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