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Technical Overview of Ecological Risk Assessment - Analysis Phase: Exposure Characterization

About Exposure Characterization

Exposure Characterization is the second major component of the analysis phase of a risk assessment. For a pesticide risk assessment, the exposure characterization describes the potential or actual contact of a pesticide with a plant, animal, or media. The objective is to describe exposure in terms of intensity, space, and time and to describe the exposure pathway(s). A complete picture of how, when, and where exposure occurs or has occurred is developed by evaluating sources and releases of the pesticide, distribution of the pesticide in the environment, and extent and pattern of contact with the pesticide.

The final product of the exposure characterization is an exposure profile that describes:

  • source(s) of the pesticide and what is exposed (e.g., plants, animals, media),

  • fate and transport of the pesticide and exposure pathways,

  • how often, how long, and amount of pesticide active ingredient and its degradates of concern to which an organism or media may be exposed.

  • impact of variability and uncertainties in the exposure estimates, and

  • conclusions about the likelihood that exposure will occur.

Risk assessors use environmental fate and transport data, usage data, monitoring data, and modeling information to estimate the exposure of various animals and plants to pesticide residues in the environment. In most cases, an exposure characterization is conducted on the pesticide active ingredient. In some cases where formulations have been shown to be toxic or where degradates occur in significant amounts or of significant toxicological concern, the exposure characterization can include a quantitative or qualitative analysis of the risk implications of exposure to these degradates or formulations.

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Pesticide Degradation / Dissipation
(Fate and Transport of Pesticides)

EPA reviews many laboratory and field studies to determine what happens to pesticides in the environment. These studies measure how pesticides interact with soil, air, sunlight, surface water, and ground water and answer questions about:

  • the degradation of the pesticide active ingredient (how fast and by what means it is degraded in the environment) and how persistent it is in the environment;

  • the breakdown products or degradates that result from the degradation processes;

  • the mobility of a pesticide active ingredient and its degradates and how these chemicals will move from the application site; these studies predict the potential of the pesticide to volatilize into the atmosphere, move into ground or surface waters, or bind to the soil; and

  • how much of a pesticide active ingredient and its degradates will accumulate in the environment.

These environmental fate studies are designed to help identify which dissipation processes are likely to occur when a pesticide is released into the environment and to characterize the breakdown products that are likely to result from these degradation processes. The diagram below illustrates the potential dissipation pathways for a pesticide after it is applied.

dissipation pathway diagram includes spray drift,surface runoff,lateral flow,sorption/retention,leaching,transformations-microbial and chemical,plant uptake,volatilization,wash-off,foliar interception and dissipation,tile drainageDissipation Pathways

Based upon results of environmental fate and transport studies, EPA can develop a preliminary, qualitative environmental fate and transport profile or assessment. This profile, in turn, can be used to design and/or trigger appropriate field studies and to provide parameters needed in simulation modeling.

Field studies are also conducted to provide a more realistic picture of what happens to the parent compound and breakdown products in the environment. Under field conditions, pesticides are exposed to several dissipation processes at the same time. The results of field studies and laboratory data are integrated to characterize the persistence and transport of a pesticide and its breakdown products. From this data, EPA produces a quantitative environmental fate profile or assessment and model estimates of exposure to the pesticide.

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Fate and Transport Studies Needed

The types of environmental fate studies required depend on the use of the pesticide. Certain laboratory studies (e.g., hydrolysis, photolysis, and soil metabolism) are routinely conducted for all outdoor use pesticides. Other studies (e.g., photodegradation in air, volatility, and droplet size) may be triggered by use/application patterns and basic product chemistry data. These studies provide the following critical information:

  • the half-life of the parent;
  • the identity of breakdown products;
  • rates of formation and decline of breakdown products; and
  • mobility of the parent and breakdown products.

The Agency's regulations found in the Code of Federal Regulations (40 CFR 158: Subpart N 158.1300) describe the types and amounts of data that the EPA needs for assessing the environmental fate of a pesticide active ingredient. In all, there are 24 studies that may be required for environmental fate testing depending on the use of the pesticide. These controlled laboratory and field studies, which are conducted under approved Harmonized Test Guidelines and Good Laboratory Practices Standards, are used to determine the persistence, mobility, and bioconcentration potential of a pesticide active ingredient and its major degradates. Degradates formed at greater than or equal to 10% of the amount of applied pesticide are considered significant (i.e., major degradate) and must be identified in the study. In addition degradates of known toxicological or ecotoxicological concern must be quantified and identified even when present at less than 10% of the applied pesticide. EPA developed guidance to use related to these degradates: Guidance for Residues of Concern in Ecological Risk Assessment. To help EPA scientists uniformly consider the issues, data, and formatting relevant to the review of environmental fate studies, the Agency has developed Guidance for Reviewing Environmental Fate Studies. The study review guides found in these guidance documents are based on the format of the Organization for Economic Co-operation and Development (OECD) Tier II summaries.

