Can persistence be considered an intrinsic property of chemicals? The need to set up relevant conditions for testing under chemical regulatory frameworks

Persistence, defined as the resistance of chemicals to environmental degradation, is a central criterion in regulatory hazard assessments under the European Union Registration, Evaluation, Authorisation, and Restriction of Chemicals and Labelling and Packaging regulations. It determines both long-term exposure potential and the reversibility of environmental contamination. The Organisation for Economic Co-operation and Development (OECD) Test Guideline (TG) 309, which assesses aerobic mineralisation in surface water, is a cornerstone method for deriving degradation half-lives used in Persistence and very Persistent (P/vP) classifications. However, despite its regulatory status, TG 309 suffers from significant variability in outcomes, raising concerns over its reliability, reproducibility, and suitability for determining an intrinsic property. This article reviews the European regulatory framework and critically examines inoculum properties (e.g.,, source, suspended solids, total and dissolved organic carbon, microbial density, pH, seasonality) and technical factors (e.g.,, inoculum storage, temperature, test concentration, adaptation vs. cometabolism) that influence test results. While research has identified several parameters affecting biodegradation kinetics, many studies deviate from regulatory conditions, limiting their applicability (e.g.,, pre-exposed inoculum, sediment addition, test temperature…). Current evidence highlights the so-called "lottery effect," whereby even readily biodegradable substances may sometimes be classified as Persistent, undermining the test's regulatory robustness. It is concluded that no set of universally relevant conditions currently ensures reproducible determination of Persistence as an intrinsic property. This uncertainty poses significant regulatory challenges, particularly as new hazard classes such as PMT/vPvM expand the scope of Persistence assessments. Research is urgently needed to clarify the role of microbial population dynamics, inoculum sources, and organic carbon composition, and to explore possible refinements that could reduce test variability while maintaining regulatory acceptability.