2026-06-21

Your body has never forgotten the forest

Editor Neokliniken
2026-06-21
18:40

It’s just waiting for you to go out there.

A Japanese immunologist named Qing Li has spent 20 years proving exactly what happens when you go into the forest. He is a clinical professor at Nippon Medical School in Tokyo and president of the Japanese Society of Forest Medicine. The Japanese government has funded his research since 2004. The results are so strong that forest bathing (shinrin-yoku) is now an officially recommended clinical treatment in both Japan and Korea.

Shinrin-yoku

It all started in 1982 when the Japanese Ministry of Agriculture and Forestry coined the term shinrin-yoku . They wanted to get people to use the vast forests instead of just sitting inside and stressing themselves out. They had no idea how powerful it actually was – until Professor Qing Li ran the first real experiments in 2005. He took 12 healthy men on a three-day forest trip. Two nights. They walked slowly, maybe two hours a day. Nothing strenuous. No breathing exercises. Just regular walks among the trees. Li took blood and urine samples before the trip, during the trip, after seven days and after 30 days. The results were surprising.

NK cells increased by 50%

The activity of NK cells (your body’s own cancer cell and virus hunters) had increased by about 50 percent. Three powerful anti-cancer proteins that the cells produce – perforin, granzymes and granulysin – went up sharply. The effect lasted when they got home. It was still there after seven days and was still partially there after 30 days. Two hours a day in the forest gave an immune system that was upgraded for a whole month. Li repeated the experiment with women a year later. The same results.

What are T cells and NK cells?
T cells and NK cells (natural killer cells) are two of your most powerful immune cells. T cells are part of the adaptive immune system – they learn to recognize specific threats and attack them with precision. NK cells are your “natural killer cells.” They patrol your blood and tissues, instantly killing virus-infected and cancer cells without warning. They are your body’s rapid first-line defense.

Then he did the crucial control test: Same type of trip, same walks, same hotel and food – but in the city instead of in the forest. No measurable changes in the NK cells at all. It wasn’t the holiday: It was the forest .

What is the biological mechanism behind it?

The mechanism is partly chemical. The trees release substances called phytoncides to protect themselves from insects and bacteria. Pine, cedar, oak and cypress emit extra amounts, especially when it’s hot or has rained. You breathe them in and they go straight into the bloodstream and stimulate the very immune cells Li measured. About half of the effect comes from the air itself. The other half comes from what the forest does to your nervous system.

What happens to cortisol during forest stays?

A large Japanese study measured cortisol (the body’s main stress hormone) in participants at 35 different forest locations. After just 30 minute walk in the forest, cortisol dropped significantly compared to the same walk in the city. Heart rate dropped. Blood pressure dropped. Parasympathetic nervous system (rest and recovery) increased. Sympathetic nervous system (fight or flight) decreased.

Then MaryCarol Hunter at the University of Michigan came along and did the cleanest version yet. She had ordinary city dwellers take a “nature pill” three times a week for eight weeks. They were allowed to choose the time, place, and duration—as long as it was outdoors during the day, without phones, without talking, and without strenuous exercise.

The result: 21.3 percent lower cortisol per hour in nature. The greatest effect came between minutes 20 and 30. After that it continued to decrease, but more slowly. The smallest dose that produced measurable stress relief was 20 minutes outside in what felt like nature.

What does this mean for my body?

This is not about nature replacing therapy or medicine when it is truly needed. Qing Li is clear about that herself – it is a powerful complement.

But it shows something much bigger:

Your body is still built for the forest. You spend over 90 percent of your life indoors. Cortisol is high. NK cells are sluggish. The parasympathetic system rarely gets a chance to take over. The nervous system that was honed over millions of years under the canopy of trees now has to live only in plaster and screens.

But it hasn’t been forgotten. It’s ready. All you need to do is go outside. Walk among the trees. Breathe. Walk slowly. Let the phytoncides do their job. Let your immune system wake up. Let the stress hormones subside.

It’s not a nice idea. It’s biology. And it works.

Start today. Your body has waited long enough.

Practical tips

Find a forest near you
Either you know where the forest is, or search Google Maps for forest, or forest walk and your location.
Add a forest walk to your week
Use some time on the weekend, or one of the days after work. Walk for at least 20 minutes or longer. The greatest effect comes between 20 and 30 minutes.
Don’t negotiate
Take a walk in the forest every week. Your body remembers. The more often you go out, the stronger and longer the effect on your immune system, stress and energy.
Forget the technology and the panting
Go slowly. No heart rate monitors, no podcasts. No phone in hand. Just breathe in the air. Look at the trees. Let the calm and the phytoncides do their job.

The effect is greater after rain or when it is warm because then the trees release extra phytoncides.

References

Li Q. Effects of forest environment (Shinrin-yoku/Forest bathing) on health promotion and disease prevention – the Establishment of “Forest Medicine”. Environmental Health and Preventive Medicine, 2022;27:43. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9665958/
Li Q et al. Forest bathing enhances human natural killer activity and expression of anti-cancer proteins. Int J Immunopathol Pharmacol. 2007
New Concept of Forest Medicine. Forests 2023;14:1024.