If environmental fate studies are conducted with foreign soils, the following guidance should be considered: Guidance for Determining the Acceptability of Environmental Fate Studies Conducted with Foreign Soils. In quantifying the degradation of a chemical in soil or sediment, it is important that rate constants, often represented as half-lives, reflect the degradation of the chemical in the total study system, both dissolved residues and weakly sorbed residues.  To assist in this evaluation, EPA has developed Guidance for Addressing Unextracted Residues in Laboratory Studies to follow when assessing residues that remain associated with soil or sediment following extraction (i.e., unextracted residues) in laboratory-based pesticide studies. This internal Unextracted Residues guidance is for immediate use in the interim until more comprehensive guidance is developed to fully address bioavailability and toxicity issues.

  • Physicochemical Degradation

    This includes hydrolysis and photodegradation in water, soil, and air. Hydrolysis studies determine the potential of the parent pesticide to degrade in water, while photodegradation studies determine the potential of the parent pesticide to degrade in water, soil, or air when exposed to sunlight. During these studies, data are also collected concerning the identity, formation, and persistence of breakdown products.

  • Biological Degradation

    These studies include aerobic and anaerobic soil metabolism, and aerobic and anaerobic aquatic metabolism. The soil metabolism studies determine the persistence of the parent pesticide when it interacts with soil microorganisms living under aerobic and anaerobic conditions. The aquatic metabolism studies produce similar data that are generated by pesticide interaction with microorganisms in a water/sediment system. These studies also identify breakdown products that result from biological degradation. Working with Health Canada's Pesticide Management Regulatory Agency (PMRA), the Environmental Protection Agency's Office of Pesticide Programs has developed Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media. This guidance establishes a procedure for determining a first-order constant from biotransformation/degradation studies, which include biotic studies conducted in soil, water, and mixed media.

  • Mobility

    These studies include leaching and adsorption/desorption, laboratory volatility, and field volatility. The leaching study assesses the mobility of the parent pesticide and its degradates through columns packed with various soils. The adsorption/desorption study determines the potential of the parent pesticide and its degradates to bind to soils of different types. The potential mobility of the parent pesticide and each breakdown product is determined by examining the data from both of these studies and may range from immobile to highly mobile.

  • Field Dissipation

    These studies assess the most probable routes and rates of pesticide dissipation under actual use conditions at representative field sites. While laboratory environmental fate studies are designed to address one dissipation process at a time, field dissipation studies assess pesticide loss as a combined result of chemical and biological processes (e.g., hydrolysis, photolysis, microbial transformation) and off-site transport (e.g., volatilization, leaching, run-off) as well as loss from plant uptake. Examples of field dissipation studies include terrestrial field dissipation, aquatic dissipation, forestry dissipation, and combination products and tank mix use dissipation. Data from these studies can reduce potential overestimation of exposure and risk and can confirm assumptions of low levels of toxic degradates. In addition, results can be used to propose scenario-specific effective risk mitigation measures. These studies provide a field dissipation half-life, a lumped parameter that includes all routes of dissipation. Typically, several field studies are conducted for a pesticide in representative use areas.

    Under the North American Free Trade Agreement (NAFTA), U.S. EPA and the ExitPest Management Regulatory Agency (PMRA) of Health Canada developed a harmonized terrestrial field dissipation (TFD) guidance document so that one set of tests for this study could be used for registration of a pesticide in Canada, the United States, and Mexico. In developing this harmonized guidance, EPA and PMRA conducted an extensive outreach and review program, soliciting input from stakeholders and the technical community through several forums: three symposia, one Scientific Advisory Panel (SAP) meeting (1998 SAP Meetings), and one Terrestrial Field Dissipation Workshop. Working closely with its stakeholders, PMRA and EPA developed a conceptual model for designing terrestrial field studies that will evaluate the overall dissipation of a pesticide in the field. The conceptual model, which is specific for each pesticide, is based on the chemical's physicochemical properties, laboratory environmental fate studies, formulation type, and intended use pattern. For the terrestrial field dissipation guidance, see NAFTA Guidance Document for Conducting Terrestrial Field Dissipation Studies.

