Health and well-being benefits of spending time in forests: systematic review

There is a growing interest in the health benefits associated with individuals undertaking outdoor activities in a natural environment [1, 2]. Forests and other natural environments are recognized as fundamental health resources and may play a role in disease prevention [3], with one population survey reporting that the average person spends almost 90% of their life indoors [4].

Since the development of the concept of nature as a therapy in the 1990s [5, 6], a number of studies using a variety of methodologies, have been conducted to examine the effect of forest environments on health promotion and well-being. Several recent environmental studies have claimed that a number of medical symptoms related to lifestyle stress can be treated by encouraging individuals to interact with nature [7]. Furthermore, one study revealed an association between positive health outcomes and the amount of exposure an individual has to a green environment [8].

Moreover, research conducted in a very specific natural context, exposure to “forests”, or “forest therapy”, have reported potential benefits in the management of psychological symptoms including anxiety [9], depression [9], mood disorder [10], burnout syndrome [11], lifestyle-related stress [12], and overall quality of life [13]. Studies investigating the effect of forest therapy on physiological well-being have also demonstrated a positive impact on cognitive function [14], immune function [15, 16], blood glucose levels in diabetic patients [17], hypertension [18], cardiovascular disease [19], cancer [20], and pain [21]. Earlier studies conducted with surgical patients suggest that exposure to a green environment is associated with the recovery of illness and even decreased mortality [22‐24].

Despite a number of studies providing preliminary evidence linking exposure to a natural, green forest environment and positive psychological and physiological benefits [25‐27], most health professionals have failed to recommend nor integrate such therapy for the management or prevention of disease during their medical consultations [24]. As yet, there are no evidence-based clinical practice guidelines for the use of such therapy for healthcare professionals. In previous literature reviews [1], authors have attempted to draw conclusions regarding the therapeutic benefits of forest therapy but doubt remains regarding its efficacy and value [28].

While the theory of forest therapy [5, 6], the belief that taking time out in nature, specifically a forest area, can affect human well-being and heath, was first proposed in Western countries, it was researchers from Japan that instigated studies to evaluate the therapeutic effect of spending time in forests on health and well-being. These Japanese studies influenced further research in Korea, China, and Europe. It was the Japanese who coined the term “shinrin-yoku” to depict activities of recreation and relaxation in a forest. Shinrin-yoku specifically relates to the activity of forest bathing, staying and/or walking in forest, and breathing in the volatile substances released by the trees [17]. Hence, we conducted a further literature review to evaluate the current evidence of spending time in natural green environment, specifically, forest bathing. Our research question is “what, if any, evidence is there that forest bathing has effects on health, and if so, what health indicators show improvement with this exposure?”

There continues to be conjecture about the value that exposure to nature plays in human health and disease. Numerous case studies and epidemiological and observational studies conducted with forest intervention reported positive health and well-being outcomes among the participants who spent time in a forest, and some benefit was shown to be derived even with simply viewing natural environments. Previous literature reviews attempted to synthesize the results of case studies and epidemiological and observational studies to demonstrate the positive effect of spending time in forest rather than providing evidence based on RCTs. To our knowledge, to date, there has been no systemic literature review conducted to assess the evidence of health and well-being benefits of forest therapy based on RCTs. Hence, this present review builds on the previous work and goes further by including RCTs in the evaluation of the evidence for the physiological and psychological health and well-being benefits of forest therapy.

