Multi-decade changes in pollen season onset, duration, and intensity: a concern for public health?

Abstract

BACKGROUND AND AIM: Climate change alters pollen seasons, a public health concern for the growing number of individuals with pollen sensitization. Prior aerobiological research has inconsistently applied definitions of the main pollen season (MPS) and drawn heterogeneous conclusions regarding the magnitude, directionality, and significance of long-term pollen trends. We aimed to better understand long-term shifts in MPS onset, duration and intensity through multi-definition analysis. METHODS: We examined national pollen data in Switzerland between 1990 and 2020, applying six MPS definitions (two retrospective percentage-based, four prospective threshold-based) to twelve relevant allergenic plant species. We analyzed long-term pollen season changes using both linear regression and locally estimated scatterplot smoothing (LOESS). RESULTS:For four of twelve plants, there is unanimity between the six definitions regarding earlier onset of the pollen season (p0.05), with magnitude of 31-year change, dependent on specific MPS definition (hazel: 9-18 days; oak: 5-13 days; grasses: 8-25 days; and nettle/hemp: 6-25 days). There is also consensus for altered MPS duration among hazel (21-104% longer), nettle/hemp (8-52% longer), and ash (18-38% shorter). Between-definition agreement is highest for MPS intensity analysis, showing significant increases in seasonal pollen quantity among hazel, birch, oak, beech, and nettle/hemp. The largest relative intensification is noted for hazel (110-146%) and beech (162-237%). LOESS analysis indicates that these multi-decade pollen changes are typically nonlinear. CONCLUSIONS:Pollen season onset, duration, and intensity have changed for multiple allergenic plants in Switzerland. We found an increase in seasonal pollen quantity emitted for five of twelve species, thus increasing the exposure burden on the allergic population. We identify systematic between-definition differences affecting magnitude and significance of long-term trends, and suggest future studies addressing aeroallergen exposure apply multiple MPS definitions to minimize bias. Further research can determine whether these pollen season shifts correlate with longitudinal differences in population pollen sensitization. KEYWORDS: Climate, Exposure assessment, Environmental epidemiology, Allergies