A 2019 systematic review and meta-analysis published in the International Journal of Biometeorology found that forest bathing, the Japanese practice of shinrin-yoku, was associated with a measurable drop in salivary cortisol, the body’s main stress hormone, compared with time spent in urban settings.1 The lead author, Michele Antonelli, and colleagues pooled data from twenty two trials and called the effect “consistent” across studies, even when protocols varied widely.
That single finding has done a lot of work in the press. The fuller picture, drawn from Japanese field experiments and a string of immunology papers out of Nippon Medical School, is more interesting. Walking slowly through a forest for a couple of hours appears to nudge several systems at once, in ways that show up on blood pressure cuffs, in heart rate variability traces, and in the activity of immune cells called natural killer cells.2,4
What is forest bathing, exactly?
The term comes from a 1982 program by Japan’s Forestry Agency. Shinrin-yoku translates roughly as “taking in the forest atmosphere,” and that is the entire prescription. You walk slowly, you breathe, you stop and look at things. There is no required distance, no app, no checklist. The Japanese government later folded the practice into a formal preventive-medicine framework, and forty eight forest-therapy bases were certified across the country, where doctors can refer patients the same way a Western GP might suggest a Mediterranean diet.6
Westerners often hear “forest bathing” and picture something more elaborate. It is not. The Japanese researcher Yoshifumi Miyazaki, who has spent two decades studying the physiology of green space, has been blunt about it in interviews. The intervention is rest in the presence of trees. The novelty is that the rest is being measured.
The cortisol question
Cortisol is the body’s morning-alarm hormone. It rises before you wake, peaks shortly after, and tapers across the day. Chronic stress flattens the curve and pushes the average level up, which is associated with worse sleep, blunted immunity, and metabolic problems over time. So when researchers measure cortisol after a forest walk, they are checking whether the body’s stress-axis dial moves at all.
In the Antonelli meta-analysis, salivary cortisol fell more after a forest visit than after a comparable visit to a city street, with a small to moderate effect size.1 Smaller does not mean trivial. A 2010 series by Bum-Jin Park and his team at Chiba University, using 24 forests across Japan, reported the same direction of effect with the added rigor of paired urban controls and a standardized 15-minute viewing plus 15-minute walking protocol.4 Heart rate dropped. Diastolic blood pressure dropped. The high-frequency component of heart rate variability, an index of parasympathetic activity, went up.
None of those changes are dramatic in any single person. They are the kind of shift you would expect from a competent meditation session, or a long unhurried meal with people you like. The reason researchers find them interesting is that they happen reliably across cohorts, and they happen quickly.
Why natural killer cells keep coming up
The most quoted forest-bathing studies are not actually about stress. They are about a particular kind of immune cell. Natural killer cells, or NK cells, are part of the innate immune system. They patrol the body looking for cells that have gone wrong, virally infected ones and tumor cells, and they kill them on contact. Their activity rises and falls with sleep, exercise, and chronic stress.
In 2007, Qing Li and colleagues at Nippon Medical School published a small but striking trial in the International Journal of Immunopathology and Pharmacology.2 Twelve healthy men were taken on a three-day, two-night trip to forests in Nagano Prefecture. Their NK cell activity rose by roughly fifty percent on day two and stayed elevated for at least seven days after the trip ended, with a smaller rise still detectable at thirty days. Levels of perforin, granzyme A, and granulysin, the proteins NK cells use to dispatch abnormal cells, also went up. A control trip to an urban tourist district produced none of those changes.
That sounds too clean. The follow-up was the interesting one. In 2009 the same group ran a hotel-room experiment, where volunteers slept three nights with a humidifier dispersing essential oil from Chamaecyparis obtusa, the Japanese hinoki cypress.3 NK activity rose, perforin and granulysin rose, and urinary stress-hormone levels fell. The volunteers never set foot in a forest. The compounds doing the work, called phytoncides, are antimicrobial volatile oils trees release into the air around them.
The hinoki experiment matters because it isolates a candidate mechanism. The forest is doing more than calming you down. The chemistry of the air seems to be part of the active ingredient.
What the trees are emitting
Phytoncide is a Russian word coined in the 1930s by the biologist Boris Tokin, who noticed that injured plant tissue released compounds that killed bacteria in vitro. Conifers are especially generous with them. Pinene, limonene, camphene, and bornyl acetate are the four most studied, and a healthy stand of cedar or cypress can perfume the surrounding air with measurable concentrations on a warm, still afternoon.
In rodent studies, phytoncides modulate the hypothalamic-pituitary-adrenal axis, the body’s stress feedback loop, and increase NK activity in cell culture.3 In humans, the data is thinner but consistent in direction. A 2012 randomized study from Zhejiang Province, China, in a broad-leaved evergreen forest rather than a Japanese conifer one, found drops in blood pressure and pro-inflammatory cytokines after a short forest stay, suggesting the effect is not unique to cypress and pine.7
The dosage matters. The concentrations measured in actual forest air are far lower than what a lab uses on cultured cells, which is one of the standing critiques of the mechanism story. Skeptics point out that the effect on a human nervous system might be partly olfactory rather than pharmacological. The smell itself, learned in childhood as the smell of safe outdoor places, could be doing some of the work the chemistry is taking credit for. Both routes are interesting. Neither has been cleanly disentangled in a controlled setting yet.
How long do you actually need?
This is where the popular advice tends to overstate a thinner evidence base. The honest answer is that nobody has nailed down a dose-response curve for forest bathing the way you can for, say, blood pressure medication. What there is, instead, is a cluster of suggestive findings.
