Long-term trends in pollen season characteristics in Stockholm, Sweden (1973–2024)

• 52 years of daily pollen monitoring (1973–2024) analysed for nine taxa • Season duration increased for birch, willow, elm, grasses, and mugwort • Seasonal Pollen Integral rose for hazel, oak, and grasses, while elm declined • Growing Degree Days (GDD) strongly predicted onset and end dates in most taxa Climate change is increasingly altering the timing and magnitude of airborne pollen exposure, with implications for population health. Long-term aerobiological monitoring provides critical evidence of these shifts and their environmental drivers. We analysed 52 years of daily ambient pollen records (1973–2024) for nine taxa ( Alnus, Artemisia, Betula, Corylus, Pinus , Poaceae, Quercus, Salix, Ulmus ) in Stockholm, Sweden, using data from a Hirst-type volumetric spore trap. Pollen season metrics (start, end, peak, duration) were calculated from standardized thresholds. Meteorological data on Growing Degree Days (GDD) and precipitation were incorporated into multivariable regression models to evaluate pollen season timing and pollen production. Marked shifts in pollen timing were observed across taxa. Most arboreal species showed substantially earlier season starts (up to 31 days) and advances in peak and end dates. Season duration increased for Poaceae and Artemisia as well as Betula, Salix , and Ulmus . GDD was strongly associated with earlier onset and, to a lesser extent, end dates, confirming the influence of accumulated heat on phenological advancement. Seasonal Pollen Integral (SPIn) increased for several taxa (e.g. Corylus, Quercus , Poaceae), while Ulmus displayed a pronounced decline. Our findings demonstrate significant long-term changes in both the timing and intensity of pollen seasons in northern Europe, consistent with regional climate change. GDD emerged as a robust predictor of pollen seasons. These results highlight the importance of sustained monitoring and provide evidence to inform public health planning and adaptation strategies in urban environments.