The importance of nature for health: is there a specific benefit of contact with green space? (systematic review)
There is increasing interest in the potential role of the natural environment in human health and well-being. Natural environments could impact on health through a number of pathways, either acting directly on health and therefore having a specific benefit, or acting indirectly, for instance, by promoting health-enhancing behaviour such as participation in physical activity. Despite the discussion on this topic, the evidence for there being a specific and direct benefit from contact with the natural environment has not been systematically reviewed.
To address the question: how effective is direct accessing of natural environments in the promotion of health and well-being compared with other forms of ‘exposure’ to the natural environment or with accessing ‘synthetic’ environments?
We searched for literature in relevant databases, websites of specialist organisations, internet search engines, and references from bibliographies. Predefined inclusion criteria were applied to each article in order to identify the subset relevant for the review. For an article to be included in the review it must have compared a health or well-being outcome following activity (passive or otherwise) in a natural environment with activity (passive or otherwise) in a synthetic environment and/or viewing a natural environment. ‘Natural environment’ was used in a broad sense to include any environment with green space. The methodology of studies included in the review was critically appraised and the most commonly measured health/well-being outcomes were synthesized with meta-analysis.
Our broad search captured over 20,000 articles, however, after applying the inclusion criteria, only 28 studies were identified as relevant for the review. These studies mainly fell into two main pools: 1) studies comparing activity in the natural environment with an indoor environment or 2) studies comparing activity in the natural environment with an outdoor, built environment. Studies were diverse in terms of the types of participants and health/well-being outcomes measured. Most studies were short-term crossover trials and took measurements before and after exposure to the different environmental settings. Meta-analyses were conducted on several physiological parameters such as blood pressure, pulse and cortisol concentrations: these showed no evidence of an effect. There was also no evidence of a consistent effect on measures of attention or concentration. The most common outcome types measured were of mood/emotions, and based on these data, there was some evidence of a positive benefit on mood after a walk or run in a natural environment when compared to a different environment.
There is some evidence that activity in a natural environment compared to a different environment can have a positive impact on mental well-being. However, this is primarily drawn from short-term tests on self-reported feelings such as ‘anger/aggression’, ‘sadness/depression’ and ‘fatigue/tiredness’. The validity of these psychological scores as measures of mental well-being is not clear. There is little evidence of an impact on physiological outcomes but this is limited by the low number of studies available which measured similar outcomes. There was insufficient data to allow comparison of differences types of exposure to nature. Clearly, a ‘natural environment’ has many components. It is likely that further investigation on this topic and the design of more appropriate studies would be aided by refining the hypotheses on how specifically nature might impact on health and which specific attributes are the most important. The evidence is suggestive that nature may be used within the context of public health promotion interventions but we require a more comprehensive evidence-base in order to make appropriate and effective use of natural resources.
The presence of a link between the natural environment and human health and well- being is of current interest to a number of organisations within the public health and environmental sectors. On the basis that this link does exist, several organisations have already invested resources in initiatives which use the natural environment in some way as a means of improving public health (e.g. British Trust for Conservation Volunteer’s Green Gym; Parks Victoria’s Graded Walks). The aim of such initiatives is not only to promote health but also to conserve biodiversity. However, these initiatives can be costly and can mean resources are diverted from other causes: therefore it is important to ensure that there is a strong and scientifically sound evidence-base supporting them.
The literature on the links between the environment and health covers a wide range of issues from effects of nature on psychological health and well-being (Bird, 2007) to the encouragement of exercise (Kaczynski and Henderson, 2007). A growing number of reviews have been produced that draw together the theory and evidence and offer conclusions on the importance of the environment for health. However, work in progress indicates that, in many cases, it is not clear how much the conclusions from these reviews reflect an objective and unbiased synthesis of the literature. Indeed, many explicitly start out with the assumption that there is a positive link and then discuss the ways in which the environment can promote health. Few reviews specifically address the effectiveness of particular interventions, which would be of particular use to decision-makers.
Systematic review methodology is widely employed in medicine and public health as a way of synthesising the evidence for the effectiveness of a particular intervention (Sackett and Rosenberg 1995; Khan et al., 2003), but is only beginning to be recognised in environmental management (Pullin & Knight 2001, Pullin & Stewart 2006). These reviews differ from narrative reviews in a number of key ways. Essentially, the systematic review process comprises the setting of a clear and focused question which the review will address, a comprehensive search of the literature (both published and unpublished), transparent criteria for including studies captured by the search, critical appraisal of studies included in the review, and extraction and synthesis of data from included studies in order to address the review question (Khan et al., 2003; CRD, 2004). Each component of this methodology is important as it allows confidence to be placed in the findings of the review being both unbiased and based on the best evidence available. Systematic reviews of the effectiveness of the natural environment in promoting health and well-being could also inform policy or practice through highlighting an evidence gap.
This project, a collaboration between Natural England and the CEBC, aims to produce systematic reviews on the effectiveness of specific aspects of the natural environment in the promotion of health and well-being. Questions for review have been formulated following consultation with the project stakeholder group through a workshop held in October 2007 together with email correspondence with others. A report on the outputs of this consultation process is available via www.cebc.bangor.ac.uk. Question formulation was further informed by an appraisal of literature reviews and other relevant reports on the topic of ‘the natural environment and promotion of human health’. A report (Bowler, Knight and Pullin, 2007) on the findings from this appraisal is in preparation and will also be published on the CEBC website. Stakeholders collectively identified ‘effectiveness of measures to improve/increase access to natural environments in order to promote health and well-being’ as a core question of importance to decision-makers. This breaks down into two sub-questions:
1a: How effective is direct accessing of natural environments1 in the promotion of health and well-being compared with other forms of ‘exposure’ to the natural environment or with accessing ‘synthetic’ environments?
1b: What is the effectiveness of interventions aimed at increasing access by people to natural environments?
Question 1a is the focus of this protocol. Direct access refers to direct contact with or physically ‘being’ within the natural environment, while other forms of exposure include viewing nature through a window, in a picture or on a video. Essentially, this question addresses whether the benefits of green space vary with the type of exposure and whether there is an added benefit to exposure to the natural environment versus an indoor or synthetic environment. Given that this question is still very broad, we propose that it be addressed initially through a two-stage review process. The first stage will be a baseline systematic review, which will allow the identification of pools of research to guide the identification of narrower, more focused questions. The second stage of this review process would focus on these narrower questions and begin with the critical appraisal of the evidence selected during the first stage, followed by data extraction and synthesis.