  • Stereoisomers

    Stereoisomers are compounds that have the same chemical formula but different three-dimensional structures. Because stereoisomers may exhibit selective biological effects towards organisms in the environment, EPA needs data to assess the risk posed to ecosystems and drinking water sources by mixtures of these stereoisomeric pesticides. EPA's interim policy for stereoisomeric pesticides can be found at: EFED INTERIM POLICY FOR STEREOISOMERIC PESTICIDES.

  • Ground Water Monitoring

    These studies include small-scale prospective ground water monitoring and small-scale retrospective monitoring. These studies, which are required on a case-by-case basis, are designed to determine whether a pesticide applied under various conditions reaches ground water and in what concentrations. Guidance for conducting prospective ground water monitoring studies can be found in the EPA docket at the following website: EPA-HQ-OPP-2007-1163.

  • Spray Drift

    These studies include droplet size spectrum and spray drift field evaluations (See 40 CFR Part 158: Subpart L 158.1100). The objective of pesticide spray drift evaluations is to determine the potential of a pesticide to drift off-site during or immediately after it is applied according to the label directions. The droplet size spectrum test provides information on the effects of pesticide application equipment and formulations on droplet sizes. Droplet size influences how readily the pesticide droplets are carried by air currents. The field drift evaluation test determines the effects of environmental conditions and application equipment on the extent of off-target transport immediately following release of the pesticide from the application equipment.

    For several years, a consortium of industry representatives and EPA have been cooperating on the development of spray drift experimental data sets and drift modeling for many spray application scenarios. As a result of this effort, the pesticide industry has produced a spray drift model called AgDRIFT.

    AGDISP (Agricultural DISPersal) is another spray drift model that EPA uses to estimate spray area deposition patterns of aerial pesticide applications and deposition off-site and downwind. This model predicts the motion of a chemical released from aircraft, including the mean position of the chemical and the position variance about the mean as a result of turbulent fluctuations. For more information regarding this model, see ExitAGDRIFT/AGDISP Model Capabilities. Guidance for the use of linked AGDISP-Gaussian extension models for estimating far field drift of pesticides can be found at: Guidance on the Use of the Linked AGDISP-Gaussian Extension Models for Estimating Far Field Drift of Pesticides in FIFRA Ecological Risk Assessments.

  • Environmental Chemistry Methods

    In accordance with data reporting guidelines for environmental field studies, pesticide manufacturers submit environmental chemistry methods (ECMs) to the Environmental Fate and Effects Division (EFED) in the Office of Pesticide Programs. These methods identify and quantify the pesticide residue(s) of interest, determining the total concentration of pesticides, including the extracted parent compound and significant degradates. In order to provide uniform procedures for processing and reviewing ECMs, EFED has developed Environmental Chemistry Methods Guidance.

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How OPP Uses Fate and Transport Data

After EPA scientists review the available fate and transport data for a pesticide, they develop a data evaluation record (DER) for each study, which summarizes the fate and transport data for the parent pesticide and its degradation products. See the list of Environmental Fate Data Evaluation Record (DER) Templates.

The conclusions from these individual DERs are then integrated and summarized in an exposure profile, which is the final product of the exposure characterization.

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Approaches for Evaluating Exposure

Aquatic Animals

For aquatic animals, such as fish and invertebrates, EPA generally uses computer simulation models to estimate exposure to a pesticide active ingredient. In situations where a pesticide formulation may be more toxic to aquatic animals than the active ingredient, EPA may consider aquatic exposure to the formulation. The Agency's approach for considering formulated product exposure in an aquatic risk assessment follows approaches developed by the European Union (ExitEU Council Directive 91/414/EEC).

EPA's aquatic models calculate estimated environmental concentrations (EECs) in surface water using fate and transport laboratory data that describe how fast the pesticide breaks down to other chemicals and how it moves in the environment. In general, EPA uses a tiered approach to estimate EECs, beginning with a screening model, such as GENEEC2, that estimates the concentration of a pesticide in water from sites that are highly vulnerable to runoff or leaching. If a more refined risk assessment is needed, a higher tiered screening model (e.g., PRZM-EXAMS) is used to estimate pesticide concentrations that are more reflective of actual use site conditions. A detailed description of these aquatic models can be found at EPA's Models for Pesticide Risk Assessment website.

