Introduction
Greenspace is related to various beneficial health outcomes, including better mental health and well-being 1, reduced risk of all-cause mortality 2 and type 2 diabetes3, improved pregnancy outcomes and decreased blood pressure 4, among others. These benefits are hypothesized to be due to the restorative potential of greenspace, reduced concomitant exposure to harmful environmental influences, as well as because greenspace provides settings for physical activity and social contacts which in turn improve physical and mental health.5
Mechanistic links between greenspace, asthma and allergic outcomes seem to be more complicated to resolve than those for other health indicators. On the one hand, greenspace was hypothesized to reduce the risk of allergic diseases by similar mechanisms as does living on a farm, 6 namely via increased contact with microbes and consequent modulating of immune responses and allergic inflammation.7 On the other hand, greenspace is a source of allergenic pollen, short-term exposure to which can exacerbate asthma and allergic symptoms as studies based on pollen counts have demonstrated. 8
It is not surprising then that greenspace research into allergic health outcomes stagnates. The published studies have typically reported associations, but those were in different directions depending on the study area. 9,10 One example for such diverging directions is our own study where higher greenness (i.e., vegetation degree) was associated with increased risk of allergic rhinoconjunctivitis and aeroallergen sensitization in the Munich study area, but with reduced risk of these outcomes in the Wesel area of the same multicentre birth cohort. 11 We failed to explain such inconsistency but managed to replicate it in five additional cohorts across three continents.12
Besides the complicated and potentially conflicting underlying mechanisms, the lack of consistency in greenspace-allergy studies was also blamed on metrics of greenspace that are too crude to differentiate between different types of vegetation. 12 Most of the existing research used either land use or land cover-derived distance to or amount of different types of green spaces, or satellite-derived vegetation indices capturing greenness. None of these metrics can distinguish trees from herbaceous vegetation, let alone allergenic from non-allergenic plant species.
Keeping all this in mind, we decided to explore whether residing in places with overall higher greenness, more trees and specifically more allergenic trees early in life can contribute to the onset of allergic diseases, or whether such exposures act as protective factors. To our knowledge, no study so far looked at the long-term exposure to allergenic trees and development of allergic diseases – partially because detailed vegetation geodata are very scarce and partially because expert knowledge is needed to classify species into allergenic and non-allergenic. This study was possible thanks to the collaboration between environmental epidemiologists and plant biologists.
Since elevated air pollution levels are observed to increase the allergenicity of pollen, 13 as a secondary objective, we investigated whether the associations between greenness, trees and allergenic trees and allergic diseases differ depending on the concentration of several air pollutants. In addition, vegetation and air pollution are inversely spatially correlated and, depending on the spatial arrangement, composition and temperature, trees can remove, trap, or even emit air pollution. 14