Why Forest Bathing Works: What Blood Pressure and Stress Hormones Reveal

What Is Forest Bathing? A Short History of Japan's "Forest Therapy"

The term "shinrin-yoku" (forest bathing) is said to have originated in 1982, coined by Japan's then Director-General of the Forestry Agency. Modeled on terms like sunbathing and sea bathing, it captured the idea of immersing the whole body in a forest's air, sounds, scents, and light. Originally little more than a public health slogan promoting recreation, it later grew into "forest medicine": a research field that scientifically measures what actually happens in the body.

The 1980s, when the term was coined, was also a period when rapid economic growth had concentrated people in cities and long working hours were becoming a recognized social issue. The Forestry Agency's focus on the recreational value of national forests reflected a broadening of forestry policy's role — using forests not just for timber, but as places for urban residents to refresh mind and body.

Akasawa Natural Recreational Forest: "The Birthplace of Forest Bathing"

One of the places most closely tied to the origin of the term is Akasawa Natural Recreational Forest, in Agematsu, Kiso County, Nagano Prefecture. Home to a natural stand of Kiso cypress trees over 300 years old, it was designated Japan's first "natural recreational forest" in 1970, and in 1982 hosted the country's first-ever "Forest Bathing Festival." For this reason it is often called "the birthplace of forest bathing," and in 2001 it was selected as one of the Ministry of the Environment's "100 Fragrant Landscapes of Japan." When the Forest Therapy Base certification system launched in 2006, it was among the very first sites certified.

The Rise of "Forest Therapy"

From around 2004, research teams led by Chiba University repeated physiological experiments in forests nationwide, quantifying changes in blood pressure and stress hormones. Building on this scientific foundation, the NPO Forest Therapy Society was established in 2005, and in 2006 a certification system began: forests where physiological experiments confirmed a healing effect were designated "Forest Therapy Bases," with their walking trails designated "Forest Therapy Roads."

What sets this certification system apart from ordinary lists of "relaxing spots" is that a physiological experiment is always required before certification. At candidate sites, participants walk a set course, and their blood pressure, salivary cortisol, and autonomic nervous activity are compared against the results of the same task performed in a city. Certification is decided not by scenic beauty or fame, but by measured data.

In other words, a Forest Therapy Base is not simply "a forest that feels vaguely healing" — it is a forest whose effects have been backed up by experiment. The next chapters look at exactly what kinds of numerical changes those experiments have observed.

How Much Does Blood Pressure Drop? Real Data from Chiba University

Chiba University's Center for Environment, Health and Field Sciences has repeatedly measured blood pressure changes among participants in Forest Therapy programs. In one representative experiment, men with elevated blood pressure took part in a one-day Forest Therapy program, and their systolic blood pressure was compared before and after.

In the two-day, one-night program, this drop in blood pressure was also found to persist for three to five days afterward. The effect does not vanish the moment the walk ends — the body seems to carry an "afterglow" for several days. It is thought that the balance of the autonomic nervous system remains adjusted for a while even after returning to daily life.

A Large-Scale Comparison of Forest and City Environments

Using a common experimental protocol, researchers also conducted a comparative study across 24 forest sites and urban environments nationwide, involving roughly 280 participants in total. The results showed statistically significant differences between forest and urban environments across all three measures: systolic blood pressure, diastolic blood pressure, and pulse rate. The fact that the same pattern reappears not just in a single experiment, but across many sites and many people, strengthens the case.

What matters most in this kind of research is the "controlled comparison" design. The same participants perform the same tasks — walking the same distance, viewing the same kind of scenery — for the same amount of time, in both a forest and a city, and only the difference between the two is analyzed. By canceling out noise from season, individual constitution, and daily condition, researchers can statistically isolate the effect of the forest environment itself.

Applications for Older Adults and Lifestyle-Disease Prevention

Given the confirmed effect on blood pressure, Forest Therapy programs are now drawing attention as one non-drug approach to preventing and managing high blood pressure and other lifestyle-related diseases. This is of course not a substitute for medical treatment, but it is increasingly positioned as an easy-to-adopt form of self-care alongside regular exercise and dietary habits.

Because blood pressure can easily be measured at home, it has become one of the most reproducible and applicable indicators in Forest Therapy research. Participants can check the numbers themselves with a portable blood pressure monitor without visiting a clinic, which means the effect can be confirmed not just as a subjective sense of "feeling good," but as a visible number — and this ease of self-measurement can also become a starting point for participants to reconsider their broader daily habits, from exercise to sleep.

