Residing near allergenic trees can increase risk of allergies later in life: LISA Leipzig study
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).
Section snippets
Study design and population
“Influence of Life-Style Factors on the Development of the Immune System and Allergies in East and West Germany” (LISA) is an ongoing multicentre population-based German birth cohort (Heinrich et al., 2017; Zutavern et al., 2006). Briefly, healthy full-term neonates with a normal birth weight were recruited at selected maternity hospitals in Munich, Leipzig, Wesel and Bad Honnef between 1997 and 1999. Parent-completed questionnaires were administered at birth and at ages 0.5, 1, 1.5, 2, 4, 6,
Descriptive characteristics of the study population and exposure variables
Our analytic sample comprised 51.3% females and 45.8% of children had parents with atopy. Over half of the children (55.2%) were from families with high level of parental education while only 2.4% were from families with low level of parental education (Table 1). This was the only systematic difference between the analytic sample compared to the original LISA Leipzig population where the above-mentioned numbers were 48% and 5%, respectively. 35.5% of children were born during the season with
Discussion
The results of our longitudinal analyses in 631 children from the city of Leipzig, Germany, show that early life residence in places with many trees and allergenic trees specifically, was consistently associated with increased prevalence of AR up to 15 years and, to a lesser extent, also with increased prevalence of allergic sensitization to aeroallergens. Associations with asthma and sensitization to food allergens were inconsistent. Associations with greenness, as measured by NDVI, were
Conclusions
We observed that early life residence in places with many trees and specifically many allergenic trees may increase prevalence of AR later in life regardless of general level of vegetation. This association, as well as its modification by nitrogen dioxide and ozone, needs replication in further studies.
Sources of funding
The LISA study was mainly supported by grants from the German Federal Ministry for Education, Science, Research and Technology and in addition from Helmholtz Zentrum Munich (former GSF), Helmholtz Centre for Environmental Research—UFZ, Leipzig, Germany, Marien-Hospital Wesel, Germany, Pediatric Practice, Bad Honnef, Germany, for the first 2 years. The 4- year, 6-year, and 10-year follow-up examinations of the LISA study were covered from the respective budgets of the involved partners
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank all children and parents for their cooperation, and all technical and administrative support staff and medical and field work teams. We are also grateful to all members of the LISA Study Group. The LISA Study group consists of the following: Helmholtz Zentrum München, German Research Centre for Environmental Health, Institute of Epidemiology, Munich (Heinrich J, Schnappinger M, Brüske I, Ferland M, Schulz H, Zeller C, Standl M, Thiering E, Tiesler C, Flexeder C); Department of
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