Guidance on the use of probabilistic methodology for modelling dietary exposure to pesticide residues

The European Food Safety Authority (EFSA) asked the Panel on Plant Protection Products and their Residues to provide guidance on methodology for performing probabilistic dietary exposure assessment of single or multiple active substances, as a potential additional tool to supplement or complement the standard deterministic methodologies which are currently used in the EU for conducting dietary exposure assessments for pesticides. Specific guidance is provided for basic assessments but not for refined assessments, where specialized expertise is required to select methods appropriate to the assessment in hand. The guidance includes probabilistic methods for quantifying some of the major sources of variability and uncertainty affecting dietary exposure to pesticides. Other important sources of variability and uncertainty might be quantified probabilistically in refined assessments but are addressed more simply in basic assessments by conducting alternative model runs with optimistic and pessimistic assumptions. Guidance is provided on problem formulation, including definition of appropriate scenarios for acute and chronic exposure assessment in the differing contexts of approval of new substances, MRL setting, authorisation of products, evaluation of residues found above the MRL, and annual reviews of residue monitoring data. Detailed guidance is provided on methodology for probabilistic modelling of acute and chronic exposures and for quantifying variability and uncertainty in food consumption and residues. For basic probabilistic assessments, optimistic and pessimistic assumptions are used for the effects of processing and for residues below the level of reporting. A separate section is devoted to additional approaches required for modelling exposure to multiple substances (cumulative assessment): the use of relative potency factors to cumulate exposures to different substances, and a basic methodology for addressing gaps in data on the co-occurrence of residues of different substances. Specific guidance is provided on the types and formats of outputs that should be produced from a probabilistic assessment. Particular emphasis is placed on characterising the upper tail of the exposure distribution and on 'drill down' techniques to evaluate the reliability of the estimates. A general approach is recommended for evaluating uncertainties affecting the model outputs. An appendix to the guidance describes uncertainties associated with the methodology recommended in this guidance, and provides a general evaluation of their potential impacts on estimated exposures. The guidance also includes a checklist of key issues to be considered when writing or peer-reviewing reports on probabilistic exposure assessments, a discussion of approaches to validating probabilistic assessment approaches, and a list of desirable characteristics of software for probabilistic exposure modelling. Some comments are provided on the interpretation of results, while recognising that risk management is outside the remit of EFSA. Case studies are included in an appendix, illustrating some but not all of the recommended approaches. Further work will be required to make the methods in this guidance available to end-users in more practical form, including software and more specific user instructions. Some recommendations on this are provided.