This present review found that six RCTs reported promising therapeutic benefits of forest exposure on several physical and psychological conditions including hypertension, cardiac and pulmonary function, immune function, inflammation, oxidative stress, stress, stress hormone, anxiety, depression, and emotional response, although outcomes of anxiety and depression had mixed results and some inflammatory biomarkers showed null results. These data show a consistent trend in a broad range of health outcomes, suggesting potential for forest bathing to improve physiological and psychological health in healthy and health-compromised individuals, but these results are drawn mostly from studies with strong to moderate design weaknesses. All studies included in this review had a high ROB (Fig. 1). Two of the studies only reported the differences pre and post intervention within each group, and even though they found significant improvements within the forest therapy group, they failed to report whether there was any difference between groups, thus not testing the significance of between-group differences expected of an rigorously evaluated RCT. Of the six RCTs, five evaluated only the immediate or short-term effect of forest intervention [1 day (n = 2), 2 days (n = 1), 7 days (n = 1), 9 days (n = 1)] without long-term follow-up. Only one RCT examined the effect of 11 weeks of forest intervention. Furthermore, five studies were conducted with small sample sizes and thus failed to meet statistical power required to detect significant therapeutic effects, if any. None of the studies were performed with blinding of subjects. Moreover, none of the studies controlled for potential bias or confounding factors (social interaction of subject, physical activities, and forest environmental factors) during the data analysis thus limiting conclusions which may be drawn regarding the true therapeutic effect of forests. Nonetheless, the present review findings are consistent with previous reviews that suggest benefits; however, the current findings need to be evaluated cautiously due to the majority of the studies having risk of bias, low sample sizes, and lack of control for participant expectation effects [34, 35].

The present review did not find convincing evidence of the benefits of forest therapy due to the lack of high-quality studies. However, we cannot disregard the potential impact of nature in the form of forest therapy on health and disease. The concept of human health and longevity and its relationship with the natural environment has a long history [36]. Several studies suggest that exposure to a green environment is associated with a positive impact on physical and psychological well-being including recovery from illness and even decrease mortality [23, 37, 38]. A recent study also revealed that therapeutic benefits from nature and more specifically a green environment may be dose-dependent [8]. It appears likely that the therapeutic benefit of forest therapy is multi-factorial. It may be induced by the complex ecosystem as a whole such as the green scenery, fresh air, sunlight, clean water, rocks, soil, soothing sounds of streams, waterfalls, birds, and natural aromas of trees, plants, and flowers [39].

Taking into account the complexity of existing research on the therapeutic benefits of nature, researchers should consider the following suggestions to improve the rigor of future studies.

First, evaluation of both short-term intervention (1 week) and long-term intervention (12 weeks) duration is recommended, with multiple follow-ups in the post intervention phase (3 months, 6 months, and 12 months). The dose-response relationship should be examined by varying length (e.g., 30 versus 60 versus 90 min), frequency (e.g., weekly versus biweekly versus every 4 weeks), and intensity of intervention, as measured by a physical activity intensity scale.

Next, the lack of participant blinding in most of these forest intervention studies could be remedied. When the study design is a forest versus city outdoor exposure comparison, having participants believe they are involved in a “walking” study would blind the urban vs. forest walkers to the expected outcome. There may be a benefit in designing a study that also controls for the outdoor activity, for example a three-arm design (forest versus urban walking versus either a waitlist group or an indoor reading group). Adequate sample size is recommended to detect statistical and clinical significance; future studies should include power calculations for sample size selection. Moreover, it is recommended that future studies utilize both quantitative and qualitative approaches that will capture the complexity of the forest environment effect. Measurement of disease-specific biomarkers (e.g., cancer makers for the oncology patient) and overall well-being biomarkers (e.g., immune function, cytokines, and DNA damage) may provide objective information on the physiological and psychological effects of forest intervention. Finally, a cost-benefit analysis of forest intervention should be conducted in order to support the possible implementation of forest therapy from a health economics perspective.

The objective in this review was to evaluate the health and well-being benefits of a specific type of exposure to nature, forest therapy based on RCTs. In conclusion, strong evidence of the benefits of forest environment on health and well-being has yet to be confirmed. The findings of this review support the premise that exposure to a forest environment may provide benefits. However, the evidence is insufficient due to methodological design flaws. Future research studies with more robust design is warranted. While the beneficial effects of forest therapy require further investigation, given that the intervention has low risk of adverse effects and the likelihood that such a recommendation would also increase outdoor activity, policy makers and health professionals should consider recommending forest therapy to their patients and the general public.