • What environmental pathways are relevant for general population and environmental exposure? (indoor air, indoor dust, indoor surfaces, outdoor air, drinking water, surface water, etc)
  • What routes of human exposure are relevant?  (inhalation, ingestion, dermal, fetal)

And also:

  • What exposure factors are needed to characterize exposure? (breathing rates, drinking water ingestion rates, skin contact surface area, body weight, etc)?
  • Do exposures occur in steady-state conditions or do they change over time?  How much exposure occurs over time (acute, sub-chronic, or chronic)?
  • Do relevant activity patterns influence exposures? (How often and for how long does exposure occur)?
  • What is the fate of a chemical once in the environment and how is the chemical transported?
  • Which receptors are important given anticipated environmental media and relevant toxicological endpoints? (children, adults, fish, invertebrates, earthworms, etc)

How and when to use predictive fate and exposure assessment models

The first step in completing an exposure assessment is to compile existing measured or monitoring data relative to the purpose of the assessment.

Exposure assessment at EPA under the Toxic Substances Control Act (TSCA) typically includes:

  • Occupational exposures in the workplace
  • Exposures to the general population from chemicals in the air and drinking water
  • Consumer exposure through the household use of products
  • Ecological Environmental exposures

Representative measured data obtained under realistic conditions are generally more accurate than modeled estimates, and are needed to develop predictive models. However, models can provide useful fate and exposure estimates where data gaps exist. EPA's models and tools are intended to be used by scientists and engineers familiar with exposure assessment principles.

Use considerations: models vs. measured or monitoring data

Under TSCA, EPA generally uses a tiered approach for developing exposure assessments depending on the chemical and decision framework.

EPA has developed a toolbox for exposure assessors called the EPA Expo-Box, which includes:

Quality assurance and quality control of measured data and models are important components of  EPA exposure and fate assessments. Understanding the equations, limitations, default values, and assumptions is important when using models.  For risk assessment purposes, a quantitative exposure assessment approach is needed and exposure information (concentrations or doses) must be clearly linked to the hazard identification and relevant toxicological endpoints.

Assessments often use a combination of monitoring data and modeled estimates where reliable measured data are lacking.

Use of Monitoring Data: The most accurate way to characterize exposure is usually to conduct a well-designed monitoring study that concurrently measures environmental concentrations and internal doses over time. 

Elements of a well-designed exposure monitoring study include:

  • Establishing quality assurance objectives that will allow exposure assessors to make estimates of average and high end exposures with a known level of reliability
  • Using, where possible, sampling and analytical chemistry methods that have been found acceptable by an independent authoritative body (e.g. American Society for Testing and Materials (ASTM), National Institute for Occupational Safety and Health (NIOSH), etc)
  • Ensuring that quality control procedures have been employed and documented. Measured data are also preferred for estimates of environmental fate and transport.

Use of Models: The screening level tools often make simplifying assumptions which are protective by design (for example, assuming that people live near chemical discharge locations). Higher tier tools are more complex and allow for more realistic exposure assessments, such as using census data and a measure of the distance between the location of the chemical release and the populations living nearby.

The exposure assessment models in this website include standard default values which can be changed by the user. Where possible, language which describes the impact of the standard assumptions and default values on the exposure result is included. The default values were carefully selected but often reflect general rather than specific exposure conditions. If the standard assumptions and default values are applied when using the model, the descriptive language should be incorporated into the results. Using specific exposure related data in the models will enable you to tailor the model to the exposure scenario of interest and refine estimates of exposure.

EPA's fate and exposure models and tools 

Approaches to Estimate Consumer Exposure: A variety of tools and models have been developed to estimate exposure to consumer products and materials.

Chemical Screening Tool for Exposures and Environmental Releases (ChemSTEER):  Estimates environmental releases and worker exposures resulting from chemical manufacture, processing, and/or use in industrial and commercial workplaces.

Estimation Programs Interface (EPI Suite)™: Estimates physical / chemical properties (melting point, water solubility, etc.) and environmental fate properties (breakdown in water or air, etc.) which can indicate where a chemical will go in the environment and how long it will stay there.

Exposure and Fate Assessment Screening Tool (E-FAST): Estimates consumer, general public and environmental exposures to chemicals.

IIOAC - Integrated Indoor-Outdoor Air Calculator:  A tool based on EPA’s AERMOD for use in assessing releases to air and exposure potential for new and existing chemicals.

Internet Graphical Exposure Modeling System (IGEMS): Is a modernization of EPA's older Graphical Exposure Modeling System and PCGEMS tools. IGEMS brings together in one system several EPA environmental fate and transport models and some of the environmental data needed to run them. EPA is requesting public comment on the use of this model.

Point Source Calculator Version 105 psc v105: An aquatic model that OPPT uses to estimate chemical concentrations in water column, porewater, and sediment from point sources. 

ReachScan: Estimates surface water chemical concentrations in stream segments downstream from industrial facilities.  ReachScan is under development and is not available yet.

Fate and exposure guidance and publications

Environmental fate and transport documents

Interim Guidance for Using Ready and Inherent Biodegradability Tests to Derive Input Data for Multimedia Models and Wastewater Treatment Plant (WWT) models

Recommendations for Estimating Transport Between Environmental Compartments (Fugacity) for Existing Chemicals, Including HPV Chemicals

Interim Technical Guidance for Assessing Screening Level Environmental Fate and Transport of, and General Population, Consumer, and Environmental Exposure to Nanomaterials

University of Minnesota Biocatalysis-Biodegradation Database - The University of Minnesota Biocatalysis/Biodegradation Database includes a searchable database of microbial biocatalytic reactions and biodegradation pathways. In addition, the website includes a Pathway Prediction System (PPS) that predicts plausible pathways for microbial degradation of chemical compounds. It is available at the University of MN Biocatalysis Biodeg DatabaseEXIT

USEPA ACToR - The Aggregated Computational Toxicology Resource (ACToR) is EPA's online warehouse of all publicly available chemical toxicity data and can be used to find all publicly available data about potential chemical risks to human health and the environment. ACToR aggregates data from over 1000 public sources on over 500,000 environmental chemicals searchable by chemical name, other identifiers and by chemical structure, and is available at ACToR.

 OECD eChemPortal - eChemPortal provides free public access to information on chemicals such as physical & chemical properties, ecotoxicity, and environmental fate. It is available at eChemPortal EXIT

Human and environmental exposure assessment documents

Considerations When Evaluating Exposure Assessments

Scenarios Documents for Screening Level Exposure and Release Assessment(46 MB) ​  Note: The document is in PDF files, which are in this downloadable zip file.  If you do not have unzip software in your computer, you may not successfully retrieve this compilation from this zip file.

Use and Exposure Information Project (UEIP)

Guidelines for Statistical Analysis of Occupational Exposure Data

Community Air Screening How-To Manual

Physical/chemical property estimation documents

Persistent, Bioaccumulative and Toxic (PBT) Policy

Chemical Fate Half-Lives for Toxics Release Inventory (TRI) Chemicals

Workplace and occupational exposure assessment documents

Interim Approaches for Assessing and Controlling Workplace Releases and Exposures to New and Existing Nanomaterials

Other publications

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