A systematic review of evidence of additional health benefits from forest exposure

Exposure to forest environment is beneficial to human health and has complex physiological and psychological effects. Here, we synthesized the results from 40 peer-reviewed publications, and conducted a meta-analysis to evaluate the general responses of 15 variables related to physiological and psychological functions to forest exposure. We summarized the evidence that forest exposure was beneficial to health, such as reducing systolic blood pressure (SMD = -0.27), diastolic blood pressure (SMD = -0.28), heart rate (SMD = -0.27), sympathetic nervous activity (SMD = -0.24), salivary cortisol (SMD = -0.26), and increasing parasympathetic nervous activity (SMD = 0.23). Evidence showed that lower levels of systolic blood pressure and sympathetic nervous activity, and higher levels of parasympathetic nervous activity were associated with duration exposure to the forest, while lower levels of heart rate and higher levels of parasympathetic nervous activity were associated with methods of intervention with forest environment. When exposed to the forest, it was positive in terms of comfortable - uncomfortable (SMD = 2.07), natural - artificial (SMD = 2.56), and soothing - awakening (SMD = 2.01). Our results also showed that significant, moderate-sized reductions in negative emotions, such as anger, confusion, depression and fatigue (SMD = - 0.73-- 1.28), but large-sized improvements in vigor (SMD = 0.64) following exposure to forest environments. Subgroup analysis of duration and intervention methods showed that the psychological effects of forest exposure had no significant difference among different groups except for the effect of duration on tension-anxiety. Publication bias was detected in some parameters. Further large-scale clinical trials should be conducted to identify the mechanism of forest exposure to human health.