The presence of green space, such as forests and parks, is now widely viewed as a health-promoting characteristic of residential environments, and has been linked to benefits such as recovery from mental fatigue [1
], stress reduction [8
] neighborhood social cohesion [11
], reductions in crime, violence and aggression [12
], reduced morbidity in multiple disease categories [16
] and better self-reported health [17
Some studies have demonstrated that the relationship between green space and health is stronger among lower socioeconomic status groups, the elderly, and others who stay home during the day, providing some support for the notion that the size of the effect of local green space on health is related to the amount of an individual’s exposure to the local environment [17
]. These findings suggest that increasing neighborhood access to green space could be a cost-effective strategy to improving health and reducing health disparities, as lower socioeconomic status groups have a more limited ability to travel beyond local neighborhoods, resulting in increased dependence on local environments for healthy lifestyles and exposures [7
]. Given evidence suggesting health benefits of exposure to green space and stronger effects among some racial, ethnic, or socioeconomic groups, it has been suggested that green spaces could be “systematically deployed to mitigate health inequalities” in addition to improving health overall [7
However, while green space holds great promise as an innovative, place-based solution to improving population health, understanding where and for whom green space confers health benefits has proven complex. Some evidence and theoretical guidance has suggested that several main pathways may be important in linking green space exposure to health benefits. Green space can have direct protective effects against health hazards posed by air pollution, extreme temperature, and noise pollution [24
]; has been associated with increased health promoting behaviors such as physical activity [23
]; and linked to increased levels of social support, social cohesion, and sense of community [11
]; and to mental health benefits such as stress reduction [8
]; buffering between stressors and health outcomes [30
]; and attention restoration that reduces mental fatigue [1
]. Recent research has linked green space directly to biomarkers of stress and attention—diurnal variation of salivary cortisol [8
] and brain waves as measured by portable EEG devices [9
]—suggesting a biologically plausible link between exposure to green space and reduction of stress and mental fatigue.
The mental health benefits conferred by green space are of particular interest given a growing body of knowledge that emphasizes stress responses as a main link between neighborhood conditions and health outcomes. Attention Restoration Theory posits that experiences in natural environments can reduce mental fatigue and restore the capability for directed attention. Directed attention is employed “when something (does) not of itself attract attention, but when it (is) important to attend nonetheless” [2
]. Maintaining this focus requires mental effort, which can lead to mental fatigue. In order to recover from mental fatigue, an individual must have the opportunity to relax directed attention. One way to accomplish this is to engage in another kind of attention—fascination attention—which occurs involuntarily and does not require the same mental effort as directed attention. Scholars argue that natural environments have the inherent capacity to fascinate, thereby providing a restorative experience that enables recovery from mental fatigue. This may be particularly relevant when considering the directed attention demands of fast-paced, modern, urban environments [2
Numerous studies, including a large body of experimental work in the field of environmental psychology, have empirically linked nature exposure with attention restoration and stress reduction [1
], as well as the buffering between stressful life events and psychological outcomes [32
]. Early work focused on visual images of nature scenes and resultant impacts on preferences for different landscapes and measures such as emotions and heart rate [33
]. More recent work has emphasized experimental designs incorporating walks/runs in natural areas and additional measurement techniques, including the Digit Span Backwards (DSB) test and other tests of cognitive function. Several studies from the Landscape and Human Health laboratory at the University of Illinois at Urbana-Champaign have provided strong evidence of nature’s benefits for children affected by Attention Deficit Hyperactivity Disorder (ADHD) [1
]. In particular, one study engaged children with ADHD in walks in several different environments, finding improvements in attention after walking in a park, as compared to walks in a downtown and neighborhood setting; the study found significant effect sizes comparable to those reported for common pharmaceutical therapies for ADHD [1
]. Several recent reviews provide good coverage of the current evidence base and may be consulted for further detail [38
]. In particular, a meta-analysis by Bowler et al.
(2010) identified effect sizes and significant levels indicative of improvement in energy, anxiety, anger, fatigue and sadness with exposure to natural environments [38
], with less evidence of an impact on attention, tranquility, blood pressure or cortisol. Findings of this 2010 review can be contrasted with those from the studies of children with ADHD described above, as well as with more recent work that has linked green space with biomarkers of stress (cortisol) [8
] and with changes in brain waves indicative of a more “meditative” cognitive state [9
Previous work has suggested that greenspace may be linked to lower levels of depression, anxiety and/or stress, and that more research is needed [41
]. More evidence is needed to inform the development and implementation of effective neighborhood-level interventions that harness the salutogenic properties of green space to improve health and reduce disparities in different geographical settings. While population level studies have examined the role of green space, stress and mental health in several European countries [8
] and New Zealand [44
], and several US studies have examined the health benefits of green space more generally [45
], less is known about green space and mental health relationships at the population level in settings across the United States (US). In the US, a recent study demonstrated that a program to green vacant lots in Philadelphia significantly reduced stress [12
] and another found a weak association between green space and mental health in Miami [41
]. A third study found a positive impact of green space for stress reduction in Chicago, with park spaces demonstrating a larger effect size than the vegetation level overall [49
]. A recent review concluded that much of the evidence of a psychological benefit of green space is from qualitative data and non-peer reviewed sources, and argued that more research is needed to quantify the association between green spaces and health [39
Given an evidence base suggesting associations between environmental green space and both physical and mental health outcomes in non-US settings, the potential for green space to contribute to reductions in health inequities, and the significant and persistent patterns of place-based health inequities associated with the history of residential racial segregation in the United States, there is a great need for more research examining the potential role for green space as a population health intervention in the US. Further, few studies have considered the role of green space in non-urban settings. This study contributes a population-level perspective from the United States to examine the relationship between environmental green space and mental health outcomes in a study area that includes a spectrum of urban to rural environments. In addition, in recognition of previous work that has found variation in impact of different types of green spaces [49
] and a body of work that emphasizes trees as a special and important category of green space that deserves particular attention [20
], we examine several measures of green space in the present study.
