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BACKGROUND: Birch pollen is highly allergic and has the potential for episodically long-range transport. Such episodes will in general occur out of the main pollen season. During this time, allergy patients are unprotected and high pollen concentrations will therefore have a full allergenic impact. OBJECTIVE: To show that Denmark obtains significant quantities of birch pollen from Poland or Germany before the local trees start to flower. METHODS: Simultaneous observations of pollen concentrations and phenology in the potential source area in Poland as well as in Denmark were performed in 2006. The Danish pollen records from 2000 to 2006 were analysed for possible long-range transport episodes and analysed with trajectories in combination with a birch tree source map. RESULTS: In 2006, high pollen concentrations were observed in Denmark with bi-hourly concentrations above 500 grains/m(3) before the local trees began to flower. Poland was identified as a source region. The analysis of the historical pollen record from Copenhagen shows significant pre-seasonal pollen episodes almost every year from 2000 to 2006. In all episodes, trajectory analysis identified Germany or Poland as source regions. CONCLUSION: Denmark obtains significant pre-seasonal quantities of birch pollen from either Poland or Germany almost every year. Forecasting of birch pollen quantities relevant to allergy patients must therefore, take into account long-range transport. This cannot be based on measured concentrations in Denmark. The most effective way to improve the current Danish pollen forecasts is to extend the current forecasts with atmospheric transport models that take into account pollen emission and transport from countries such as Germany and Poland. Unless long-range transport is taken into account, pre-seasonal pollen episodes will have a full allergic impact, as the allergy patients in general will be unprotected during that time.
Skjøth et al. (Wed,) studied this question.