Belgium is a highly industrialized and densely populated country with substantial air pollution and at least ~10% of the people develop allergic rhinitis symptoms due to birch tree pollen. The only information on airborne birch pollen is coming from five offline monitoring stations, lacking spatial and timely details of the pollen levels. In contrast, Chemistry Transport Models (CTM’s) are able to quantify the spatial and temporal distributions of airborne birch pollen levels a few days ahead. Recently, a new dataset on the birch pollen emission sources has been developed for Belgium. Here we show the preliminary results of the forecast set-up for Belgium using the CTM SILAM (System for Integrated modelling of Atmospheric composition, http://silam.fmi.fi) driven by ECMWF meteorological fields and an updated map of birch pollen emission sources to produce the spatio-temporal distributions of airborne birch pollen levels for the current birch pollen season of 2021 four days ahead. We have evaluated the model performance by comparing the forecasted airborne birch pollen levels time series with four offline birch pollen monitoring stations in Belgium and with the CAMS forecast data.