Roadside trees along school routes enhance foliar particulate-matter retention but increase children's...

Children's school commutes represent microenvironments where exposure to traffic-related particulate matter (PM) occurs at breathing height. Roadside trees are often promoted to reduce PM by capturing particles on their leaves, but dense tree crowns can also limit ventilation, increasing concentrations at pedestrian level. We quantified these effects along frequently used access routes to primary schools in Łódź, Poland. In June 2022, we measured PM2.5 and PM10 at 1.4 m above ground during commuting periods (n = 242). We assessed tree crown structures using two indicators: horizontal crown density (leaf area index, LAI) and vertical crown density (the proportion of tree crowns obstructing visibility). Our mixed-effects models showed that greater vertical crown density was associated with higher airborne PM: each one percentage-point increase in vertical crown density corresponded to roughly 1.9% more PM at child height. Higher horizontal crown density (LAI), by contrast, improved foliar retention - a 1% increase in LAI was associated with 0.52% and 0.56% higher foliar PM₁₀ and PM₂.₅, respectively. Thus, while roadside trees offer particulate deposition benefits, they may also increase pedestrian exposure. Urban planning should consider the balance between horizontal crown density and vertical crown density to limit airflow obstruction, rather than assuming higher vegetation density always enhances air quality.