Biological plausibility
Several mechanisms may be relevant for explaining the observed
associations between green spaces and allergic rhinitis. The mediation
analysis revealed that associations between NDVI and allergic rhinitis
do not seem to be mediated by air pollution. However, our results
suggest that this association may be modified by the levels of
PM10, PM2.5, NO2, and
SO2 exposures. Green spaces may remove pollutants from
the atmosphere through deposition on the tree surfaces and/or by
adsorption and absorption processes.51,52 Moreover,
the presence of healthy trees or vegetation could improve air quality by
dispersing local pollutants or by limiting dispersion towards
sidewalks.53 During the summer, the highest NDVI
levels, which indicate a higher density of vegetation, may reduce the
levels of air pollutants,51,52 as well as create a
cooling effect through shade and evapotranspiration, and thus generate
airflows and disperse the concentrations of pollutants
more.54 However, during the spring, the higher levels
of air pollutants in Espoo during the study period (previously reported
by Siddika et al. 55) may induce changes into the
chemical composition of pollens, intensifying their allergenic potential
and thus, increasing allergen-induced inflammatory and cellular immune
responses,7,56 which consequently increase the risk of
allergic rhinitis. There is some previous evidence that observed
associations between exposure to green spaces and allergic diseases may
be mediated or modified by air pollution levels.35,36Furthermore, the results observed during spring and summer may be
related to changes in the type and abundance of
vegetation,51 which cannot be distinguished by using
the NDVI value as an indicator of exposure.