Urban airborne allergenic pollen shows highly local patterns correlated with public green spaces

Ongoing climate change is increasing global temperatures, with cities experiencing amplified warming due to the urban heat island effect. Urban trees, forbs, and grasses are widely planted to mitigate heating. However, many plant species produce allergenic pollen, causing hay fever symptoms that can discourage people from using green spaces. This avoidance may increase heat exposure and reduce the physical and mental benefits of urban greenery. Despite this, optimal urban green design that minimizes allergy risks remains unclear, partly due to limited knowledge of local airborne pollen dispersion in urban heat islands. To address this gap, we investigated whether (1) local vegetation influences urban pollen concentrations and composition, (2) urban trees, forbs, or grasses contribute most to allergenic pollen loads, and (3) public or private urban spaces are the main source of airborne allergenic pollen. We conducted spatial and temporal street-level pollen monitoring at 10 sites across two neighborhoods in Leiden, the Netherlands, over 12 months. Samples were analyzed using DNA metabarcoding and microscopy. We identified 366 plant species across 105 families. Results show that public green spaces are the primary source of allergenic pollen in cities. Grasses dominated pollen loads in summer, whereas trees contributed most in winter, spring, and autumn. Notably, allergenic tree pollen was detected well beyond peak flowering periods. These findings highlight the importance for critically selecting urban tree species and the use of more non-allergenic forbs and less grasses when designing healthy urban green infrastructure.