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.