Published on 13 June 2025
Thanks to the RetroPollen project, the impact of weather conditions, air quality, and birch and grass pollen on our health in Belgium has been quantified for the first time.
In Belgium, it is estimated that at least 10% of the population is likely to develop allergic reactions or rhinitis due to birch pollen and at least 15% to grass pollen. Furthermore, in a densely populated and industrialized country like Belgium, air pollution can increase the number of patients suffering from allergic respiratory conditions. The ultimate goal of the research project was to develop a warning system for expected pollen concentrations, which has been available since 2023 on the IRM and Sciensano websites.
What role does weather play in the spread of pollen?
Much of the difference in pollen concentrations during and between seasons can be attributed to weather conditions. Wind is the main factor determining the transport of pollen in the air. The distance that different pollen species can travel is generally around 30 km, or even 1,000 km for birch. Wind direction largely determines the pollen's destination. For birch pollen, temperature is also important because it determines the start and end of the pollen season. The study also showed that the predicted concentrations of birch pollen in the air in Belgium increased by 13% every ten years between 1982 and 2019. This increase is mainly due to climate change. Scientists did not expect that climate change (higher temperatures and dry periods during the pollen season) could increase the pollen concentration in the air to this extent. Scientist Willem Verstraeten explains: "Climate change is leading to an increase in the amount of pollen in the air. Vegetation dynamics can counter this phenomenon, but cannot fully compensate for it.
Why is a dispersion model such as SILAM important?
In Belgium, pollen concentrations are only measured at a limited number of sites. However, this does not allow patients to avoid pollen exposure and does not provide any information on the temporal and spatial distribution of pollen throughout the country. A dispersion model, on the other hand, allows for the calculation of pollen spread under the influence of weather conditions. During rainy and very humid periods, trees and grasses release less pollen, while the wind disperses pollen from birch catkins or grass spikelets. Based on weather forecasts, a dispersion model also allows for the estimation of expected pollen spread.
In addition, spatial models produce the daily pollen distribution and make it possible to estimate the effect on the health of the population.
Excessive concentration of birch and grass pollen poses a health risk
This study shows that lung function in asthmatic and cystic fibrosis patients decreases when exposed to high concentrations of birch and grass pollen. Air pollution further enhances this effect. Furthermore, exposure to large amounts of birch pollen increases the risk of death in the general population.
How to get informed and protect yourself?
Thanks to this project, citizens can now consult four-day forecasts for all pollen types, both on the IRM website and in the IRM app, for their specific place of residence in Belgium. The AirAllergy app and the Sciensano website have also been updated and supplemented with the IRM pollen forecasts.

A consortium of Belgian partners
The Belgian BRAIN RetroPollen project consortium, funded by BELSPO and led by the Royal Meteorological Institute (RMI), assembled a large collection of data from various sources for this study. The ERASME Hospital in Brussels and the Zeepreventorium (convalescence center) in De Haan collected more than 25 years of medical patient data. STATBEL provided more than 30 years of statistical data on the Belgian population. Time series of birch and grass pollen concentrations from Sciensano measuring stations dating back to 1982 were processed by RMI with meteorological data into a pollen transport model. This made it possible to calculate the daily spatial distribution of birch and grass pollen concentrations in the air for the whole of Belgium. IRCELINE provided maps of air pollution measurements. Statistical models from Hasselt University pooled all the data to analyze the influence of pollen, air pollution, and weather on our health. Thanks to the RetroPollen project, the impact of weather, air quality, and pollen on our health in Belgium has been quantified for the first time.
Source:
Belgian Federal Science Policy. (2025, May 8). Verschillende Belgische instituten bundelen hun krachten om de risico’s van weer, luchtvervuiling en pollen op de volksgezondheid in kaart te brengen. BELSPO Persberichten (translated to English)