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Technical Overview of Ecological Risk Assessment

Analysis Phase: Exposure Characterization

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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 regulations found in the Code of Federal Regulations (40 CFR 158: Subpart N 158.1300) describe the types and amounts of data that the Agency 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 Chemicals Testing - Guidelines Exit EPA Disclaimer 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 signicant (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. If 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.

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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/EEC).

OPP'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.

EPA uses a tiered approach to estimate EECs, generally 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 Web site.

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.

Terrestrial Animals

For terrestrial animals, EPA estimates dietary exposure for birds and small mammals by calculating residues of pesticides on food items. At the present time, scientists assume that the organisms are exposed to a single pesticide residue in a given exposure scenario. Estimation of exposure and effects from multiple chemical residues is an area under development.

In addition, EPA is currently exploring methods for estimating exposure from other pathways, such as inhalation and dermal adsorption. Recently, EPA scientists presented preliminary terrestrial models for estimating exposure from inhalation and dermal adsorption at an international scientific meeting.

EPA recognizes the importance of drift for spray application scenarios and has cooperated with industry in the development of spray drift experimental data sets and drift modeling. See Spray Drift Task Force Exit EPA Disclaimer for information about an industry-sponsored spray drift model.

In situations where a pesticide formulation may be more toxic to terrestrial animals than the active ingredient, EPA may consider terrestrial exposure to the formulation. The Agency's approach for considering formulated product exposure in a terrestrial risk assessment follows approaches developed by the European Union (EU Council Directive 91/414/EEC).

The approach EPA uses for estimating exposure to terrestrial animals depends on the application method. The two types of application methods include:

  1. spray applications and
  2. granular, bait, and treated seed applications.
  1. Spray Applications

    For spray applications, EPA estimates pesticide concentrations in animal food items. In this approach, the focus is on quantifying possible dietary ingestion of residues on vegetative matter and insects.

    Residue estimates are based on a nomogram (chart) that relates food item residues to pesticide application rate. 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.

    • The first tier of the nomogram uses the maximum predicted residues; subsequent refinements may consider mean or average residues.

    • Residues may be compared directly with dietary toxicity data or converted to an oral dose.

    • 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
    (mg/kg)     		Average EEC
    (mg/kg)
    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)

    How EECs from Spray Application Analyses Are Used

    • Avian Assessments

      • Concentration is compared to a toxicity endpoint.

        (EEC/LC50 or NOEC) or

      • Concentration is converted to a daily dose and compared to a toxicity endpoint.

        (EEC x Ingestion Rate)/BW/LD50

        where IR = (0.648(BW)0.651)/1000

      EPA uses three body weight categories for birds: 20g, 100g, 1000g

    • Mammal Assessments

      Concentration is converted to daily dose and compared to the toxicity endpoint.

      Three body weights are used.

      (EEC x percent body weight consumed daily)/LD50 or NOEL

      Ingestion Rate (% of body weight)
      Body Weight
      (grams)      		 Herbivore/Insectivore
      (% body weight consumed/day)      		 Granivore
      (% body weight consumed/day)
      159521
      356615
      1000153
  2. Granular, Bait, and Treated Seed Applications

    For granular, bait, and treated seed applications, EPA estimates the amount of pesticide per unit area for avian and mammal assessments. This approach considers observed effects in field studies and relates them to pesticide applied to surface area of the field. It is intended to represent exposure via multiple routes and not just direct ingestion. In developing its exposure assessments, EPA uses the following assumptions:

    • The label rate of application for the active ingredient (a.i.) is the basis for the exposure term.

    • In-furrow applications assume 1% of granules, bait, or seed are unincorporated.

    • Banded treatments assume 15% of granules, bait, seeds are unincorporated.

    • Broadcast treatment without incorporation assumes 100% of granules, bait, seeds are unincorporated.

    Equations for Calculating Product and Toxicant per Square Foot

    Exposed toxicant per square foot requires several types of calculation methods depending on whether the material is applied in rows or broadcast over the entire application site.

    • Broadcast

      [(lbs product/acre)(% a.i.) (453,590 mg/lb)] / 43,560 ft2/acre = mg a.i./ft2

    • Row/band/in-furrow

      • [(oz product/1000 ft row)(28,349 mg/oz)(% a.i.)(1-% incorporated)] / (1,000 ft)(band width ft) = exposed a.i. mg/ft2 or

      • [(lbs product/acre)(% a.i.) (453,580 mg/lb)(1-% incorporated)] / {[(#rows)(row width ft)(row length ft)] / 43,560 ft2/acre } = exposed a.i. mg/ft2

        Where:

        # rows = (square root 43,560 ft2)/row space ft

        row width = width of band or furrow in feet (crop specific)

        row length = square root 43,560 ft2

    How EEC's from Granular, Bait and Treated Seed Application Analyses Are Used

    After the amount of pesticide per unit area is calculated, it is divided by the LD50.

    [ (mg/ft2) x (% of available pesticide) ] / LD50 ] = RQ

    The results of this comparison is a risk quotient (RQ), which is explained in the next chapter on risk characterization.

Non-Target Plants

For aquatic plants, EPA uses the aquatic model GENEEC2 and the spray drift model Ag DRIFT 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 and semi-aquatic plants, EPA has developed runoff and spray drift scenarios that are based on a pesticide's water solubility and the amount of pesticide on the soil surface and its top one inch layer.

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.

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.

Water Resources

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

REFERENCE INFORMATION

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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