The results of five years (1977#x2013;1981) of continuous monitoring of airborne pollen concentrations with a volumetric trap in Leiden, indicate that usually about 37 of the total pollen count comes from a group of 24 taxa, trees and shrubs flowering in the spring, from January to June. The main taxa in this group, representing over 90 of this pollen count, are:Alnus, Betula, Cupressaceae (including Taxaceae),Fraxinus, Pinaceae,Populus, Quercus, Salix, andUlmus; almost 50 belongs toAlnus, Betula, andSalix. Comparison of the counts for the various years reveals that both the total number and the time of airborne presence of the different pollen types show great and irregular variations. Cases of pollinosis with clinical significance are rare, and have been ascribed to allergy to pollen fromBetulaandAlnus, although allergic sensitization to other pollen types without severe clinical manifestations do occur.
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