Airborne Pollen Monitoring and Forecasting: Methods and Data Sources
Airborne pollen from wind-pollinated plants is among the most significant environmental aeroallergens globally. According to the World Health Organization, allergic rhinitis — for which airborne pollen is a primary trigger — affects an estimated 10–30% of the global population. This article describes how pollen concentrations are measured, what the forecast data represents, and which data sources currently underpin public pollen services.
How Airborne Pollen Is Measured
The internationally standardised method for pollen monitoring uses volumetric air samplers, most commonly the Hirst-type volumetric spore trap. These instruments draw ambient air past an adhesive-coated drum at a calibrated flow rate of 10 litres per minute. Pollen grains deposited on the surface are identified and counted by trained aerobiologists using optical microscopy.
Results are expressed in grains per cubic metre of air (grains/m³), which is the standard international unit adopted by monitoring networks including the European Aeroallergen Network (EAN). EAN aggregates data from stations across more than 30 European countries and publishes standardised daily pollen counts that form a key reference for model validation.
Pollen Concentration Categories
Threshold values separating Low / Moderate / High / Very High pollen categories vary by plant type, because different species produce markedly different absolute concentrations in the air while causing qualitatively similar allergenic impact. The European Academy of Allergy and Clinical Immunology (EAACI) has published separate threshold guidelines for major species: birch, grass, ragweed, olive, mugwort, and others. Forecast services use these thresholds to translate model output (grains/m³) into the categorical severity scale displayed to end users.
Major Allergenic Pollen Types and Their Seasons
Pollen release follows predictable phenological calendars driven by temperature accumulation and day length. The following table shows approximate seasons for the primary allergenic types in northern and central Europe:
| Pollen Type | Typical Season | Notes |
|---|---|---|
| Alder (Alnus) | January–April | Early spring, often the first major release |
| Birch (Betula) | March–May | High potency; major source in northern Europe |
| Grass (Poaceae) | May–August | Longest season; widest geographic spread |
| Olive (Olea europaea) | April–June | Dominant in Mediterranean climates |
| Mugwort (Artemisia) | July–October | Late summer; weed category |
| Ragweed (Ambrosia) | August–October | Invasive in central/eastern Europe; high potency |
Exact season timing shifts by several weeks across latitudes and varies year-to-year with accumulated growing-degree temperatures. Long-term phenological datasets, maintained by the European Phenology Network and national meteorological services, are used to calibrate emission models.
How Computer Models Forecast Pollen
Physical pollen forecast models combine three components:
- Phenological emission models — calculate pollen release rates based on plant species distribution maps, temperature accumulation, and degree-day thresholds that trigger anthesis (flowering)
- Atmospheric transport and dispersion — NWP models simulate pollen movement by wind, including long-range transport of hundreds of kilometres; daily concentration patterns are strongly influenced by boundary layer mixing height
- Wet deposition — precipitation scavenges pollen from the atmosphere; rainfall events are a primary driver of rapid day-to-day concentration changes
The Copernicus Atmosphere Monitoring Service (CAMS), operated by ECMWF, runs the CAMS European pollen forecasting system. This system uses the SILAM (System for Integrated modeLling of Atmospheric coMposition) dispersion model, developed by the Finnish Meteorological Institute, to produce daily multi-day forecasts for six major pollen types across Europe. CAMS pollen data are publicly available via the CAMS API.
Open-Meteo and Pollen Data
The Open-Meteo API provides freely accessible pollen forecast data derived from CAMS global and European model outputs. It covers alder, birch, grass, mugwort, olive, and ragweed pollen for European locations with hourly resolution and a multi-day forecast horizon, available under an open-source licence for both non-commercial and commercial use. Pollen Today uses Open-Meteo to serve location-specific daily pollen forecasts.
Long-Term Pollen Season Trends
Research published in the Proceedings of the National Academy of Sciences (Anderegg et al., 2021, doi:10.1073/pnas.2013284118) analysed pollen monitoring data from 60 North American stations over three decades and found that pollen seasons have lengthened by an average of 20 days and total pollen concentrations have increased by approximately 21% since 1990. The study attributed these changes primarily to rising temperatures associated with climate change. Similar phenological shifts have been documented in European monitoring network data.
Further Reference Documents
- WHO — Asthma — WHO fact sheet covering asthma and allergic rhinitis
- ARIA (Allergic Rhinitis and its Impact on Asthma) — WHO-initiated clinical guideline initiative for allergic rhinitis
- European Aeroallergen Network — standardised European pollen monitoring data
- Copernicus CAMS pollen forecasts — operational forecast data and documentation