Stress Hormones and the Autonomic Nervous System

In Chiba University's research, sitting quietly in a forest for just 15 minutes was found to lower salivary concentrations of the stress hormone cortisol, and to raise parasympathetic nervous activity — the branch associated with relaxation — by 55% compared with a control experiment conducted in a city. No lengthy exercise or special training was needed; a short period of simply sitting produced a measurable change in the autonomic nervous system.

What Is Cortisol?

Cortisol is a hormone secreted by the adrenal glands that helps regulate blood sugar and energy metabolism in response to stress. As a short-term stress response it is necessary, but chronically elevated levels are known to contribute to fatigue, poorer sleep quality, and impaired immune function. Salivary cortisol is used as an indicator in forest-bathing research because it can be measured without drawing blood, allowing stress levels to be assessed under near-everyday conditions.

Suppressed Sympathetic Activity and a Calmer Prefrontal Cortex

In forest environments, "sympathetic nervous activity" — which dominates under high stress — has repeatedly been observed to be suppressed, while "parasympathetic nervous activity" — which dominates during relaxation — increases. At the same time, activity in the prefrontal cortex, the brain region responsible for thought and judgment, has also been reported to calm down, which may correspond to the subjective sense of one's "head clearing."

The autonomic nervous system normally balances sympathetic and parasympathetic activity like a seesaw. Urban life — commuting, work, an overload of information — tends to tip that balance toward the sympathetic side, whereas being in a forest environment appears to shift the balance back toward the parasympathetic side, according to the interpretation drawn from these experimental results.

This chain of responses is also closely tied to the so-called "HPA axis," the pathway through which signals travel from the hypothalamus to the pituitary gland and then the adrenal glands. Under strong stress, this pathway becomes more active and cortisol secretion increases; in forest environments, by contrast, this pathway appears to move in a calmer direction compared with urban settings. The fact that blood pressure, cortisol, and autonomic activity — seemingly unrelated indicators — all move in the same direction (toward relaxation) suggests that multiple regulatory systems in the body are responding in a coordinated way.

Psychological Effects

Beyond physiological indicators, studies combining psychological tests have also confirmed significant reductions in feelings of depression and anxiety before and after forest bathing. The fact that objective measures like blood pressure and hormones move in the same direction as a person's own reported mood is part of what makes forest-bathing research so persuasive.

The link between nature and mood has also been independently verified outside Japan. In a 2015 study published in the journal PNAS, a team at Stanford University found that participants who took a 90-minute walk in a natural setting showed reduced rumination — the tendency to dwell on negative thoughts — and lower activity in a brain region linked to depression risk (the subgenual prefrontal cortex), compared with participants who took a 90-minute walk in an urban setting. The fact that independent studies, using different countries and different methods, arrive at the same conclusion reinforces that the psychological benefits of forest bathing are not a fluke.

Effects on Immunity: The Science of NK Cell Activation

The immune-related effects of forest bathing have been studied extensively by Dr. Qing Li of Nippon Medical School Hospital's Department of Rehabilitation. After conducting cancer immunology research at Stanford University School of Medicine in 2001, Dr. Li began studying the health-promoting effects of forest bathing from 2004 onward.

In one of Dr. Li's studies, 12 healthy men aged 35 to 56 took part in a two- to three-day forest-bathing program, and the activity of natural killer (NK) cells in their blood was measured. NK cells are a type of immune cell that attacks virus-infected cells and cancer cells, and are one of the main players in the body's innate immune system.

NK Cell Activity Rose 1.5-fold

The results showed that NK cell activity rose to roughly 1.5 times its pre-trip level, and the amount of proteins such as perforin — which NK cells secrete to attack cancer cells — also increased. Importantly, this was not simply an increase in the number of cells, but an increase in the attacking power of each individual cell.

Even more notably, this elevated activity remained higher than the pre-trip baseline even 30 days after the forest-bathing trip. Rather than a brief, one-off reaction, this suggests a change in the immune system that may persist for a relatively long period — a finding that has since been followed up by further studies.

The Phytoncide Hypothesis

One substance thought to play a role in this immune effect is a group of compounds known as "phytoncides" — a general term for the volatile aromatic compounds released by trees. Experiments exposing participants to air containing phytoncides have also reported increases in NK cell activity, but scent alone likely cannot explain the entire effect; light, sound, temperature, and physical activity may all combine to play a role. The full mechanism is still under investigation, and it should be noted that no definitive conclusion has yet been reached.

