Forest Bathing (Shinrin-yoku): What Happens in the Body When We Go into the Forest
The forest may measurably change your cortisol, your heart rhythm and your defence cells. That is not a mood. That is physiology, with all the question marks that belong to it.
Forest bathing has an image problem. It sounds like incense sticks and hugging trees. Yet it is one of the best measured natural phenomena in stress research. You only have to be willing to look at the numbers instead of laughing at the name.
I bet you know this moment. You go into the forest, really just to move a little. After ten minutes you notice that your shoulders are hanging lower. After twenty minutes you are breathing differently. Not deliberately, it simply happens.
And then comes the thought that makes it all smaller: it is just imagination. Fresh air, that is all. A bit of movement.
I find that thought interesting. We immediately accept that noise, screen light and open plan offices can act on the body. But as soon as the environment does something pleasant, it gets declared esoteric. That is not a particularly logical line to draw.
In Japan this line was never drawn. The term Shinrin-yoku, literally bathing in the atmosphere of the forest, was coined in 1982 by the Japanese Ministry of Agriculture, Forestry and Fisheries. The term became a research programme. People were sent into the forest and their saliva, pulse, blood pressure and defence cells were measured. And they were compared with people who walked the same distance in the city.
I would like to show you what came out of that. And just as honestly, where the data become thin, where expectation effects may be at work and where a third of the participants simply did not respond the way people had hoped.
What awaits you in this article
- What happens to cortisol and blood pressure in the forest
- Why the vagus plays along and how it is measured
- Terpenes: the scent of the forest as a molecule
- How tree oils were tested in a hotel room
- Natural killer cells and their defence proteins
- Why rumination gets quieter in the forest
- What large population data show and what they do not
- Where the evidence is honestly weak
- Three levers you can put into practice this week
- For whom caution is appropriate
The forest talks to your nervous system first
May I ask you an uncomfortable question? When did you last notice your body coming out of alarm mode?
Most people who come to me know the opposite state very well. Awake in the morning, but not rested. The pulse a little too high. The shoulders pulled up, for weeks now. The body has forgotten how powering down works.
It is exactly this switch that the Japanese research groups measured. And not in the laboratory, but outdoors.
Who: Park and colleagues ran field experiments in 24 forests across Japan and published the results in 2010 in Environmental Health and Preventive Medicine. A total of 280 young adults took part, twelve per forest.
What they observed: Each group went into the forest on one day and into an urban area on the other, as a crossover comparison. Walking lasted 16 ± 5 minutes, followed by viewing the surroundings for 14 ± 2 minutes. In the forest environment, salivary cortisol, pulse rate and blood pressure were lower than in the city. Parasympathetic activity was higher, sympathetic activity lower.
What that may mean for you: Twenty minutes. Not two weeks off. If that order of magnitude holds, then the forest is not the big holiday but a short, repeatable braking. These are snapshots, not statements about the course of a disease.
Park BJ, Tsunetsugu Y, Kasetani T, Kagawa T, Miyazaki Y. The physiological effects of Shinrin-yoku. Environ Health Prev Med. 2010;15(1):18-26. DOI: 10.1007/s12199-009-0086-9What heart rate variability actually measures
Your heart does not beat like a metronome. Between two beats there are sometimes 840 milliseconds, sometimes 870, sometimes 810. These small fluctuations are called heart rate variability (HRV).
Imagine two hands on a steering wheel. The sympathetic nervous system pulls in one direction, the parasympathetic nervous system in the other. When both make fine adjustments against each other, a lively, irregular pattern emerges. When the stress system sits at the wheel alone, the rhythm becomes rigid. A rigid heart is not a calm heart. It is a tense one.
The share of the fast fluctuations, in technical language the high frequency band, is considered a fairly good marker for the activity of the vagus nerve. The vagus is the main line of the parasympathetic nervous system, that is, of rest and digestion mode.
Who: Kobayashi and colleagues analysed the heart rate variability of 485 men in 2018 in Frontiers in Public Health, measured during roughly 15 minute walks in 57 forests and 57 urban areas in Japan.
What they observed: On average the values differed clearly between forest and city. What I find more important, though, is the breakdown: 65.2 percent of participants showed the expected rise in the parasympathetic marker, 67.0 percent the expected fall in the sympathetic ratio.
