In their study, Astell-Burt and Feng1 address 2 important limitations of much of the research on contact with nearby nature and human health and well-being.2 The first one is its cross-sectional design, preventing the ability to draw firm conclusions regarding the causality of observed associations. The second one is that most studies do not distinguish between different types of green and/or blue space and consequently tell us little about which type of nature is most beneficial. In their study, they begin to address this gap by making efficient use of longitudinal health data already collected for other purposes and enriching these data with land-use data on the availability of green and blue space in the residential environment. Consequently, in addition to prevalence data, incidence data were available on high risk of psychological distress, physician-diagnosed anxiety and/or depression, and less than good self-rated health. Furthermore, they distinguished 3 types of vegetation: tree canopy, grass, and low-lying vegetation. They concluded that tree canopy may be more beneficial than the other 2 types.
The study by Astell-Burt and Feng1 is definitely a step in the right direction. At the same time, it bears the markings of a journey in uncharted territory. As with many studies on this topic and despite distinguishing different types of green space, the problematization of how to best characterize access to green space is limited. For example, with regard to the presence of nature translating to actual exposure in terms of contact with or use of the green space, its distance to one’s home is of great importance. A buffer size of 1.6 km seems rather large because most people will consider this distance too far to walk to and will therefore have a low probability of frequently coming in contact with the green space located in the more distant part of the buffer. Egorov et al3 concluded that the positive effects of vegetation (on allostatic load) already decrease gradually with distance from the residence within a 500-m radius. Therefore, a smaller buffer size might have resulted in a stronger association with mental health.
Furthermore, the study’s information on which green spaces are included is limited. It remains unclear whether vegetation in agricultural areas within the 1.6-km buffer is included. Agricultural grasslands may not be as accessible to the general public as other types of grasslands (eg, in parks) or as wooded areas (eg, parks, forests). In addition, the spatial resolution of the land-use data used to determine the amount of different types of green space is not mentioned. This omission makes it unclear whether the sometimes isolated street trees that the authors refer to as important were actually included in the tree cover percentage. Even if they were, street trees may have constituted only a small part of the tree cover within the buffer. In their discussion, the authors mention that the level of biodiversity of the green space may be important based on a study conducted in 2007. However, more recently, Lovell et al4 performed a systematic review and concluded that overall the evidence is inconclusive. There is also no clear theoretical argument provided as to why high-biodiversity areas would be more beneficial than low-biodiversity areas. Note that visible variation in plants and/or animals, especially of likeable species, may make a green space more attractive and thereby generate more visits to it. Zoos and arboreta offer a lot of variation in this regard, but is this the type of biodiversity that is intended here?
In general, many epidemiologic studies5,6 in this area come across as still rather data driven. Theory development could help to guide empirical studies and increase our knowledge more efficiently and faster. With regard to the etiology of disorders and diseases and how access to green (or blue) space might intervene, one of the promising pathways is the effect of contact with nature on chronic stress levels, allostatic load, and immune system functioning, which can help to account for the large array of health benefits associated with access to green space.5 From an applied perspective, one of the main reasons for people to visit green spaces is to experience peace and quiet. The ongoing densification of cities is likely to result in less nearby green space, at least on a per capita basis. This lower availability of green space may interfere with the sought-after experience and thereby the health benefits provided by the remaining green space.6 If so, an adequate provision of green space is perhaps better defined in terms of amounts per capita rather than in terms of amounts as such. Given this potential consequence, the relevance of green space offering peace and quiet for its mental health benefits seems an important topic for further investigation.
Published: July 26, 2019. doi:10.1001/jamanetworkopen.2019.8215
Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2019 de Vries S. JAMA Network Open.
Corresponding Author: Sjerp de Vries, PhD, Wageningen Environmental Research, Wageningen University & Research, PO Box 47, 6700 AA Wageningen, the Netherlands (email@example.com).
Conflict of Interest Disclosures: None reported.
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de Vries S. Comparison of the Health Benefits of Different Types of Nature. JAMA Netw Open. Published online July 26, 20192(7):e198215. doi:10.1001/jamanetworkopen.2019.8215
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