Childhood exposure to green space – A novel risk-decreasing mechanism for schizophrenia?

Abstract

Schizophrenia risk has been linked to urbanization, but the underlying mechanism remains unknown. Green space is hypothesized to positively influence mental health and might mediate risk of schizophrenia by mitigating noise and particle pollution exposure, stress relief, or other unknown mechanisms. The objectives for this study were to determine if green space are associated with schizophrenia risk, and if different measures of green space associate differently with risk. We used satellite data from the Landsat program to quantify green space in a new data set for Denmark at 30 × 30 m resolution for the years 1985–2013. The effect of green space at different ages and within different distances from each person's place of residence on schizophrenia risk was estimated using Cox regression on a very large longitudinal population-based sample of the Danish population (943,027 persons). Living at the lowest amount of green space was associated with a 1.52-fold increased risk of developing schizophrenia compared to persons living at the highest level of green space. This association remained after adjusting for known risk factors for schizophrenia: urbanization, age, sex, and socioeconomic status. The strongest protective association was observed during the earliest childhood years and closest to place of residence. This is the first nationwide population-based study to demonstrate a protective association between green space during childhood and schizophrenia risk; suggesting limited green space as a novel environmental risk factor for schizophrenia. This study supports findings from other studies highlighting positive effects of exposure to natural environments for human health.

Introduction

Globally, 450 million people are estimated to suffer from some form of mental illness and the number is expected to increase.(World Health Organization, 2003) Loss of productivity costs national economies billions of dollars, and the economic burden is estimated to 3–4% of GNP for developed countries (World Health Organization, 2003). Increasing rates of mental illnesses have been linked to increasing urbanization and environmental degradation whereas access to green space has been linked to mental health benefits (James et al., 2015; Lee and Maheswaran, 2011; Sandifer et al., 2015; Sugiyama et al., 2008) but the mechanistic link remains unknown.

Since the beginning of the 19th century a number of studies have documented urban rural differences in the occurrence of schizophrenia, generally showing increasing occurrence of schizophrenia in urbanized areas (Faris and Dunham, 1939; Haukka et al., 2001; March et al., 2008; Mortensen et al., 1999; Pedersen and Mortensen, 2001). The underlying mechanism is likely linked to multiple factors, but hypotheses roughly fall within one of two main explanatory categories (DeVerteuil et al., 2007): 1) individuals with schizophrenia migrate into inner-city areas, or 2) disease development is disproportional in inner-cities because of environmental risk factors. Recent studies have shown only partial effects of selective migration, indicating that the urban environment itself is a risk factor (DeVerteuil et al., 2007; Pedersen, 2015; Vassos et al., 2016). A clear difference between urban and rural environments is the amount and types of green space. Green space is known to decrease air and noise pollution (Gidlöf-Gunnarsson and Öhrström, 2007; Nowak et al., 2006) and increase stress restoration (Annerstedt et al., 2013; Beyer et al., 2014; Grahn and Stigsdotter, 2010) – factors that have been linked to mental health and risk of developing schizophrenia (Freeman, 1994; Mortensen, 2000). However, with few exceptions, studies investigating which characteristics of green space are important for mental health are lacking (Annerstedt et al., 2012).

Schizophrenia risk could be associated with different measures of green space as well as the spatial distribution of green space – e.g., the quantity or heterogeneity of green space within a given distance from a person's place of residence. We would expect that the quantity is related to availability and density of the surrounding green space, which could be important for e.g. air and noise pollution levels, whereas the heterogeneity is related to variation of the surrounding green space, which could be important for the viewscape and the restorative qualities of the surrounding green space. Expansion of remote sensing programs and increased investments in satellites now provide users with high-resolution earth observation data that can be used to calculate objective measures of green space with broad temporal and spatial coverage (Henke and Petropoulos, 2013; Li and Weng, 2007; Ryznar and Wagner, 2001; Wu et al., 2014). However, satellite data has not been used before in studies of schizophrenia risk.

Assessing the influence of environmental risk factors, such as green space, on schizophrenia requires estimation of incidence rates in representative samples of the general population (Mortensen et al., 1999). Denmark offers a unique opportunity to study the potential association between schizophrenia and green space since place of residence and health of all citizens are recorded longitudinally in national registers. By using data from the Danish national registers we can account for effects of socioeconomic factors and family history allowing more robust estimation of the potential influence of green space on schizophrenia risk.

This is the first nationwide population-based study assessing the potential impact of green space on schizophrenia risk. Most studies of health and green space have focussed on quantity (van den Berg et al., 2015), but it is still unclear how green space is linked to mental health. Studies on landscape preference, restorative environments, and usability of green space suggest that the type and characteristics of green space is also important (Bratman et al., 2015; Carrus et al., 2015; Lee and Maheswaran, 2011). We addressed this question by linking the rich Danish population-based register on health and socioeconomic status with two different individual-level exposure during childhood to quantity and heterogeneity of green space from a new high-resolution data set. We used a measure of vegetation greenness, the normalized difference vegetation index (NDVI), calculated from remotely-sensed satellite images at each place of residence for all Danish residents covering the years 1985–2012. It is unclear at what distance green space potentially affects mental health (e.g. Ekkel and de Vries, 2017; Maas et al., 2009; Nutsford et al., 2013), and whether green space surrounding a persons' residence or green space within the neighbourhood is potentially more important. To test this, we examine the effect of green space at different residential proximities. We hypothesize that more green space (higher mean value of NDVI around the residence) reduces the risk of schizophrenia, and that this effect is strongest closest to the residence. Furthermore, as vulnerability to environmental risk factors may vary with age (Pedersen and Mortensen, 2001), we also examine at what age green space exposure is more important for the risk of schizophrenia.