Historically chronic or long-term studies performed for the authorisation of plant protection products (PPP) result in the reporting of endpoint values in terms of No-Observed Effect Concentration (NOEC). The related Test Guidelines (OECD, ISO, EPA) were designed to fulfil the data requirement for NOEC determination. NOEC values are however criticised since their values strongly depends on the experimental study design, and actually the extrapolation of Effect Concentration affecting a percentage x of the test organisms or where a percentage x of an effects is observed (Ecx) is considered more appropriate since it takes into account the whole concentration-response curve. The new Regulation (EC) No. 1107/2009 for the authorisation of PPPs and the related data requirements (Commission Regulation (EU) No. 283/2013 and 284/2013) require that ecotoxicological endpoint data from chronic or long-term studies provided by the Applicant are reported as EC10 or EC20 values together with the NOEC. However, there is no systematic comparison available to compare NOEC values to EC10 and EC20 values derived from the same study. In the present work, 200 long term and chronic studies on aquatic organisms, equally distributed among fish, daphnids, algae and aquatic macrophytes, are re-analysed in order to calculate NOEC, EC10, EC20, EC50 and their limit of confidence (from the same study) using appropriate statistical analyses. A comparison of NOEC with Ecx values and their lower limit of confidence is performed by analysing the distribution of the NOEC/Ecx ratios. Considerations are made on studies based on the same organisms and on the study design (appropriately developed to calculate EC50 for algae and macrophytes and NOEC for fish and daphnids).
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