What that may mean for you: Roughly one in three did not respond in the expected direction. That is not a weakness of the study, that is biology. An average describes a group, not you. If the forest gives you little, there is nothing wrong with you.
Kobayashi H, Song C, Ikei H, Park BJ, Lee J, Kagawa T, Miyazaki Y. Forest Walking Affects Autonomic Nervous Activity: A Population-Based Study. Front Public Health. 2018;6:278. DOI: 10.3389/fpubh.2018.00278The stimulus comes in
Soft, irregular shapes, dimmed light, broadband sounds without sudden peaks, aroma molecules via the olfactory nerve. All stimuli that do not force vigilance.
The amygdala signals all clear
The alarm centre rates the environment as harmless. The constant impulse to the hypothalamus grows weaker. Physiologically plausible, but not directly measured in humans in the forest.
The stress axis eases off
Via hypothalamus, pituitary and adrenal cortex, in short the HPA axis, cortisol release falls. Exactly that can be measured in saliva and has been described many times.
The vagus gets room
Parasympathetic activity rises, pulse and blood pressure fall, heart rate variability increases. Urinary adrenaline and noradrenaline were lower in several studies.
Forest bathing is not a relaxation technique you have to learn. It is rather the opposite: the removal of stimuli that keep your nervous system permanently on standby.
You do nothing. You stop doing something. And the body takes over a regulation it is capable of anyway.
And now you know why the shoulders drop after just ten minutes: the stimulus that kept them up is gone.
The scent of the forest is a molecule
Next time you pass a spruce, take a deliberate sniff. That resinous, slightly lemony smell is not a mood. It is chemistry in the air.
Trees release volatile compounds, above all terpenes. Part of them can act antimicrobially and protects the tree from germs and pests. For this fraction the term phytoncides has become established.
In the forest air of the Japanese investigations, alpha-pinene, beta-pinene and isoprene were measured above all. In city air the same substances were practically undetectable. That is an important point: the difference between forest and city is not only a difference in what you see. It is also a difference in what you breathe in.
How to take the forest out of forest bathing
When someone is more relaxed in the forest, that can have many reasons. The movement, the holiday feeling, the absent work, the better air, the expectation. So the interesting question is: can the contribution of the aroma molecules alone be tested?
That is exactly what was attempted. Twelve healthy men aged 37 to 60 spent three nights in a city hotel. No forest, no hike, no holiday feeling. In the room, stem oil of the hinoki cypress was vaporised through a humidifier. Alpha-pinene and beta-pinene could subsequently be detected in the room air.
The result: the activity of natural killer cells rose, as did their proportion and the amount of perforin, granulysin and granzyme in the cells. The proportion of T cells fell, as did urinary adrenaline and noradrenaline. The authors conclude cautiously that phytoncide exposure and reduced stress hormones might contribute in part to the effect.
Twelve participants are few. There was no control group in the sense of a blinded study. But as a signal that there may be something to the terpene hypothesis, this experiment is elegant.
Who: Yang and colleagues investigated in 2016 in Molecular Pharmacology how alpha-pinene acts on the sleep of mice. They combined sleep measurements in the living animal, electrophysiological recordings from brain slices and molecular modelling.
What they observed: (-)-alpha-pinene lengthened the duration of deep sleep and shortened the time to fall asleep. The effect could be abolished by flumazenil, an antagonist at the benzodiazepine binding site of the GABA-A receptor. That suggests alpha-pinene docks exactly there, as a partial modulator.
What that may mean for you: It makes it biologically plausible why a scent can be calming, namely via a very specific receptor. But this is a mouse study with oral administration. Whether the concentrations you breathe in the forest could do anything similar in humans is not shown by it.
Yang H, Woo J, Pae AN et al. α-Pinene, a Major Constituent of Pine Tree Oils, Enhances Non-Rapid Eye Movement Sleep in Mice through GABAA-benzodiazepine Receptors. Mol Pharmacol. 2016;90(5):530-539. DOI: 10.1124/mol.116.105080Before the impression arises that every tree oil pulls in the same direction: in a small investigation with 16 healthy adults, two cypress oils were inhaled for five minutes each. With one oil, sympathetic activity fell and parasympathetic activity rose. With the other oil it was exactly the other way round, even though both influenced mood pleasantly.
That is a good antidote to overly smooth stories. Scent is not simply scent. And an aroma oil from the shelf is not the same thing as the mixture that hangs in the air of a living stand of trees.