We found in our sample that higher levels of neighborhood green space correspond to better mental health outcomes, when controlling for a wide range of confounding factors. The associations between green space and mental health are significant and sizeable and persist with different measurement techniques. Furthermore, the estimated effect of environmental green space is similar in magnitude to that of other well-known and studied contributors to symptomology for depression, anxiety and stress. For example, results indicate that the difference in depressive symptoms between an individual living in an environment with no tree canopy and an environment with 100% tree canopy is larger than the difference in symptoms associated with an individual who is uninsured compared to an individual with private insurance.
This study also revealed interesting differences in access to percentage of tree canopy based on demographic variations in population length of residence, age, income, marital status, and insurance type. Our multivariate adjusted results highlight that those from lower income brackets and without private health insurance experience greater anxiety, stress, and depression, thus supporting the notion that low socioeconomic populations could benefit more from increased exposure to green space [19
]. This study found a higher percentage of tree canopy and more positive mental health among populations age 55 and older. In other literature, older populations have been identified along with individuals of low socioeconomic status as being vulnerable and likely to reap increased benefits from green space based on increased time spent in their neighborhoods [19
]. In this study, it appears that younger adults may currently experience greater need to receive the mental health benefits conferred by greener environments. Future research should continue to explore how demographic characteristics such as race, age, and income contribute to both access to green space and mental health benefits of green spaces.
Another important contribution of this study is the examination of neighborhood environmental factors other than green space. The correlation between higher unemployment levels and greater anxiety and stress and the correlation between higher levels of residential segregation and increased anxiety, stress, and depression support the need for research on “greening” and other neighborhood strategies to counteract spatial aspects of socioeconomic disadvantage [7
]. Our finding of higher levels of depression among rural populations highlights potential benefits of environmental green spaces for diverse populations and motivates the expansion of green space research to non-urban environments, as well as the examination of benefits by socio-demographic and economic characteristics of residents.
Strengths of this study include the population-based sample, the use of state-of-the art and detailed measures of green space, the use of well-known and validated outcome measures (DASS instruments), and the consideration of a large number of covariates to control for confounding influences. Several potential limitations of this study are worth considering. SHOW data are based on self-reported health outcomes and non-response might introduce selection bias. The use of SHOW data from 2008–2011 with satellite data from 2009 and tree canopy data from 2001 might introduce some error stemming from temporal misalignment between survey responses and neighborhood greenness. We limited NDVI analyses to 2009 due to the availability of images that were not significantly affected by cloud cover. As NDVI calculations for 2009 and percent tree canopy for 2001 are highly correlated (r = 0.83), we assume that temporal misalignment does not alter the results of the study significantly. Similarly, the unavailability of Census Block Group information and resultant use of Census Tract data for estimating neighborhood socioeconomic characteristics may introduce some error stemming from spatial misalignment of predictors and outcomes.
A final and important limitation is the problem of reverse causality or neighborhood selection bias, whereby it is possible that associations found could in part be attributable to populations with fewer mental health problems moving into greener environments, given that this is a cross-sectional study [64
]. While this possibility cannot be ruled out, the body of evidence, including experimental work, linking exposure to green space and nature to mental health benefits provides strong support for a causal relationship between greenspace and mental health, which should be further investigated. In addition, we controlled for length of residence in the neighborhood to reduce the impact of neighborhood selection bias.
Previous work has identified “a positive relation between perceived general health and both agricultural green and natural green in a person’s living environment” [19
]. Building upon this previous work, we contribute, to our knowledge, the first investigation of the relationships between green space and mental health in a study area that includes urban and rural landscapes common across the United States. Our work indicates that “greening” could be considered a potentially low cost, high return investment among urban and regional planners to positively influence population mental health. Further, it is know that mental health conditions such as stress, anxiety and depression can be associated with a myriad of other adverse health conditions, missed days of work and low productivity, indicating the benefits of such a strategy could be diverse and numerous. More research in this area is needed, particularly as some have suggested the potentially negative aspects of greener environments, including their association with sprawl and possibly diseases associated with sedentary behaviors encouraged by car dependent environments [48
Given an evidence base suggesting associations between environmental green space and both physical and mental health outcomes in non-US settings, this work provides further evidence to support neighborhood level greening as a potentially important intervention for improving health and reducing persistent disparities. Furthermore, because of significant and persistent patterns of place-based health inequities associated with the history of residential racial segregation in the US, which often coincide with adverse social and build environments, there is a great need for continued research examining greening as a population health intervention in the US.