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

Doyun Song,Sieon Kim,Minseo Park,Choyun Kim,Chorong Song,Geonwoo Kim

Published 2025 in Scientific Reports

ABSTRACT

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.

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