Shinrin-yoku: the forest as recognised medicine — NK cells and anti-cancer proteins

Shinrin-yoku — “forest bathing” in Japanese — is not a poetic metaphor. It is a medical practice recognised in Japan since 2004, underpinned by rigorous physiological measurements.

A review published in 2022 in Environmental Health and Preventive Medicine by Qing Li (Nippon Medical School, Tokyo) summarises the clinical data accumulated on the practice. The picture is precise: shinrin-yoku reduces blood pressure and heart rate, and increases the activity of NK (natural killer) cells. These natural killer cells are at the core of innate immunity: the review notes a rise in “intracellular levels of anti-cancer proteins” — a biological signal that warrants clinical studies, not yet proof of cancer prevention. On cortisol, the data are less clear-cut: a GRADE meta-analysis published in 2026 covering 11 studies found only negligible variation, in contrast to what older studies — including Qing Li’s — had reported.

At the level of mood, the POMS (Profile of Mood States) test records falls in scores of anxiety, depression, anger, fatigue and confusion — and a rise in the vigour score.

What distinguishes shinrin-yoku from a simple walk is intention: slow walking, attention given to smells, sounds, bark textures. No equipment required. The effects are measurable even during brief exposures.

For anyone with access to a forest or an urban woodland, this is probably one of the most accessible preventive interventions available — and one of the least prescribed.

Further reading: Nature as Medicine: What Science Now Measures