And now you know why I am reluctant to speak of wellness when it comes to forest bathing. This is about molecules with names and receptors with addresses.
Why the immune system is listening in
This is where it gets really interesting from the perspective of clinical psychoneuroimmunology. Because the nervous system and the immune system are not separate departments. They speak the same language.
Natural killer cells, NK cells for short, are a kind of patrol force of innate immunity. They do not need a wanted poster. They recognise from surface features whether a cell has been altered, for example by a virus. Then they deploy their tools: perforin drills pores into the target cell, granzymes enter and initiate controlled cell death, granulysin additionally attacks microbes.
And this patrol force has receptors for stress hormones. Adrenaline and cortisol can dampen its activity. So if stress hormones fall, it would be plausible that NK activity rises.
Who: Li and colleagues published a comparison in 2008 in the International Journal of Immunopathology and Pharmacology in which twelve healthy men aged 35 to 56 took both a three day forest trip and a city trip with the same level of activity.
What they observed: After the forest trip, NK activity, NK cell numbers and the number of cells carrying perforin, granulysin and granzyme A and B rose. Urinary adrenaline fell. The elevated value was still measurable more than seven days later. After the city trip none of this appeared. Alpha-pinene and beta-pinene were found in the forest air, in city air almost not at all.
What that may mean for you: The comparison arm is the decisive part. It was not the travelling and not the walking, otherwise the city would have produced the same result. Twelve people nevertheless remain twelve people. And laboratory values say nothing about infections or cancer.
Li Q, Morimoto K, Kobayashi M et al. Visiting a forest, but not a city, increases human natural killer activity and expression of anti-cancer proteins. Int J Immunopathol Pharmacol. 2008;21(1):117-127. DOI: 10.1177/039463200802100113A year later the same pattern was described in thirteen nurses aged 25 to 43. There too the increase lasted more than seven days. And in a further investigation, a single day trip to a forest park with two walks of two hours each was already enough to raise NK activity and defence proteins. Blood cortisol and urinary adrenaline were lower afterwards.
Who: Li and colleagues tested in 2006 in Immunopharmacology and Immunotoxicology whether phytoncides also act on NK cells outside the body. A human NK cell line in the test tube was used.
What they observed: The phytoncides increased cell lysing activity in a dose dependent way and raised perforin, granzyme A and granulysin in the cells. They were also able to partly restore NK activity that had been dampened by an insecticide.
What that may mean for you: This is cell culture, not a human being. It proves nothing about your walk in the forest. But it does show that a direct point of attack at the cell is conceivable and not only the detour via reduced stress hormones.
Li Q, Nakadai A, Matsushima H et al. Phytoncides (wood essential oils) induce human natural killer cell activity. Immunopharmacol Immunotoxicol. 2006;28(2):319-333. DOI: 10.1080/08923970600809439Path one: via the hormones
Less stimulus pressure, less adrenaline and cortisol. Since NK cells carry receptors for these hormones, a dampening influence could fall away. This path is the best described one in humans.
Path two: via the molecules
Terpenes from forest air reach the body via lungs and blood and could act directly on defence cells. So far documented mainly in cell culture and in the hotel room experiment.
Path three: via lifestyle
More movement, more daylight, often better sleep the following night. All factors with good evidence of their own. In forest studies they can never be fully separated out.
Your immune system is not a closed system on the inside. It reads along with the environment you are in. The forest is simply a particularly well studied example of that reading.
And now you know why nervous and immune systems are never looked at separately in KPNI (Clinical Psychoneuroimmunology).
Why the head gets quieter in the forest
There is a state that almost everyone knows and hardly anyone names. You lie awake and the same thought goes round its loop. For the fourth time. For the twelfth time. Nothing new is added, but stopping is not possible either.
Psychology calls this rumination. It is considered a risk factor for depressive developments. And it has a fairly clear address in the brain.
Who: Bratman and colleagues published an investigation in healthy participants with functional magnetic resonance imaging in 2015 in the Proceedings of the National Academy of Sciences.
What they observed: A 90 minute walk in a natural environment lowered both self reported rumination and activity in the subgenual prefrontal cortex. An equally long walk in an urban environment changed neither the one nor the other.
