What you've heard
Forest bathing works because trees release chemicals, called phytoncides, that boost your immune system and fight cancer
Verdict
Enjoy the forest as relaxation, plausible though the size is uncertain; the tree-chemical immune story is unproven
Does forest bathing work through tree chemicals?
Enjoy the forest as relaxation, plausible though the size is uncertain; the tree-chemical immune story is unproven.
The chemical story traces to one Japanese laboratory. Its best-known test put a dozen men in a Tokyo hotel room for three nights with cypress oil vaporized in the air, and found their immune killer cells more active than on a normal working day. There was no placebo scent, nobody was blinded, and a hotel stay is itself a break from work. The laboratory's leader sits on forest-therapy boards. Killer-cell activity is only a stand-in, and no cancer outcome was ever measured. Even for short-term effects on the heart and mood, a recent pooled analysis rated the certainty of the evidence "very low." Forests may still help people relax for a while: pooled small trials suggest blood pressure runs a little lower on the same day. The likeliest reasons are ordinary ones: shade, quiet, walking and company.9
Where the book tests it
Sources
- 9.Li, Q., Kobayashi, M., Wakayama, Y., Inagaki, H., et al. (2009). "Effect of phytoncide from trees on human natural killer cell function." International Journal of Immunopathology and Pharmacology, 22(4), 951-959. https://doi.org/10.1177/039463200902200410 (12 men, three nights, hinoki cypress oil.) ; Li, Q. (2022). "Effects of forest environment (Shinrin-yoku/Forest bathing) on health promotion and disease prevention: the establishment of 'Forest Medicine'." Environmental Health and Preventive Medicine, 27, 43. https://doi.org/10.1265/ehpm.22-00160 (board memberships declared) ; Andrade, A.M., Durães, S.D.F., Franco Netto, L.C.R., Fenner, A.L.D., et al. (2026). "Short-term cardiovascular and mental health responses to Shinrin-Yoku (forest bathing): a systematic review and meta-analysis." Frontiers in Psychology, 17, 1707829. https://doi.org/10.3389/fpsyg.2026.1707829 (Published 2026.) ; Ideno, Y., Hayashi, K., Abe, Y., Ueda, K., et al. (2017). "Blood pressure-lowering effect of Shinrin-yoku (forest bathing): a systematic review and meta-analysis." BMC Complementary and Alternative Medicine, 17(1), 409. https://doi.org/10.1186/s12906-017-1912-z ; Markevych, I., Schoierer, J., Hartig, T., Chudnovsky, A., et al. (2017). "Exploring pathways linking greenspace to health: theoretical and methodological guidance." Environmental Research, 158, 301-317. https://doi.org/10.1016/j.envres.2017.06.028
Note numbers are those of Chapter 10, Green and Quiet.
This verdict is one of the "What you've heard" boxes in Chapter 10, Green and Quiet, where the book tests the claims people repeat against the studies behind them.
Educational content, not medical advice. Disclaimer.