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08/04/2026
A Japanese immunologist spent more than twenty years studying what happens inside the human body when people spend time in forests.
What he discovered surprised even many researchers.
Trees release natural airborne compounds called phytoncides.
As we walk through forests and breathe the air, we inhale some of these microscopic plant compounds.
Research led by Dr. Qing Li found that spending time in forests was associated with lower levels of stress hormones such as cortisol and adrenaline, along with increases in the activity of natural killer (NK) cells, an important part of the immune system.
Some of those immune-related changes were still measurable several days after participants had returned home.
In another experiment, researchers diffused phytoncide-rich tree oils into hotel rooms and observed some similar biological responses, suggesting the compounds themselves may contribute to the effects.
That doesn't mean forests are a medicine or that phytoncides directly eliminate stress hormones.
Physical activity, cleaner air, quieter surroundings, and time away from everyday stress all likely play important roles as well.
But the research suggests that forests may be doing more than simply helping us relax.
Every walk beneath the trees could be exposing our bodies to a complex mix of natural chemicals that subtly influence how we feel and how our bodies respond.
