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Technical Overview of Ecological Risk Assessment
Risk Characterization
Contents
About Risk Characterization
Risk characterization is the final phase of the ecological risk assessment. The risk characterization:
integrates the analyses from the exposure characterization and ecological effects characterization;
describes the uncertainties, assumptions, and strengths and limitations of the analyses; and
synthesizes the overall conclusion about risk that is used by risk managers in making risk management decisions.
Risk characterization has two major components: risk estimation and risk description. Risk estimation compares exposure and effects data, considers integrated exposure and effects data in context of Levels of Concern (LOCs), and states the potential for risk. The risk description interprets risks based on assessment endpoints. In interpreting the risk, the risk assessor evaluates the lines of evidence supporting or refuting risk estimates in terms of the following factors:
Adequacy and quality of data
Degree and type of uncertainty
Relationship of evidence to risk assessment questions
For a risk characterization to be useful to risk managers, it must be transparent, clear, consistent, and reasonable (the TCCR principles). Once the risk characterization is finalized, it may be used as the basis for producing fact sheets, press releases, technical briefings, and other communication products.
Deterministic Approach
For most risk assessments, EPA uses a deterministic approach or the quotient method to compare toxicity to environmental exposure. In the deterministic approach, a risk quotient (RQ) is calculated by dividing a point estimate of exposure by a point estimate of effects. This ratio is a simple, screening-level estimate that identifies high- or low-risk situations.
Calculation of risk quotients are based upon ecological effects data, pesticide use data, fate and transport data, and estimates of exposure to the pesticide. In this method, the estimated environmental concentration (EEC) is compared to an effect level, such as an LC50 ( the concentration of a pesticide where 50% of the organisms die.)
RISK QUOTIENT = EXPOSURE / TOXICITY
Terrestrial Animals
Acute risk quotients for birds are calculated using the following formulas:
Initial screening assessment
Peak nomogram concentration on short grass / Most sensitive bird LC50 = RQ
Granular formulation mg a.i./ft2 / Most sensitive bird LD50 = RQ
More comprehensive dietary risk screen
Peak nomogram concentration on each food item / Most sensitive bird LC50 = RQ
Chronic risk quotients for birds are calculated using the following formulas:
Initial screening assessment
Peak concentration on each food item* / Most sensitive bird reproduction NOEC = RQ
*Multiple application peaks are added.
More comprehensive assessment
Peak concentration food item** / Most sensitive bird reproduction NOEC = RQ
**Assessment considers pesticide dissipation for multiple application scenarios. Risk characterization may include discussion of the duration of exposure above the toxicity thresholds.
- OPP uses the Terrestrial Residue EXposure (T-REX) model to estimate acute and reproductive risk quotients for birds and mammals for each type of pesticide application. Go to the Terrestrial Models Web site for the T-REX spreadsheet and User's Guide.
Aquatic Animals
Acute Risk Quotients in fish and invertebrates are calculated using the following formula:
Peak water concentration / Most sensitive organism LC50 or EC50 = RQ
Chronic Risk Quotients in invertebrates are calculated using the following formula:
21-day average water concentration / Aquatic invertebrate chronic toxicity NOEC = RQ
Chronic Risk Quotients in fish are calculated using the following formula:
56-day or 60-day average water concentration / Fish early life-stage or full life-cycle toxicity NOEC = RQ
Plants
Non-Target Plant Risk Quotients are based on the most sensitive toxicity endpoint (e.g., plant height, dry weight, etc.).
For multi-use risk assessments, EPA scientists may calculate multiple RQ values for each labeled rate, method of application, interval, or formula. This allows the risk managers to see what uses offer the highest risk potential and those that do not offer high risk potential.
After the risk quotient is calculated, it is compared to the Agency's Level of Concern (LOC). An LOC is a policy tool that the Agency uses to interpret the risk quotient and to analyze potential risk to non-target organisms and the need to consider regulatory action. Several ecological LOCs, which are used in regulatory decision-making, are listed below:
Risk Presumptions for Terrestrial Animals
| Risk Presumption | RQ | LOC |
|---|---|---|
| Acute Risk | EEC/LC50 or LD50/ft2 or LD50/day | 0.5 |
| Acute Restricted Use | EEC/LC50 or LD50/ft2 or LD50/day or LD50<50 mg/kg | 0.2 |
| Acute Endangered Species | EEC/LC50 or LD50/ft2 or LD50/day | 0.1 |
| Chronic Risk | EEC/NOEC | 1.0 |
| Risk Presumption | RQ | LOC |
|---|---|---|
| Acute High Risk | EEC/LC50 or LD50/ft2 or LD50/day | 0.5 |
| Acute Restricted Use | EEC/LC50 or LD50/ft2 or LD50/day or LD50<50 mg/kg | 0.2 |
| Acute Endangered Species | EEC/LC50 or LD50/ft2 or LD50/day | 0.1 |
| Chronic Risk | EEC/NOEC | 1.0 |
Risk Presumptions for Aquatic Animals
| Risk Presumption | RQ | LOC |
|---|---|---|
| Acute High Risk | EEC/LC50 or EC50 | 0.5 |
| Acute Restricted Use | EEC/LC50 or EC50 | 0.1 |
| Acute Endangered Species | EEC/LC50 or EC50 | 0.05 |
| Chronic Risk | EEC/NOEC | 1.0 |
Risk Presumptions for Plants
| Risk Presumption | RQ | LOC |
|---|---|---|
| Acute High Risk | EEC/EC25 | 1.0 |
| Acute Endangered Species | EEC/EC05 or NOEC | 1.0 |
| Risk Presumption | RQ | LOC |
|---|---|---|
| Acute High Risk | EEC/EC50 | 1.0 |
| Acute Endangered Species | EEC/EC05 or NOEC | 1.0 |
Although the quotient method is useful for screening purposes, it only gives one point estimate of environmental risk.
Probabilistic Approach
With the publication of the "EPA Framework for Ecological Risk Assessment" and subsequent guidance from the Scientific Advisory Panel (SAP), EPA has moved towards developing new, more sophisticated methodologies for conducting probabilistic or refined risk assessments (RRA).
In a refined risk assessment, EPA incorporates probabilistic tools and methods to predict the magnitude of the expected impact of pesticide use on non-target organisms as well as the uncertainty and variability involved in these estimates.
A refined or probabilistic risk assessment produces a distribution or range of values instead of one fixed value. The assessment becomes more refined as the levels increase, presenting increasingly focused effects and exposure scenarios. Because the results of the refined risk assessment show the range of possible environmental impacts and which ones are most likely to occur, they provide a better basis for decision-making.
See the Initiative to Revise the Ecological Assessment Process for Pesticides: About Ecological Risk Assessment Web site for more detailed information on the refined risk assessment process.
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