Global microplastic contamination levels indexing: A new approach using bivalve as sentinels

Microplastic (MP) contamination is a major component of the triple planetary crisis, of omnipresence posing substantial risks to biodiversity and food security, but global overviews of MPs contamination patterns are limited, with concentration comparison lacking categorization, especially across different environmental matrices. As the need for standardized monitoring intensifies, bivalves emerge as globally reliable sentinels, overall due to their filtration capacity, wide distribution, and economic relevance. Here, we proposed the Microplastic Contamination Index (MPCI), based on a simple yet effective separatrix approach of global data, categorizing concentrations from very low to severe. The index was applied for bivalves to introduce the concept worldwide, and aquaculture farms and protected areas were used as a study-case to assess food safety and ecological risks. We synthesized data from 234 articles, encompassing 3404 MP concentration records (particles.g^-1 ww) for 150 bivalve species across 49 countries from 2011 to 2024. Severe concentrations occurred across 24 countries, particularly in Argentina, Brazil, China, Indonesia and India. A total of 13 countries had bivalve farms under severe or very high contamination levels, and protected areas with severe of MP contamination. Global patterns revealed the consistent dominance of polyethylene, small (<500 µm), fibers, and blue MPs, mirroring other environmental matrices' trends. The MPCI_Bivalve provided an overview on global MP levels, identifying severe levels, research gaps, geographical distribution. This approach can be extended to other environmental matrices, offering a powerful tool to monitor progress and guide decision-makers as support evidence-based policies to mitigate MPs contamination through international agreements.