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Toxicokinetic/Toxicodynamic (TKTD) effect models for regulatory risk assessment of pesticides for aquatic organisms

EFSA has recently published a Scientific Opinion on the state of the art of Toxicokinetic/Toxicodynamic (TKTD) effect models for regulatory risk assessment of pesticides for aquatic organisms (, developed by the Panel on Plant Protection Products and their Residues (PPR). This Scientific Opinion focuses on the regulatory questions that can be answered by TKTD modelling, the available TKTD models for aquatic organisms, the selection of species to be modelled and the model parameters that need to be included and checked in evaluating the acceptability of regulatory relevant TKTD models.

TKTD models may be used to refine the risk assessment when experimental effect assessment approaches in Tier‐1 (based on standard test species) and Tier‐2 (based on standard and additional test species), in combination with an appropriate exposure assessment, trigger potential risks. This EFSA Scientific Opinion mainly deals with TKTD models as Tier-2C tools of the acute and chronic Environmental Risk Assessment (ERA) schemes for pesticides and water organisms in edge-of-field surface waters (EFSA Journal 2013; 11(7):3290). It establishes three different classes of TKTD models that are intended to be used in risk assessment. These are ‘General Unified Threshold models of Survival’ (GUTS) to predict survival of individuals (influenced by mortality/immobility), toxicity models derived from the Dynamic Energy Budget theory (DEBtox models) for sublethal endpoints (growth and reproduction) and models for primary producers that measure aquatic macrophytes and algal growth. TKTD models need to be calibrated and validated for the Plant Protection Product (PPP) and surrogate species of concern in order to be used directly in prospective ERA, with no extrapolation across levels of biological organisation (i.e. from individual to population level) allowed by the use of TKTD models alone.

Finally, it is concluded that the GUTS model and the primary producer Lemna model are considered ready to use in risk assessment, while the DEBtox modelling approach is, so far, limited to research applications based on the current lack of documented and evaluated examples, although its great potential for future use in ERA of PPPs is recognised.

For advice on risk assessment and further information on the registration of plant protection products, biocides or general chemicals, or any other regulatory issues, please contact ERM on +44 (0)1423 520245,

Published 31st August 2018
Categories Agrochemicals, Links, News
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