Residing near allergenic trees can increase risk of allergies later in life: LISA Leipzig study

Highlights

• We conducted longitudinal analyses in 631 children from the city of Leipzig, Germany.

• We classified all trees from the tree registry around birth by allergenicity.

• Allergenic trees early in life increased the risk of allergic rhinitis up to 15 years.

• This association was independent of general level of vegetation.

• The association tended to be stronger in high NO₂ and low ozone settings.

Abstract

Background

We investigated whether residing in places with higher greenness, more trees and more allergenic trees early in life increases the risk of allergic outcomes, and whether these associations differ depending on the concentration of air pollutants.

Methods

The analytic sample included 631 children from the German birth cohort LISA Leipzig. Asthma and allergic rhinitis, sensitization to aeroallergens and food allergens, as well as confounders, were collected prospectively up to 15 years. Greenness was assessed by Normalized Difference Vegetation Index (NDVI). A tree registry was used to derive information on trees, which were classified into allergenic and non-allergenic. Annual average concentrations of nitrogen dioxide (NO₂) and ozone were also used. Geographic exposures were assigned to home addresses at birth. Longitudinal associations were analysed using generalized estimating equations.

Results

Medium and high numbers (tertiles) of trees and allergenic trees in a 500 m buffer around birth addresses were associated with increased odds of allergic rhinitis up to 15 years regardless of NDVI. These exposures were also related to higher odds of sensitization to aeroallergens. Associations with asthma and sensitization to food allergens were less consistent. Effect estimates for allergic rhinitis were stronger in the high tertile of NO₂ compared to the low tertile, while an opposite tendency was observed for ozone.

Conclusion

We observed that early life residence in places with many trees, and allergenic trees specifically, may increase the prevalence of allergic rhinitis later in life. This association and its modification by air pollution should be pursued in further studies.

Introduction

Greenspace is related to various beneficial health outcomes, including better mental health and well-being (Houlden et al., 2018), reduced risk of all-cause mortality (Rojas-Rueda et al., 2019) and type 2 diabetes (den Braver et al., 2018), improved pregnancy outcomes and decreased blood pressure (Twohig-Bennett and Jones, 2018), 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 (Markevych et al., 2017).

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 (von Mutius and Vercelli, 2010), namely via increased contact with microbes and consequent modulating of immune responses and allergic inflammation (Rook, 2013). 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 (Erbas et al., 2018).

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 (Lambert et al., 2017, 2018). 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 (Fuertes et al., 2014). We failed to explain such inconsistency but managed to replicate it in five additional cohorts across three continents (Fuertes et al., 2016).

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 (Fuertes et al., 2016). 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 (Beck et al., 2013), 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 (Eisenman et al., 2019).