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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:

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:

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 drainage
Dissipation 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 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 (17pp, 160 KB, About PDF). 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 (144pp, 3.52 M, About PDF) and Additional Guidance for Reviewing Environmental Fate Studies (84pp, 764 KB, About PDF ). 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.

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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 (EU Council Directive 91/414/EECExit EPA Disclaimer).

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 Water Models 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.

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 Terrestrial Models 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 Terrestrial Models 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 Terrestrial Models 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 Terrestrial Models 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 Terrestrial Models 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.

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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.


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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