The Park 24-forest study used roughly 30 minutes of forest exposure, split between viewing and walking, and saw acute physiological effects.4 Smaller observational reports have flagged anxiety reductions after sessions as brief as 15 minutes. The Li immune studies used multi-day immersions, which complicates comparisons.
A different kind of study, by Mathew White at the University of Exeter, took the question sideways. Using a survey of nearly 20,000 people in England, his team asked how much time per week respondents spent in any natural setting (parks, woodland, beach), and compared that to self-reported health and wellbeing.5 The threshold that emerged, after adjustment for age, deprivation, and other factors, was 120 minutes a week. Below that, no association. Above it, a steady plateau of better outcomes that held up to about 200 to 300 minutes.
White was careful to note that this is correlational. People who spend two hours a week in nature might be healthier for many reasons, and the survey cannot prove direction. Still, the threshold replicated across age groups, occupation types, and the chronically ill, which is unusual for soft-outcome research.
Walking matters less than you would think
One of the more counterintuitive findings is that you do not need to hike. The Japanese protocols are explicitly leisurely. Park’s group reported physiological benefits from sitting and viewing the forest for fifteen minutes, before any walking happened.4 If anything, vigorous exercise raises cortisol acutely, which would muddy the picture.
Cardiologists asked about this often point out that the obvious confound, simply being outdoors and moving, has been partially controlled in the better studies by comparing forest walks with city walks of identical duration and exertion. The forest arm wins on most stress markers.1,4 That does not mean the city walk is useless. It means the trees seem to add something on top.
Is any of this clinically meaningful?
The Japanese health system thinks so, at least at the prevention end. Forest therapy bases are inspected for things like the volatile-oil profile of the air, accessibility for older walkers, and the presence of trained guides. Insurance does not generally pay for prescriptions, but the cultural framing is that the forest is a place a doctor can responsibly send you, the same way an American physician might tell you to walk thirty minutes a day.6
Outside of Japan and South Korea, where Chungnam National University runs a comparable program, the evidence has not yet pushed Western medical bodies to formal recommendations. The studies are small, often unblinded, and run by enthusiasts. That is a fair caveat. It is also not a reason to ignore a simple, almost free intervention with a plausible mechanism and no obvious downside.
The honest framing is the one the original Facebook post landed on. Forest bathing is not a treatment. It is a tool that supports the systems your body already uses to handle stress. If you have a panic disorder, untreated hypertension, or active cancer, you need a clinician, not a hike.
Common questions about forest bathing
Do I need a real forest, or will a city park do?
A real forest produces a stronger phytoncide signal, but parks with mature trees, especially conifers, still appear in observational data as helpful for stress and mood. Density of greenery and time spent there matter more than the formal label of the place.
How often should I go to see a benefit?
The clearest dose finding in the literature is the 120 minutes a week threshold from White’s 2019 paper, drawn from English survey data.5 That can be one long visit or split across days; both worked in the analysis.
Do I have to walk slowly, or can I run or hike?
Slow movement is what the Japanese protocols use, and it is what the cortisol and blood pressure studies measured. Running is fine for fitness, but it raises stress hormones acutely, which is not what forest bathing is targeting.
Will the immune effect really last 30 days?
That figure comes from a small Li study with twelve men and a three-day forest trip.2 The signal is real but the sample is tiny, and longer follow-up has not been replicated in larger trials. Treat it as suggestive, not settled.
Are there any risks?
For most people, no, beyond the usual outdoor concerns: tick-borne illness in some regions, allergens in spring, getting lost. Anyone with severe asthma triggered by tree pollen should pick the season carefully.
Where the evidence actually leaves us
The fairest summary is unflashy. Several decades of mostly Japanese research, plus a handful of European replications, point in the same direction. Spending two unhurried hours a week among trees is associated with lower stress markers, slightly lower blood pressure, and, in immunology studies that need replication, more active NK cells. The mechanism likely involves both parasympathetic activation, which any quiet outdoor activity might trigger, and inhalation of phytoncides, which seems to be specific to forested air.
None of that turns a walk into medicine. It does mean that the often-mocked Japanese practice of writing forest visits onto preventive-care plans has more behind it than novelty. If your nervous system has been running hot for months, the cheapest thing the literature suggests trying is two hours a week somewhere with trees, and a willingness to stop checking your phone while you are there.
Sources
- Antonelli M, Barbieri G, Donelli D. Effects of forest bathing (shinrin-yoku) on levels of cortisol as a stress biomarker: a systematic review and meta-analysis. International Journal of Biometeorology. 2019. PubMed: 31001682
- Li Q, Morimoto K, Nakadai A, et al. Forest bathing enhances human natural killer activity and expression of anti-cancer proteins. International Journal of Immunopathology and Pharmacology. 2007. PubMed: 17903349
- Li Q. Effect of phytoncide from trees on human natural killer cell function. International Journal of Immunopathology and Pharmacology. 2009. PubMed: 20074458
- Park BJ, Tsunetsugu Y, Kasetani T, Kagawa T, Miyazaki Y. The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan. Environmental Health and Preventive Medicine. 2010. PubMed: 19568835
- White MP, Alcock I, Grellier J, et al. Spending at least 120 minutes a week in nature is associated with good health and wellbeing. Scientific Reports. 2019. PubMed: 31197192
- Miyazaki Y, Ikei H, Song C. Preventive medical effects of nature therapy. Nihon Eiseigaku Zasshi. 2011. PubMed: 21996763
- Mao GX, Lan XG, Cao YB, et al. Effects of short-term forest bathing on human health in a broad-leaved evergreen forest in Zhejiang Province, China. Biomedical and Environmental Sciences. 2012. PubMed: 22840583
