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.