The connection to cancer also deserves a cautious reading. Because NK cells attack cancer cells, forest bathing is sometimes framed as a way to prevent cancer, but current research has only shown changes in immune markers in healthy adults — it has not proven that forest bathing treats or prevents any specific disease. It is best understood as research showing "a positive change in part of the body's defense system," rather than as a medical claim.

A Note on Study Size

The NK cell study cited above involved just 12 participants, a small number compared with large-scale clinical trials. That said, similar trends have been reported in follow-up studies, including studies of women and replications in different forests, so the finding does not rest on a single experiment alone. Even so, given the limited scale and diversity of participants studied so far, it is worth being cautious not to overstate these results as a guaranteed health benefit.

Why Do Forests Heal Us? Hypotheses and the Five Senses

Several hypotheses have been proposed to explain why forest bathing produces these physiological changes. One frequently cited idea is the "biophilia hypothesis," proposed by the biologist Edward O. Wilson. It holds that because humans spent most of their evolutionary history in natural environments, we have evolved an innate affinity and sense of safety toward environments filled with greenery, water, and living things.

Combined Stimulation of the Five Senses

A forest environment offers not a single stimulus but a combination of stimuli across the five senses: visual input such as green foliage and dappled sunlight, auditory input such as birdsong, rustling leaves, and flowing streams, olfactory input from phytoncides and other tree-derived scents, and tactile input from bark, soil, and fallen leaves. These are thought to act together to bring about changes in autonomic balance and hormone levels.

• Sight: green foliage, dappled sunlight, and open, far-reaching views
• Hearing: birdsong, rustling leaves, and the sound of flowing streams
• Smell: phytoncides and other tree-derived scents
• Touch: the natural textures of bark, fallen leaves, and soil

Is Looking at Nature Alone Enough?

Some experiments have tested conditions where participants do not actually walk, but simply look at forest scenery or listen to forest sounds. These studies, too, report greater parasympathetic activity compared with viewing or hearing urban scenes, suggesting that even a single sense — sight or hearing alone — can produce some effect. That said, engaging multiple senses at once appears to lead to larger changes.

One internationally well-known study on the effect of "looking alone" is a landmark 1984 study published in the journal Science by the American environmental psychologist Roger Ulrich. Comparing patients who had all undergone the same gallbladder surgery, Ulrich found that those assigned to rooms with a window view of trees had shorter hospital stays, needed fewer strong painkillers, and received fewer negative comments in nurses' notes than matched patients whose windows faced a brick wall. This pioneering study — showing that simply "seeing" greenery, without ever walking through a forest, may aid recovery — has since been widely cited in healthcare design and environmental psychology research.

What the Forest-Versus-City Comparisons Show

As with the 24-site, roughly 280-person comparison mentioned earlier, the practice of comparing forest and urban environments under matched conditions is a key design choice that underpins the credibility of this field. By having the same individual walk through both a forest and a city at the same time of day and measuring both sets of physiological indicators, researchers can strip out the effects of season and individual variation and isolate the effect of the forest environment itself.

Attention Restoration Theory

In psychology, another frequently cited framework is "Attention Restoration Theory," proposed by Rachel and Stephen Kaplan. It holds that urban life demands constant "directed attention" — the effortful focus needed to filter out noise and distraction while concentrating on a task — and that overusing this capacity contributes to fatigue. Natural environments, by contrast, tend to engage a gentler, more effortless form of attention that draws interest without deliberate focus, allowing that overtaxed capacity to recover. Combining physiological findings like blood pressure and hormone changes with this kind of psychological framework gives a fuller picture of how forest bathing works.

Shinrin-yoku Goes Global: South Korea, the US, and Europe

Japan's research into forest medicine has influenced practices abroad as well. Dr. Qing Li's book "Shinrin-Yoku" was published by a major international publisher, translated into more than 25 languages, and released in more than 30 countries and regions, becoming a bestseller. The Japanese word "shinrin-yoku" has itself become a recognized term in English-speaking countries.

South Korea's "Healing Forest" System

In South Korea, the Korea Forest Service has led the development of its own version of Japan's Forest Therapy bases, known as "healing forests." Facilities have been established across the country, offering programs that train forest healing instructors and support cancer patients, in collaboration with the medical and welfare sectors.

The Spread of "Forest Therapy Guides" in the United States

In the United States, Amos Clifford, a wilderness guide, founded the Association of Nature and Forest Therapy (ANFT) in 2012, inspired by Japan's shinrin-yoku. After holding its first guide-training program in California in 2014, the organization has grown to train nearly 3,000 guides across more than 70 countries. National park services and local governments in various countries have also begun incorporating guided forest-bathing programs.

