Synchronized inter-annual fluctuation of flowering intensity affects the exposure to allergenic tree pollen in North Europe

Many anemophilous, early-flowering tree genera include allergy plants of world-wide significance. We studied the synchronisation of high and low pollen years in the genera Betula, Alnus, Corylus, Salix and Populus and the cumulative effects that an increasing number of taxa has on the number of days of exposure to different levels of allergenic pollen in North Europe. The proximal causes of the inter-annual variations of airborne pollen loads were analysed with a multiple regression analysis. The annual fluctuations of airborne pollen sums were compared between genera and found to be positively correlated among all combinations of genera at the three study sites. Most correlations were statistically significant (p < 0.05). The comparison between Betula and Alnus is discussed first. Betula pollen was clearly the most abundant airborne pollen type. The presence of Alnus pollen, however, significantly increased the predisposal to allergenic pollen. At all sites, the number of days per year when the Betula and Alnus pollen counts together exceeded 10 and 100 grains M 3 of air, was found to be greater than the number of days when the Betula pollen counts alone exceeded 10 and 100 m(-3) of air. The difference was statistically significant. In Kuopio, the difference was found to be statistically significant even for grains per 1 000 m(-3) of air of Betula and Alnus together compared with the same count of Betula pollen alone. Betula, Alnus and Corylus belong to the order Fagales and have cross-reacting main allergens. The flowering of Alder and Corylus culminate at the same time, two to four weeks earlier than that of Betula. Due to synchronization of high and low years and the mostly non-overlapping flowering seasons, the time of exposure to pollen may be very long during the high years. Furthermore, Alnus and Corylus pollen may prime allergic people before the onset of the Betula season.