Critical predicted no effect concentrations (PNECs) should not be based on a single toxicity test

Area of Science:

• Environmental toxicology
• Ecotoxicology
• Risk assessment

Background:

• Predicted no-effect concentrations (PNECs) are crucial for environmental risk assessment, policy, and regulation.
• PNECs are typically derived from single-species laboratory toxicity tests, often relying on the most sensitive endpoints.
• The reliability of these sensitive endpoints is critical for accurate risk assessment.

Purpose of the Study:

• To emphasize the importance of assessing the reliability of sensitive endpoints used in PNEC derivation.
• To present a case study on the variability of toxicity test results for Daphnia magna.
• To provide recommendations for improving the robustness of PNEC calculations.

Main Methods:

• Conducted five 21-day Daphnia magna toxicity tests using identical procedures across two laboratories.
• Assessed the 20% inhibitory concentration (IC20) response to a specific ionic composition of total dissolved solids.
• Analyzed the variability in IC20 results and the characteristics of the concentration-response curve.

Main Results:

• IC20 values for total dissolved solids varied significantly, ranging from 684 mg/L to over 1510 mg/L.
• The concentration-response curve was observed to be shallow, indicating potential for high variability.
• Differences in results could be attributed to random chance due to the shallow curve.

Conclusions:

• Relying on a single toxicity test result for sensitive endpoints can lead to unreliable PNECs.
• The geometric mean of at least three test results should be used to determine PNECs.
• This approach helps to adequately assess and bound test variability, particularly with shallow concentration-response curves.