Scientific concepts and methods for moving persistence assessments into the 21st century
Authors: Davenport Russell , Curtis-Jackson Pippa , Dalkmann Philipp , Davies Jordan , Fenner Kathrin , Hand Laurence , McDonough Kathleen , Ott Amelie , Ortega-Calvo Jose Julio , Parsons John R. , Schäffer Andreas , Sweetlove Cyril , Trapp Stefan , Wang Neil , Redman Aaron 
Published: January 2022

Abstract
The evaluation of a chemical substance's persistence is key to understanding its environmental fate, exposure concentration, and, ultimately, environmental risk. Traditional biodegradation test methods were developed many years ago for soluble, nonvolatile, single-constituent test substances, which do not represent the wide range of manufactured chemical substances. In addition, the Organisation for Economic Co-operation and Development (OECD) screening and simulation test methods do not fully reflect the environmental conditions into which substances are released and, therefore, estimates of chemical degradation half-lives can be very uncertain and may misrepresent real environmental processes. In this paper, we address the challenges and limitations facing current test methods and the scientific advances that are helping to both understand and provide solutions to them. Some of these advancements include the following: (1) robust methods that provide a deeper understanding of microbial composition, diversity, and abundance to ensure consistency and/or interpret variability between tests; (2) benchmarking tools and reference substances that aid in persistence evaluations through comparison against substances with well-quantified degradation profiles; (3) analytical methods that allow quantification for parent and metabolites at environmentally relevant concentrations, and inform on test substance bioavailability, biochemical pathways, rates of primary versus overall degradation, and rates of metabolite formation and decay; (4) modeling tools that predict the likelihood of microbial biotransformation, as well as biochemical pathways; and (5) modeling approaches that allow for derivation of more generally applicable biotransformation rate constants, by accounting for physical and/or chemical processes and test system design when evaluating test data. We also identify that, while such advancements could improve the certainty and accuracy of persistence assessments, the mechanisms and processes by which they are translated into regulatory practice and development of new OECD test guidelines need improving and accelerating. Where uncertainty remains, holistic weight of evidence approaches may be required to accurately assess the persistence of chemicals. Integr Environ Assess Manag 2022;18:1454–1487. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).
KEY POINTS
This critical review addresses the challenges and limitations facing current test methods used in persistence assessment and the scientific advances that are helping to both understand and provide solutions to them. Robust methods are now available to provide a deeper understanding of microbial composition, diversity, and abundance, ensuring consistency and/or interpret variability between tests; and quantify parent and/or metabolites at low concentrations informing bioavailability and biotransformation rates/paths. Benchmarking tools and reference substances are now available that aid in persistence evaluations through comparison against substances with well-quantified degradation profiles. Modeling tools are available that predict the likelihood of microbial biotransformation and biochemical pathways, and allow for derivation of more applicable biotransformation rate constants, by accounting for physical and/or chemical processes and test system design when evaluating test data.