Few countries yet have a certification system as rigorous as Japan's, built on physiological experiments, but the basic approach — slowing your pace and engaging the five senses in nature — is shared across countries. It is also worth noting that Japan's original research data is often cited as a scientific foundation for guide training and program design abroad.

Visiting a Forest Therapy Base: A Network Spanning the Country

Forest Therapy bases and roads now span the country, from Hokkaido to Kyushu. Each base has been certified through physiological experiments, and many are developed in tandem with regional revitalization efforts. Some prefectures, such as Nagano, host multiple certified bases and roads and promote themselves as a "Forest Therapy Prefecture."

The First Experimental Site Was Yakushima

The starting point of forest medicine research was Yakushima Island, one of the sites where a Chiba University research team conducted experiments in 1990. Registered as a UNESCO World Heritage Site, Yakushima's vertically layered forest and its ancient cedars hold a symbolic place in Japan's forest medicine research. The island's diverse forests, whose vegetation shifts from subtropical to subalpine with altitude, also made for a fascinating research site.

How Bases and Roads Are Selected

Certification as a Forest Therapy Base requires more than natural beauty. Alongside physiological evidence, factors such as gentle elevation changes along walking trails and the availability of accommodation and guide services are also taken into account. Certification is granted only when both scientific evidence and practical accessibility are in place.

Akasawa Natural Recreational Forest, introduced earlier, is a prime example. Its natural stand of Kiso cypress trees, some over 300 years old, has walking trails that draw roughly 100,000 visitors a year, while also carrying the physiological backing of Forest Therapy Base certification. It combines a historic role as the "birthplace of forest bathing" with an ongoing scientific basis as a certified base today.

For certified municipalities, this designation also serves as a tool for tourism promotion and encouraging relocation. As health consciousness has grown, framing a location as "a scientifically backed healing forest" tends to be a stronger motivator for visits than simply promoting scenic beauty.

When visiting a base, many regions offer guided programs led by registered Forest Therapy Society guides that can be booked in advance. Guides not only bring knowledge of local flora, fauna, and geology, but also adjust the pace of the walk to participants and prompt them, at key points, to pay attention with all five senses. Walking alone freely is fine, but for a first visit to any given base, a guided program can help visitors more deeply appreciate what makes that particular forest distinctive.

Practicing Forest Bathing in the City

Even on days when visiting a Forest Therapy base isn't possible, a growing body of research suggests that simply being around greenery has some effect. Reports suggest that even urban green spaces — like those discussed in our article on how street trees and parks cool cities — can bring positive changes in heart rate and mood, so it appears that a forest specifically is not the only place these benefits occur.

Of course, many of the numbers cited in physiological experiments come from environments with especially high green coverage, like Forest Therapy bases, so it would be an oversimplification to claim that urban parks deliver exactly the same magnitude of effect. Still, deliberately increasing time spent around nearby greenery is increasingly seen as something that can offer psychological benefits.

Certification systems like the Forest Therapy Base are meant to highlight "environments where the effect is easy to confirm scientifically" — but deliberately walking through nearby greenery can itself be a gateway to the benefits that forest medicine research has documented. Even in a week with no time for a trip out of town, small choices — a greener route to work, a walk through a park at lunch — can be a starting point.

What matters most is not treating it as a one-time experience, but continuing it without overexertion. Given that Chiba University's research found the blood-pressure-lowering effect lasts several days, combining practices of different frequency — an occasional visit to a Forest Therapy base once a month, alongside a walk through a nearby park most weekends — may help the benefits accumulate over time.

Conclusion: Forest Bathing as "Evidence-Based Self-Care"

Forest bathing is not simply a pleasant walk for a change of mood — it is one of the few forms of self-care whose effects have been measured through concrete indicators such as blood pressure, stress hormones, and immune cells. As the research from Chiba University and Nippon Medical School shows, its major appeal is that it requires no special equipment or training and can be started by anyone, starting today. The three indicators covered here — blood pressure, cortisol, and NK cells — correspond to three distinct bodily systems: the cardiovascular, endocrine, and immune systems, and the fact that positive changes appear across multiple systems at once speaks to the depth of research in this field.

Forests support more than just our physical and mental health. Just as headwater forests store rainfall and release it into rivers, forests also sustain the very foundations of our daily lives. Whether you plan a trip to a certified Forest Therapy base or simply take a slower walk through your neighborhood park, the data accumulated by forest medicine research gives you a reason to take that first step.