Unveiling the hidden pollutants in the indoor environment: Focus on microplastic pollution and its related risks in the educational institutions of megacity, Bangladesh

Microplastics (MPs), defined as plastic particles smaller than 5 mm, are an emerging environmental contaminant of increasing concern due to their potential adverse effects on human health. While indoor environments represent a significant location for human exposure, data on MP occurrence and characteristics in these settings remain limited, particularly in densely populated urban areas. This study first investigated the abundance, distribution, characteristics, and potential risks of MPs in settled dust collected from classrooms within educational institutions in the megacity of Bangladesh. Microplastics were extracted from dust samples using wet peroxide oxidation. Quantification and morphological characterization (shape, size, color) were performed using stereomicroscopy. Polymer identification was conducted using Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). Scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS) was employed to analyze surface morphology and elemental composition. The mean abundance of MPs was 16436.67 ± 8534.06 n/kg, with considerable variability observed across sampling locations. Fibers comprised the predominant shape (89 %), with the 100–500 μm size range being most prevalent (44 %). Black was the most frequently observed color (35.5 %). Polystyrene (PS) was the most abundant polymer type (40.6 %), followed by polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polyamide (PA), polyvinyl chloride (PVC), and polyurethane (PUR). The pollution load index (PLI) values exceeded 1, indicating significant MP contamination. Other risk assessment indices, including the contamination factor (range: 1.00–6.14) and the Nemerow pollution index (range: 4.40–6.14), suggested moderate to high levels of MP pollution. The polymeric hazard index (269.33–1150.73) indicated considerable to high polymer-associated risks. SEM-EDS analysis revealed the presence of secondary MPs and associated potentially toxic elements (e.g., Hg, Pb, As). Principal component analysis (PCA) suggested that both indoor activities and the outdoor environment contribute to MP pollution in classrooms. Estimated daily intakes (EDIs) via dust ingestion were 0.39 ± 0.20 particles/day for children and 0.29 ± 0.15 particles/day for adults. Lifetime cancer risk (LTCR) assessments indicated a potential for moderate carcinogenic risk for both exposure groups, with children exhibiting a higher estimated risk. These findings highlight the need for further research to fully characterize indoor MP exposure's sources, pathways, and health risks and develop effective mitigation strategies.