Factors beyond anthropogenic climate change affecting airborne pollen levels in urban areas

Climate change is increasing airborne pollen concentrations, exposing populations to unprecedented allergenic levels and intensifying respiratory diseases. However, these changes are not exclusively climate-driven, particularly in urban environments where the structure, distribution, abundance and management of the vegetation is conditioned by human activity. This study provides an analysis of the temporal trends of airborne pollen levels from anthropically managed tree species in centres of high population density such as the metropolitan area of the Madrid Region (central Spain), one of the most densely populated in Europe. The study was done by evaluating the time trends in the Annual Pollen Integral and the number of days of allergenic risk throughout the 1996-2024 period, along with meteorological variables and changes in land cover, then integrating all this information using a Random Forest machine learning model. Results show a widespread increase over the two last decades. Notably, significant APIn increases reached up to 1200 and 2000 pollen grains*day/m³/year for Cupressaceae and Platanus, respectively. Moraceae showed the most consistent APIn increase with a maximum rate of change of 250 pollen grains*day/m³/year. Maximum temperature emerged as a key meteorological driver, showing consistent upward trends. Crucially, land-cover changes constitute a primary factor in airborne pollen loads and act synergistically with climate change. This highlights the critical role of urban growth and tree management. Consequently, mitigating this complex public health challenge requires multi-scale interventions, from large-scale urban planning to local green-space actions.