Index-based pollution and environmental risk assessment of microplastics: Handbook for a mindful practice

Microplastic (MP) pollution is a major environmental issue due to its potential impacts on ecosystems and human health. After more than 20 years of research and over 7000 publications, sufficient knowledge is now available to support the first ecological risk assessments. In this context, risk indices have emerged as tools for evaluating environmental quality and MP contamination. This review analyzed 361 studies investigating MP pollution and risk assessment indices, highlighting their strengths, limitations, and future development needs. Since the first publication in 2018, research activity has grown exponentially. Initially dominated by studies from Asia, the topic has recently expanded into a global research field. Most studies have been published in high-impact journals, reflecting the importance of risk indices in MP monitoring and assessment. However, this growth has led to the proliferation of numerous and highly similar indices (31 metrics proposed), sometimes applied with methodological inconsistencies, creating confusion among researchers and practitioners. The most widely used are the Pollution Load Index (PLI), the Polymer Hazard Index (PHI), and the Potential Ecological Risk Index (PERI), mainly applied to water samples, with 73.9% of studies used in combination. Overall, MP indices can be classified into three groups: those based on MP abundance, those focusing on a single MP characteristic, and multidimensional indices integrating multiple traits, such as size, shape, and polymer type. We conclude that multidimensional approaches should be prioritized, supported by a shared assessment framework and the consistent application of existing indices rather than the continued development of new overlapping metrics.