Thermal conditions modulate urban forest therapy outcomes: a meta-analytic review

Nature-based interventions (NBIs) such as forest therapy improve stress-related outcomes; however, their efficacy may depend on thermal conditions. This systematic review and meta-analysis investigated the moderating role of thermal conditions on the psycho-physiological outcomes of forest-based interventions. Following PRISMA guidelines (PROSPERO: CRD42022353168), studies of general populations exposed to forests under varying Discomfort Index (DI) conditions were synthesized. Outcomes included heart rate (HR) and Profile of Mood States (POMS; tension, fatigue, confusion, and vigor). A random-effects meta-analysis with subgroup tests by DI category was performed; risk of bias was assessed with ROB 2/ROBINS-I, and certainty of evidence with GRADE. From 3,384 records, 9 studies (30 cases) were included. Under thermally neutral conditions, forest exposure reduced HR (SMD - 1.19, 95% CI - 1.72 to - 0.66) and improved POMS tension (- 1.36, - 2.08 to - 0.64), fatigue (- 0.90, - 1.24 to - 0.57), confusion (- 1.10, - 1.61 to - 0.60), while increasing vigor (+ 1.23, 0.37 to 2.09). In cold and warm-hot conditions, effects were inconsistent. Evidence certainty was moderate for psychological outcomes and very low for physiological outcomes. These results suggest that psycho-physiological benefits of forest exposure appear contingent on thermal conditions. Because the included studies are geographically concentrated and between-study heterogeneity is substantial, the pooled estimates should be interpreted with caution. Prioritizing thermally neutral DI conditions (16-24) may enhance the effectiveness of NBIs, and future trials using standardized thermal indices in more diverse populations are warranted.