When reliable surface water monitoring data are available, EPA uses it to help characterize the levels of pesticide that are being detected in the environment. Water monitoring data may be available from EPA databases, U.S. Geological Survey - National Water-Quality Assessment Program, industry, states, and academia.

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Terrestrial Animals

Dietary Exposure for Birds and Mammals through Food Items

Using the computer model T-REX (Terrestrial Residue Exposure), EPA estimates dietary exposure for birds and mammals for foliar, granular, and seed treatment applications. For most foliar applications. T-REX calculates estimated environment concentrations (EECs) by calculating residues of pesticides on food items, including grasses, plants, insects, seeds, and fruits.

  • Terrestrial animal EECs are based on both the upper bound and mean residue concentrations and are determined using nomograms (charts) that relate application rate of a pesticide to residues remaining on dietary items of terrestrial organisms.

  • The nomogram is based on an EPA database called UTAB (Uptake, Translocation, Accumulation, and Biotransformation) and work from Fletcher et al. (1994)1. The UTAB database is a compilation of actual measured pesticide residue values on plants.

  • Dietary concentrations on food items are calculated on each selected food item on a daily interval for one year.

  • For small mammals, the residue concentration is converted to a daily oral dose based on the fractions of body weight consumed daily as estimated through mammalian allometric2 relationships in EPA's Wildlife Exposure Factors Handbook.

Wildlife Food Item Nomogram
Food Item Maximum EEC
Average EEC
short grass 240 85
tall grass 110 36
broadleaf forage 135 45
small insects, seeds, fruits, large insects 15 7
Residues expressed on a 1 lb a.i./acre application basis
Hoerger and Kenaga (1972); Fletcher et al. (1994)

When multiple applications are modeled, residue concentrations resulting from the final application and remaining residue from previous applications are summed. The maximum concentration calculated (out of 365 days) is returned as the EEC used to estimate potential risk to birds and mammals.

For granular and liquid banded applications and granular and liquid broadcast applications, EECs are based on the milligram active ingredient of the pesticide per square foot (mg a.i./ft2) based on the application rate of the pesticide.

For seed treatment, the EECs are based on the concentration of the active ingredient of the pesticide on the seed plus the seeding rate.

The T-REX User's Guide and spreadsheet can be found on the Models for Pesticide Risk Assessment website.

Dietary Exposure for Reptiles and Amphibians through Food Items

For terrestrial reptiles and amphibians, EPA uses a modified version of T-REX called T-HERPS (Terrestrial Herpetofaunal Exposure Residue Program Simulation) to estimate dietary exposure. The allometric equations in T-REX have been adjusted to account for the lower metabolic rate and food intake of herptiles compared to birds. Information concerning T-HERPS can be found on the Models for Pesticide Risk Assessment website.

Dietary Exposure of Birds and Mammals through Drinking Water

Exposure of birds and mammals to pesticides can also occur through drinking water. Using the model SIP (Screening Imbibition Program), EPA derives upper bound exposure estimates of pesticides through drinking water alone. This model is intended for use in problem formulation to determine whether or not drinking water exposure is a potential path of concern. It is not used for aggregating drinking water exposure with other exposure routes (i.e., diet, inhalation, dermal). The SIP User's Guide and spreadsheet can be found on the Models for Pesticide Risk Assessment website.

Inhalation Exposure for Birds and Mammals

Another important route of exposure of terrestrial animals to pesticides is through inhalation. Using the model STIR (Screening Tool for Inhalation Risk), EPA estimates inhalation-type exposure based on the physical properties of the chemical and on the spray droplet exposure. Spray droplet exposure is estimated by considering the pesticide application method (e.g., ground versus aerial spray) and the rate of application. If the application type is spray, the model estimates both the droplet inhalation and the vapor phase inhalation doses. For non-foliar applications (i.e., granular, seed treatment), the model only calculates the vapor phase inhalation dose.

After estimating exposure (EECs), STIR compares these exposure values to toxicity endpoints such as mammalian-oral, mammalian-inhalation, and avian-oral values. If avian-inhalation toxicity values are not available, the model estimates an avian-inhalation LD50 value by using an adjustment factor that accounts for the relationship between the mammalian oral and inhalation LD50 values to the most sensitive avian oral LD50 value. The Models for Pesticide Risk Assessment website contains more information regarding STIR.

