A growing body of evidence suggests that contact with nature, often referred to as green or blue space (ie, vegetated areas or water bodies), is associated with multiple health benefits. Recent reviews identify various mechanisms that may underlie the observed associations. In addition to encouraging healthy behaviors (eg, physical activity), nature exposure may have an effect on biological mechanisms by acting on the autonomous nervous system and reducing chronic stress, which can prevent systemic inflammation, a common cause of many noncommunicable diseases and related mortality. Nature exposure may also have direct structural and functional effects on the brain, resulting in reduced symptoms of depression (eg, rumination).
South et al implement, to our knowledge, the first cluster randomized trial in general populations to assess whether a greening intervention near the residence improves mental health. By working with a nongovernmental organization focused on greening blighted vacant lots in Philadelphia, Pennsylvania, South and colleagues were able to capitalize on the timing of greening interventions so they could conduct an RCT. The trial had 3 study groups: (1) areas that experienced an increase in greenness near the home when the vacant lots were cleaned and converted to small green spaces, surrounded by a short fence; (2) areas that experienced litter cleanup but no greening; and (3) control areas that experienced neither greening nor cleaning. The researchers enrolled 442 participants in total, with roughly similar sizes of each study group. Participants were surveyed for mental health using the Kessler-6 Psychological Distress Scale both prior to the interventions and 18 months afterward.
Using intention-to-treat analyses, South and colleagues found that compared with the control group, the greening intervention group had a significant reductions in feeling depressed (−41.5%; 95% CI, −63.6% to −5.9%; P = .03) and feeling worthless (−50.9%; 95% CI, −74.7% to −4.7%; P = .04). The effects were stronger in poorer neighborhoods than in wealthier areas. They combined the 2 intervention groups in the analysis and compared them with the control group resulted in similar reductions but nonsignificant results.
The study is potentially important for several reasons. It is likely the first RCT to investigate mental health benefits of green space. In so doing, South et al avoid the self-selection bias limitation, which strengthens the evidence around the positive effect of green space on mental health. The greening intervention in this study specifically limited active use of the green space, as all greened lots had fences placed around them to restrict entry. This suggests an effect of simply seeing nature as opposed to interacting with nature through, for instance, physical activity. This improves the evidence that merely being visually exposed to nature may evoke critical biological responses. Because greening existing landscapes is usually less expensive than, say, constructing new urban parks, the finding may imply a cost-efficient way of improving mental health, especially in deprived neighborhoods. Additionally, the study design could be replicated in numerous cities around the world where various types of greening interventions occur.
The study design also demonstrates the reciprocal benefits when academics collaborate with governmental and nongovernmental organizations. South et al8 appear to have worked with a nongovernmental organization to stage the greening intervention in a way that they could conduct an RCT, which ultimately provides better evidence for optimizing policies and planning. For the most part, such cooperation between bodies implementing changes to the built environment and researchers remains a rarity.