What that may mean for you: The subgenual prefrontal cortex is linked with self focused withdrawal and rumination. That an environment can change this activity makes the everyday experience tangible. It is a single study in healthy people, not a treatment study in depression.
Bratman GN, Hamilton JP, Hahn KS, Daily GC, Gross JJ. Nature experience reduces rumination and subgenual prefrontal cortex activation. Proc Natl Acad Sci U S A. 2015;112(28):8567-8572. DOI: 10.1073/pnas.1510459112Two explanations that do not exclude each other
Attention recovers
- Attention Restoration Theory assumes that directed attention fatigues
- The city demands it constantly: traffic lights, signs, people, decisions
- Nature holds attention gently and without effort
- Voluntary control gets a break as a result
- Prediction: better concentration after the visit
Stress subsides faster
- Stress Recovery Theory starts deeper, at the autonomic nervous system
- Certain environmental features are considered evolutionarily familiar
- The alarm appraisal drops faster than in a built environment
- The effect comes before conscious appraisal
- Prediction: faster recovery of pulse, blood pressure, skin conductance
The historical starting point of this second line of thinking is a surprisingly simple observation from an American hospital. Medical records of gallbladder operations from the years 1972 to 1981 were compared. 23 patients with a window view of a natural scene stayed in hospital for a shorter time, received weaker painkillers and were described negatively less often in the nursing notes than 23 comparable people with a view of a brick wall.
This window study is cited very often, and it did set the research going. But it covers 46 people and is a retrospective review of records. It proves nothing, it laid a trail.
And something similar applies to the attention theory: a systematic review with 31 studies found a clear advantage for nature in only three test procedures within the pooled analyses. In ten further analyses no pronounced benefit appeared. The authors themselves speak of uncertainty about which components of attention are affected at all.
The interesting question is not whether the forest makes you happy. It is: what are you actually recovering from?
Your nervous system is not built for uninterrupted appraisal. It needs times in which nothing wants anything from you. The forest is a place where that works without discipline. That is exactly what makes it fit for everyday life.
And now you know why you can think better in the forest, even though you were not thinking at all.
What the large population numbers say
So far we have talked about small field experiments. A few dozen people, a few hours, laboratory values. A fair question: does anything remain of that when you look at millions of people over years?
Who: Rojas-Rueda and colleagues pooled nine cohort studies with a total of 8,324,652 people from seven countries in 2019 in The Lancet Planetary Health.
What they observed: Per 0.1 more greenness within 500 metres of the home, the pooled ratio for all cause mortality was 0.96 with a range of 0.94 to 0.97. Seven of the nine studies found such an association, two found none. The spread between the studies was very high at a value of 95 percent.
What that may mean for you: This is an observation, not an experiment. People who live in the green are often wealthier, breathe cleaner air and move more. The authors adjusted for that, but it can never be ruled out entirely. The association is stable, the cause remains open.
Rojas-Rueda D, Nieuwenhuijsen MJ, Gascon M, Perez-Leon D, Mudu P. Green spaces and mortality: a systematic review and meta-analysis of cohort studies. Lancet Planet Health. 2019;3(11):e469-e477. DOI: 10.1016/S2542-5196(19)30215-3A second large synthesis evaluated 103 observational studies and 40 intervention studies with around 100 outcome measures. More contact with greenery went along with lower salivary cortisol, lower heart rate, lower diastolic blood pressure and higher activity in the fast frequency band of heart rate variability. For all cause mortality, cardiovascular mortality and type 2 diabetes, lower risk ratios were found. The authors themselves add a clear caveat: several of these analyses are limited by weak study quality and high heterogeneity.
Who: Nguyen and colleagues examined in 2023 in The Lancet Planetary Health what happens when time in nature is deliberately recommended and accompanied by professionals. 92 studies were included, 28 of them entered the pooled calculations.
What they observed: Compared with control conditions, systolic blood pressure was on average 4.82 mmHg lower and diastolic 3.82 mmHg lower. Depression and anxiety scores fell moderately to clearly. Daily step counts rose by about 900 steps. For weekly time of moderate activity there was no reliable difference. Most studies had a moderate to high risk of bias.
What that may mean for you: The blood pressure value lies in an order of magnitude that can be clinically relevant. It does not replace treatment or medication. What is more remarkable is that the forest was not what got prescribed here, but simply time outdoors, organised and accompanied.
