The EU regulates chemicals to protect the environment, but different regulations apply depending on the intended use of the substances in question. Advances in chemical monitoring have revealed inconsistencies in environmental risk assessments, prompting the European Commission to advocate for a "one substance, one assessment" (OS-OA) approach. As part of the EU Partnership for the Assessment of the Risk from Chemicals (PARC), this study examines the implementation and outcome of environmental risk assessments carried out within different regulatory frameworks, focusing on the aquatic environment. Using case studies on imidacloprid and deltamethrin, we highlight methodological differences and their implications for OS-OA. Our analysis reveals little consistency in the data used by assessments in different regulatory processes. This is partially due to the historical development of risk assessments. In addition, we show that new scientific data is not consistently incorporated into risk assessments, leading to delays and prolonged environmental risks. Finally, we demonstrate that low analytical sensitivities contribute to low detection rates in monitoring programs and hinder comparisons between prospective and retrospective risk assessments. Differences in data accessibility and validity assessments, and lack of collaboration and varying involvement of Member States contribute to these inconsistencies and underscore the need for standardized methods. As a result of different data requirements, models, and protection goals, isolated risk assessments of the same substance lead to divergent outcomes. In cases where data gaps exist, and safe uses cannot be demonstrated, we strongly recommend read-across of hazard assessments between regulations to support more efficient regulatory conclusions. In summary, our findings highlight the need for coherent regulation and risk assessment to improve environmental protection in the EU.
Kröll et al. (2026) studied this question.