Small-sized microplastics (< 500 μm) in roadside soils of Beijing, China: Accumulation, stability, and human exposure risk

The potential threats of microplastics (MPs) to human health in urban environment have received increasing attention. Small MPs substantially threaten soil organisms and human beings because they are easily ingested and may adsorb other pollutants to form composite compounds. However, roadside soils as a source and a sink of small MPs in urban environments have not been reported. Here, we studied the small-sized MPs (SMPs, < 500 μm) pollution and human exposure analysis by investigating the MPs in roadside soils in Beijing. SMPs contributed to 88.06% of the high average MPs abundance (22001.67 ± 24482.34 items/kg), and were categorized as uncontaminated to moderately contaminated, assessed by the geo-accumulation index (I). SMPs mainly consisted of polypropylene (PP) and polyethylene (PE) with morphotypes of fragment and granule. In particular, highly stable and fragmented MPs were discovered by the conditional fragmentation model. And normal-sized MPs (NMPs 500-5000 μm) NMPs could further fragment into SMPs, indicating that SMPs in roadside soil were further enriched. The estimated infant exposure to SMPs was greater than that in adults, and independent of the polymer types and morphotypes. PP is currently the main pollutant, contributing 65% to the estimated human exposure. Combining the conditional fragmentation model and main polymer types, SMPs likely derived from containers/packaging and fertilizers, and affected by atmospheric transport. This study demonstrated the dominance and potential risk of SMPs in roadside soil, suggesting that health risk assessment for SMPs in urban environment necessitates critical investigation.