Nguyen PY, Astell-Burt T, Rahimi-Ardabili H, Feng X. Effect of nature prescriptions on cardiometabolic and mental health, and physical activity: a systematic review. Lancet Planet Health. 2023;7(4):e313-e328. DOI: 10.1016/S2542-5196(23)00025-6And how much time does it take? The most cited orientation at present comes from an English survey with 19,806 participants. From about 120 minutes of nature contact per week, the probability of good self reported health and high wellbeing was clearly higher than without nature contact. The maximum lay between 200 and 300 minutes, after which nothing more was added. And it made no difference whether the time came in one block or across several visits.
And now you know why I do not talk about retreats when it comes to forest bathing, but about a habit.
Where the evidence honestly gets thin
Now comes the part that forest bathing articles usually leave out. I consider it the most important one.
If I show you ten studies with matching results above, an impression of certainty arises that the data cannot carry. So let us sort things out.
Five limitations you should know
- Small samples. Many of the impressive immune findings rest on twelve or thirteen people. That is enough for a hypothesis, not for a recommendation.
- Narrow selection. Mostly young or middle aged men, mostly in Japan, mostly from a small number of research groups. Whether that transfers to a 68 year old woman in Berlin is open.
- Blinding is impossible. Nobody can fail to notice whether they are standing in a forest or on a main road. So expectations always flow in.
- Mixed influences. Movement, daylight, particulate matter, noise, holiday feeling. In a forest study all of these change at the same time.
- High spread. In the large syntheses, heterogeneity between studies is sometimes very high. A shared average becomes less meaningful as a result.
Who: Wen and colleagues screened 210 papers from the years 2015 to 2019 in 2019 in Environmental Health and Preventive Medicine and included 28 of them. Methodological quality was rated with an established checklist.
What they observed: Across the studies, improvements appeared in circulatory, metabolic, immune and mood measures. At the same time they found that randomised investigations scored significantly better methodologically than non randomised ones, and they explicitly call for higher methodological quality with a lower risk of bias.
What that may mean for you: The field itself says that it is still methodologically young. That is no reason to dismiss forest bathing. It is a reason not to build promises of healing out of it.
Wen Y, Yan Q, Pan Y, Gu X, Liu Y. Medical empirical research on forest bathing (Shinrin-yoku): a systematic review. Environ Health Prev Med. 2019;24(1):70. DOI: 10.1186/s12199-019-0822-8I find one side finding from the cortisol meta-analysis especially instructive. There, the salivary cortisol values of the forest groups were lower than those of the city groups not only after the visit, but already beforehand. The authors explicitly name anticipated expectation effects as an explanation.
What an expectation effect means and what it does not
In public debate, placebo is a term of abuse. If something is only placebo, it counts as worthless. I see that differently, and so does physiology.
If the mere prospect of going into the forest lowers your cortisol, then that is not a deception. It is evidence of how far expectation reaches into the stress axis. The body responds to a prediction, not only to the event.
For you in everyday life, that changes little. For the question of how much the terpenes, the movement or the anticipation contribute, it changes everything. And that is exactly why I like to name names when it comes to forest bathing.
In a randomised investigation with 24 older people with high blood pressure, one half spent seven days in an evergreen broadleaf forest, the other in the city. In the forest group, blood pressure and several inflammatory and circulatory markers fell, among them interleukin-6 and TNF-alpha. Sounds impressive.
But the same paper also states that air quality at the forest site was clearly better, measured by particulate matter. Twelve people per group, different air, different place, different daily routine. I cite the study because it fits the hypothesis. It cannot prove what the forest as such contributes.
And now you know why I say: the direction of the findings is consistent, the strength of evidence of the individual study is not.
What you can make of this for your week
That leaves the question that always comes at the end: and now what?
I do not want to give a set of instructions here with a breath counter and a mindfulness exercise. The appeal of this thing lies precisely in the fact that it works without technique. Three levers are enough.
Lever 1: Twenty minutes, not two hours
The measured changes in cortisol and heart rate variability appeared in the Japanese field experiments after 15 to 20 minutes. That is a lunch break. If instead you plan to hike for three hours at the weekend, it mostly does not happen. Small and regular beats big and rare.
Lever 2: Walk slowly and then stand still
In the studies, forest bathing consisted of two parts: a short, slow walk and a following phase of quiet viewing. The second part is the first to be dropped in everyday life. Yet it is probably the more effective part, because it takes away the last demand, namely making progress.
