Challenges in human micro- and nanoplastic risk assessment: stocktaking and the road ahead

Abstract Environmental hazards of micro- and nanoplastics (MNPs) and associated chemicals have been investigated for more than a decade. However, the field of human MNPs health risk assessment is still confronted by many uncertainties regarding exposure, fate, as well as the identification of potential health effects. Among others, the lack of reference test MNPs, natural particles as validated reference control, and toxicological test methods that are tailored for MNPs and specifically address the characteristic properties of MNPs, further contribute to data scarcity and hence complicate human risk assessment. Elucidating the complex relationship between MNPs and human health, from early life to adulthood, has been addressed by five European research projects. The aim of these projects organized within the European research cluster to understand the health impacts of MNPs (CUSP) was to provide reliable data regarding human exposure and hazard posed by MNPs. However, in view of the numerous properties of MNP, it is impossible to test all variants and the mixture of associated chemicals. Hence, pragmatic approaches are needed as substitute for experimental testing, including strategies to deal with data gaps and the associated uncertainties, whilst considering the specific properties of MNPs. In this work, partners from the CUSP projects PlasticsFatE and POLYRISK critically review and structure the data needs for human MNP risk assessment and propose options for prioritization to overcome the identified general and material-specific challenges. We identified six priority areas for future research, covering, (1) Quality assurance and quality control (QA/QC), 2) MNP material provision with various compositions and properties, (3) Appropriate control particles for toxicity testing, (4) Exposure and hazard assessment for plastic associated chemicals (incl. plastic additives), (5) Assessing the joint effects of MNP and associated chemicals, and (6) Characterization of functional biomarkers for MNP exposure and effect. These priority areas can be used to structure future research in the field.