What the Forest Does to Your Immune System

Forest bathing, the Japanese practice of shinrin-yoku, means unhurried, attentive time among trees, and over two decades it has accumulated a surprisingly physiological evidence base rather than a purely psychological one. This review draws that literature together across three systems. On immunity, forest exposure is repeatedly associated with increased activity and number of natural killer cells, the immune cells that patrol for virus-infected and tumour cells, an effect some studies found still measurable a week after a multi-day forest trip. On stress physiology, it tracks with lower cortisol and a tilt of the autonomic nervous system toward parasympathetic, rest-and-restore dominance, alongside reductions in heart rate and blood pressure. A plausible mechanism is proposed in phytoncides, the airborne compounds trees release, which appear to contribute to the immune effects when breathed in. Where the evidence stops: this is a narrative review, and the authors are candid that the underlying studies vary widely in design, dose and quality, that samples are often small, and that firm causal claims are not yet warranted. The honest reading is that the direction of effect is consistent and biologically coherent even where the precise magnitude is not settled. Time in a forest appears to do something real to the body, not merely to the mood.