The connection between nature and human health: Space, scale, and time!
Updated: Nov 14, 2019
With increasing concern for healthy living and the innate connection between humans and the natural ecosystem, scholars are focusing greater attention on how nature can be used as a therapeutic element to improve physical and mental health while also achieving wider sustainability goals for a healthy planet . Many studies on the health benefits of exposure to nature (dose) have explored direct and indirect benefits of green space, yet the causal relationship and policy formulation guidance remain ambiguous [2,3].
Figure 1: Direct and indirect benefits of dose of nature to human health at different spatial scales and time.
In The Lancet Planetary Health, Sarkar and colleagues  explored the relationship between residential green space and depression disorder among 94,879 adult Biobank participants in the UK. In their cross-sectional study, the authors reported that increasing exposure to residential greenness had a proactive effect of lowing the odds ratio of major depressive disorders (odds ratio 0·960, 95% CI 0·93–0·99). Furthermore, they indicated residential greenness effect is more dominant in women, younger people, and individuals with low socio-economic status. A wide body of research supports the positive therapeutic influence of greenspace on mental health and indicates that integrating green urbanisms, Biophilic design, and the integration of therapeutic landscaping in policies and planning would create healthier cities and improve healthy living outcomes [1,5,6].
Despite such significant evidence supporting the importance of greenness, incorporating this evidence when formulating spatially explicit urban development policy is challenging due to inherent methodological limitations. These limitations complicate determinations about where, how much, and when greenness is needed to produce direct or indirect benefits (Figure 1). The challenge starts with determining the effects of exposure to greenness (measured by NDVI) at based on location and scale. Sarkar and colleagues investigated the relationship between residential green space and depressive disorders at the neighbourhood scale using a 500 meter network-buffer . The use of 500m as a standard assumes that people are static and only residential exposure matters. Thus, exposure to greenness outside of this zone remains unknown and uncontrolled. This approach poses challenges to the creation of a healthy urban environment in places where people mostly spent time outside their home . Additionally, NDVI cannot suggest what type of green space (e.g., Parks, grassland) should be considered when planning for the creation of salutogenic environments [5,8].
Furthermore, the selection of a 500 m buffer is an ad-hoc approach. With accumulated buffers, studies tend to provide greater positive associations between green space and positive health outcomes due to the increase of overall greenness in a wider area up to certain distance then no significant relation found. This also raises issues related to how much greenness a planner or landscape architect should consider when designing overall cityscapes, not just residential areas. In this regard, finer scale measurement at the body level using small buffers may provide better insights into peoples’ interaction in different spaces and their requirements for natural exposure .
Types of exposure to nature have both direct and indirect benefits [6,9,10]. Chinmoy and colleagues only considered the availability of greenness in terms of NDVI. NDVI does not illustrate whether green areas are accessible or the extent of participants’ contact with nature . Time-dependent exposure and greater accessibility are known to be profoundly beneficial to mental health, and in some cases, the benefits of exposure length and accessibility are greater than those of availability [6,9]. Thus, only using availability as a measure may be insufficient to determine what constitutes a therapeutic landscape. Composites of key exposures, availability, accessibility, visibility, and time should also be developed to better investigate this issue [8, 10].
Time is one of the most important variables missing in most environmental health studies. Sarkar et al., work was no different. They assumed people are static at home; however, in reality, people are neither static nor fixed in time in their movement in space. Assumptions of static exposure ignore dynamic direct and indirect benefits that vary based on time and place [7,10]. Direct benefits such as the reduction of heat, noise, air pollution and indirect benefits from green space exposure via physical activities vary over time [2,5]. Even with low green space exposure around a home, a person can be exposed to increased greenness during their daily mobility. Studies need to critically integrate temporal and spatial aspects in order to understand the impact of spatial planning policies. It is not enough to just improve residential greenness. Rather, the goal should be to improve the overall quality and quantity of greenness in different temporal and spatial contexts [7,2,10].
The connection between nature and health is inherently complex and multi-dimensional. Sarkar and colleagues provided an improved demonstration of possible links between greenness and health, and their findings aid in the formulation of practical policies and planning guidelines. However, to better translate evidence into practice, it is necessary to improve methodological approaches to answer where, when, and how much greenness is necessary for better health and healthy living .
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