Lever 3: Nose open, headphones off
If there is something to the terpene hypothesis, then part of the effect runs through the air you breathe. Breathe through the nose, stop at a resin scented spot, take a few calm breaths. And the sound of a forest belongs to the stimuli that demand no attention. A podcast does exactly the opposite.
A small self experiment for two weeks
- Choose a fixed time of day, ideally the same one as an existing appointment
- Look for the nearest stand of trees, a park is enough to start with
- Walk slowly for twelve minutes, then stand or sit for eight minutes
- Leave the phone in your pocket, no music and no speech in your ears
- Afterwards note one line: how does the neck feel, how does the breath feel
- After two weeks read the notes and decide for yourself
What I expect from it
Somewhat calmer evenings, falling asleep more easily, less mental spinning after work. In some people clearly, in others hardly at all. Both are normal.
What I do not expect from it
That forest bathing could replace the treatment of high blood pressure, depression, autoimmune disease or chronic exhaustion. It is one building block, not a foundation and not a substitute.
The forest asks nothing of you. That is not a side note. It is probably the active ingredient.
Shukri JarmoukliWhen caution is appropriate
With pollen allergy or allergic asthma, a flowering stand in the wrong week can be a real burden. In parts of Germany, above all in the south and south east, ticks are a real issue: cover up, check yourself afterwards, and clarify the question of the TBE vaccination with your doctor. With cardiovascular disease, the same applies to gradient and distance as to any other physical load. And in an acute mental health crisis the forest is no substitute for help, at most an addition to it.
This is not about another item on your optimisation list. It is about something we have almost unlearned in cities: having a place where nobody wants anything from you.
The capacity to regulate is not a luxury. It is the basis for making sure that strain does not change you permanently. The forest is one of the few places where it is available free of charge and without instruction.
And now you know why I take forest bathing seriously, even though the name sounds as if you would not have to.
Frequently asked questions about forest bathing and Shinrin-yoku
What exactly is forest bathing (Shinrin-yoku)?
Shinrin-yoku translates roughly as bathing in the atmosphere of the forest. The term was coined in 1982 by the Japanese Ministry of Agriculture, Forestry and Fisheries and simply describes spending time in the forest with all your senses.
It is not about athletic hiking and not about a technique with a fixed sequence. In the field experiments by Park and colleagues, forest bathing usually meant two things: walking slowly for about 15 to 20 minutes and then sitting quietly while looking into the stand of trees.
It was exactly this unspectacular combination that was measured. Everything offered today as a course or a certificate is an interpretation of that, not what took place in the studies.
What happens in the body during forest bathing?
In field experiments in 24 Japanese forests with a total of 280 participants, salivary cortisol, pulse rate and blood pressure were lower in the forest than in the city. At the same time, parasympathetic activity was higher and sympathetic activity lower.
In the investigations by Li and colleagues, urinary excretion of adrenaline and noradrenaline was also lower. Translated, that means the stress system eases off to some degree and the calming nerve gets a larger share.
These are snapshots of short visits. They say nothing about how diseases develop over years.
How long do you have to stay in the forest for anything to change?
The measured effects on cortisol and heart rate variability (HRV) appeared in the Japanese field experiments after just 15 to 20 minutes.
For general wellbeing there is a larger number from England: in a survey of 19,806 people, the probability of good self reported health was clearly higher from about 120 minutes of nature contact per week, with a maximum between 200 and 300 minutes. It made no difference whether that time came in one block or spread out.
This study is a cross section and cannot prove a cause. As an orientation I still find it usable: around twenty minutes a day.
What are terpenes and phytoncides?
Terpenes are volatile aroma molecules released by trees. Phytoncides is the term for the fraction that can act antimicrobially and is thought to protect the tree from germs and pests.
In the forest air of the Japanese studies, alpha-pinene, beta-pinene and isoprene were measured above all. In city air they were practically undetectable.
When you smell a coniferous forest, you really are breathing in these molecules. They are not an image and not a metaphor, they are chemistry with a measurable concentration in the air.
Can forest bathing strengthen the immune system?
There are signals pointing that way, although they come from small studies. In twelve men, natural killer cell activity rose after a three day stay in the forest, as did their number and the amount of the defence proteins perforin, granulysin and granzyme. After a comparable city trip this increase did not appear.