Dermal Exposure to Birds, Mammals, Reptiles, and Amphibians

Currently, EPA is developing a model (Dermal Uptake Screening Tool (DUST)) to estimate exposure to birds, mammals, reptiles, and amphibians through the dermal route. DUST compares a ratio of exposure to toxicity and then compares this ratio to a limit of concern to determine if dermal exposure warrants further exploration. After the model is finalized, it will be used as a qualitative tool to screen out pesticides that are not of concern when considering exposure through the dermal route.

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Non-Target Plants

For aquatic plants, EPA uses the aquatic model PRZM-EXAMS and the spray drift model AgDRIFT to calculate estimated environmental concentrations (EECs). Exposure for non-target aquatic plants is assessed in a manner consistent with exposure for aquatic animals.

For terrestrial plants, EPA uses the model TerrPlant to estimate screening-level environmental concentrations for single pesticide applications. In TerrPlant, EECs for a pesticide are derived from runoff and drift estimates.

For terrestrial plants inhabiting dry areas adjacent to the treatment area, runoff exposure is estimated as sheet runoff. Sheet runoff includes the maximum application rate (lbs a.i./A) times the amount of pesticide in water that runs off1 of the soil surface of a target area of land that is equal in size to the non-target areas (1:1 ratio of areas). These runoff values are combined with drift2 estimates to calculate EECs.

EEC = Max application rate x runoff value + drift

For semi-aquatic areas, runoff exposure is estimated as channel runoff. Channel runoff is the amount of pesticide that runs off of a target area 10 times the size of the non-target area (10:1 ratio of areas). As with sheet runoff, channel runoff values are combined with drift estimates to calculate EECs.

EEC = Max application rate x runoff value x 10 acres + drift

For more information concerning TerrPlant, visit the Models for Pesticide Risk Assessment website.

1 The runoff value is based on the solubility of the pesticide

2 Spray drift exposure to plants from ground applications is assumed to be 1% of the application rate and 5% for aerial, airblast, forced air, and chemigation applications. Drift is not calculated if the pesticide is incorporated into the ground.

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Amphibians and Reptiles

In general, EPA scientists use the same acute EEC exposure values as fish or invertebrates for amphibians. When amphibian and reptile data are available, the Agency will consider them in its risk assessment.

  • When toxicity information on amphibians is available, it is compared to fish or invertebrate exposure values. If no data are available, the Agency relies on fish data as surrogates for aquatic-phase amphibians and bird data as surrogates for terrestrial-phase amphibians and reptiles.

  • Exposure patterns for reptiles are generally considered to be comparable to birds although exceptions may occur with certain aquatic organisms that lay eggs in terrestrial areas.

  • At a screening level, the Agency uses the terrestrial model, T-REX, to estimate risk to terrestrial amphibians and reptiles. When greater refinement is needed, the Agency uses the model T-HERPS.

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Non-Target Insects

Currently, EPA does not characterize residue exposure for honey bees and other beneficial insects. EPA scientists do characterize toxicity to the honey bee from direct application of pesticide droplets on the body using the acute contact LD50 study. They also look at foliar exposure LD50 studies that measure the lethality of aged residues on foliage when exposed to or ingested by bees.

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Water Resources

EPA generally uses computer simulation models to estimate exposure of water resources to pesticides.

  • EPA uses a tiered approach to estimate environmental concentrations (EECs), generally beginning with screening models, such as FIRST and GENEEC and moving to higher-tiered screening models, such as PRZM-EXAMS when a more refined risk assessment is needed.

  • A detailed description of these aquatic models can be found at EPA's Models for Pesticide Risk Assessments website.

  • When reliable aquatic monitoring data are available, EPA uses these data to help characterize the levels of pesticides that are being detected in the environment.


1 Fletcher, J.S., J.E. Nellessen, and T.G. Pfleeger (1994). Literature Review and Evaluation of the EPA Food-Chain (Kenaga) Nomogram, an Instrument for Estimating Pesticide Residues on Plants. Environ. Tox. and Chem. 13,9: pp. 1383-1391.
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2 Allometry is the study of the relationships between the growth and size of one body part to the growth and size of the whole organism. Allometric relationships also exist between body size and other biological parameters (e.g., metabolic rate).
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