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 . 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 . 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 .
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.