Seasonal variation of microplastics in indoor environments from Birmingham, UK: Implication for ingestion and inhalation exposure

Despite growing scientific and public awareness and concern over microplastics (MPs) as emerging contaminants, human exposure in indoor environments remains poorly understood. This study investigates seasonal variation of MP concentrations and characteristics in five indoor microenvironments over a 12-month monitoring period for the first time, evaluating impacts on human exposure via inhalation and inadvertent dust ingestion. Average MP concentrations were 176.2 ± 96.7 MP/mg in settled dust and 9 ± 2.3 MP/m³ in air. Statistical analysis revealed significantly higher concentrations (P < 0.05) in summer compared to autumn and spring, respectively. Substantial seasonal variation occurred in MP profile characteristics (shape and size distribution), with several differences proving statistically significant at 95% confidence. These observed differences, attributed to factors including ventilation, cleaning frequency, humidity, variable indoor sources, and human activity causing wear of plastic items, resulted in statistically significant seasonal variability in estimated daily intakes for both adults and toddlers. This indicates that prolonged monitoring studies are required to accurately represent human exposure to MPs indoors and raises concern over the representativity and comparability of existing human exposure data from different studies based on single "spot" samples collected from various indoor microenvironments in different seasons.