Effects of forest bathing (shinrin-yoku) on levels of cortisol as a stress biomarker: a systematic review and meta-analysis.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of controlled intervention studies
PubMed 31001682 · doi:10.1007/s00484-019-01717-x
What was done
Authors searched Medline/PubMed, Embase, Scopus, Web of Science, Cochrane Library, and Google Scholar to evaluate the effect of forest bathing (shinrin-yoku) versus control/comparison environments (primarily urban settings) on salivary or serum cortisol. Quality was assessed using NIH tools. Of 971 screened articles, 22 were included in the qualitative synthesis and 8 in the quantitative meta-analysis.
What was found
In 20 of 22 included studies, cortisol levels were significantly lower in forest groups versus controls or showed significant within-group pre-post reductions. The meta-analysis of salivary cortisol showed significantly lower levels in forest groups compared to urban groups both before intervention (mean difference = -0.08 μg/dl [95% CI -0.11 to -0.05], p < 0.01; I² = 46%) and after intervention (mean difference = -0.05 μg/dl [95% CI -0.06 to -0.04], p < 0.01; I² = 88%).
Why it matters
This review provides quantitative evidence that brief immersion in forest environments correlates with reduced physiological stress markers compared to urban exposure.
Limits
Salivary cortisol was already significantly lower in the forest groups prior to the intervention, pointing to expectation effects, baseline imbalances, or inadequate randomization. Post-intervention outcomes exhibited high statistical heterogeneity (I² = 88%). Only 8 of 22 identified studies could be pooled quantitatively, and long-term effects were not evaluated.
Cited by
- partial Japanese shinrin-yoku (forest bathing), visiting urban parks, and having houseplants reduce salivary cortisol levels.