The effect was still measurable after seven days. A second investigation in thirteen nurses showed the same pattern, and so did an experiment with vaporised cypress oil in a hotel room.
These are laboratory values. They say nothing about infections, cancer or lifespan. Anyone who turns them into a promise goes far beyond the data.
Can forest bathing lower blood pressure?
A systematic review with meta-analysis pooled 20 investigations with 732 participants. Systolic and diastolic blood pressure were significantly lower in the forest environment than in the comparison environment.
A more recent meta-analysis of medically or socially recommended time in nature found on average 4.82 mmHg less systolic and 3.82 mmHg less diastolic compared with control conditions.
That can be a sensible building block, precisely because it has few side effects. It does not replace blood pressure treatment or medication, which your doctor decides on.
Is forest bathing the same as going for a walk?
Not quite, and this is where the methodologically most interesting point sits. In the comparison studies the physical load between forest and city was deliberately kept equal. Even so, cortisol, pulse rate and heart rate variability differed.
That suggests part of the effect comes from the environment and not only from the movement. It cannot be proven conclusively, because air quality, light, sound and expectation can never be cleanly separated.
Movement remains a well documented factor in its own right anyway. The two things are not in competition, they probably add up.
How good is the evidence on forest bathing?
Mixed, and that deserves to be said honestly. A systematic review by Wen and colleagues examined 28 papers from the years 2015 to 2019 and found that randomised studies scored methodologically much better than non randomised ones.
Many investigations have participant numbers in the double digits, mostly young men, and come from a small number of research groups. A forest cannot be blinded. The authors of the cortisol meta-analysis themselves point out that anticipated expectation effects may play a role.
The direction of the findings is consistent across many papers. The strength of evidence of the individual study is limited. Saying both at once is not a weakness, it is the honest state of affairs.
Does everyone respond to the forest in the same way?
No, and this is one of the most honest numbers in the whole field. In a field study with 485 men in 57 forests and 57 urban areas, 65.2 percent showed the expected rise in the parasympathetic marker and 67.0 percent the expected fall in the sympathetic ratio.
Put the other way round: roughly one third did not respond in the expected direction. An average describes a group, not you.
If the forest gives you nothing, there is nothing wrong with you. For some people water is the better place, for others a mountain, for others a garden. The environment that lets your system power down is individual.
Can forest bathing play a role in stress and exhaustion?
It can be one building block. In a meta-analysis of recommended time in nature, depression and anxiety scores fell moderately to clearly, and step counts rose by an average of about 900 steps per day.
In a controlled investigation with functional imaging, rumination decreased after a 90 minute walk in nature, as did activity in the subgenual prefrontal cortex. This region is linked with self focused circling thoughts. After an equally long city walk, neither changed.
In pronounced exhaustion or depression this does not replace treatment. It can accompany it. And it is one of the few recommendations that costs nobody any money.
Is a city park enough or does it have to be a real forest?
For the terpene hypothesis the difference matters, because alpha-pinene and beta-pinene were measured in forest air and practically not in city air.
For the nervous system and attention level, the data point more towards a continuum. Large analyses of green space work with satellite data on greenness around the home, not with forest visits. There, all cause mortality was statistically lower per 0.1 more greenness within 500 metres.
So the park around the corner is not worthless. It is simply something different from a spruce stand. And it has the great advantage that you actually go there.
Who should avoid forest bathing or approach it with caution?
With pollen allergy or allergic asthma, a flowering stand in the wrong season can be a burden. In that case it is better to adapt the season and the kind of trees.
In parts of Germany, above all in the south and south east, ticks are a real issue. Cover up, check yourself afterwards and clarify the question of the TBE vaccination with your doctor.
With cardiovascular disease, the same applies to walking distance and terrain as to any other physical load. And in an acute mental health crisis the forest is no substitute for help, at most an addition to it.
More in the Guide Forces of Nature
Related topics from other areas
Burnout
When the stress axis stays permanently on standby
Awake at 3 in the morning
Cortisol, the HPA axis and the organ clock myth
Sleep disorders
The map of causes behind bad nights
Light and the body clock
Why daylight outdoors counts for more than indoors
Depression holistically
Rumination, drive and the levels before them
Fasting and autophagy
Another stimulus the body answers by itself
Sources
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