Effect modification by air pollutants
Trees and allergenic trees were associated with a higher prevalence of AR when NO2 levels were comparatively high or ozone was comparatively low. Apart from the direct detrimental effect of nitric oxides on respiratory health, 39,40 these substances may have varying effects on the allergenicity and amount of pollen depending on plant species and environmental conditions. For example, a number of molecular studies reviewed by Frank and Ernst41 found increasing allergenicity (tested by increased IgE reactivity) of NO2-treated pollen of the common species Acer negundo , Betula pendula , Ostrya carpinifolia and Carpinus betulus , 42,43 while Beck et al.13 did not find a correlation of birch (Betula pendula ) pollen allergenicity with NO2levels. Divergent findings were also observed for ozone treatments.41 In addition, NO2 and ozone may have different effects on pollen allergenicity even within the same species.44 The effect of these pollutants on a larger scale is further blurred by the chemical reactions between ozone and NO2 that depend atmospheric conditions.45,46 This makes it difficult for epidemiological studies like ours to investigate their influence on the associations of trees and allergenic trees with allergic outcomes.