Airborne Microplastics in indoor environments: current knowledge, methodological challenges, and future directions

Indoor environments are increasingly recognized as notable reservoirs and exposure pathways for airborne microplastics (AMPs). This narrative review shows the outcomes from 57 observational studies published between 2017 and 2025, focusing on the occurrence, abundance, and characteristics of MPs in residential and collective indoor spaces. Methodological variability strongly affects the comparability of results. In sampling, 56 % of studies used active air collection, 37 % passive techniques, and 5 % combined strategies. Different pretreatment and analytical approaches were employed, including microscopy, & micro;-FTIR, & micro;-Raman, and Py-GC-MS. Reported concentrations range from 0.5 to 14,088 MPs/m & sup3; in residential settings and from 2 to 8305 MPs/m & sup3; in collective environments, exceeding outdoor control levels (mean similar to 260 MPs/m & sup3;) by 1 to 2 orders of magnitude. Temporal and spatial variability is a key factor influencing concentrations, along with synthetic textiles, limited ventilation, finishing materials, the number of occupants, and seasonal effects. Fibers dominate in most indoor spaces, with polyester, polyethylene, polypropylene, and nylon as the most frequently identified polymers. The human exposure assessment suggests AMP intake ranging from hundreds to several thousand particles per day, with children potentially assuming higher doses per body weight. However, variability in dose metrics limits comparability across studies. Overall, the findings underscore indoor air as a critical but underexplored exposure pathway, with significant knowledge gaps regarding methodologies and toxicological implications. This review outlines unmet needs and proposes a roadmap to move towards harmonized protocols integrating toxicological assays as priorities to strengthen risk assessment and inform mitigation strategies for AMPs in indoor environments.