Forest Bathing: How A Walk In The Woods Can Slash Stress And Boost Immunity

“Walking just for the pleasure of walking, freely and firmly, without hurrying. We are present in every step. When we wish to speak, we stop walking and lend all our attention to the person before us, to speaking and to listening… Stop, look around, and see how wonderful life is: the trees, the white clouds, the infinite sky. Listen to the birds, delight in the light breeze. Let us walk as free people and feel our steps growing lighter as we walk. Let us appreciate every step we take.” — Thich Nhat Hanh

Humans are innately aware of the therapeutic value of a brisk walk, but now science is showing that both the mental and physical benefits can be significantly augmented if such exercise is performed within a forest setting. These benefits range from improved mood and sleep to slashed stress levels and — perhaps more impressively — a boost in immune system function that may even help to fight cancer.

Photo: The ancient forest of Yakushima, Japan. Via: F.Mann | Shutterstock.

In 1982 the Forest Agency of the Japanese Government presented its Shinrin-yoku initiative. Shinrin-yoku can be defined as “making contact with and taking in the atmosphere of the forest.” This plan was introduced to encourage the people of Japan to get outside and immerse themselves in nature in an act that is also referred to as “forest bathing.” Innitially the reasoning for introducing the initiative was based on the common sense speculation that time spent outdoors, surrounded by fresh air and lush forest, would inevitably benefit the bodies and minds of the public.

Mother Earth News reports that research to confirm the credibility of these assumptions emerged in 1990 when Dr. Yoshifumi Miyazaki of Chiba University performed a study in the ancient Japanese forests of Yakushima. Miyazaki found that physical activity in the from of a 40 minute walk in the forest was associated with improved mood and feelings of health and robustness. On top of this, the results of Dr. Miyazaki’s study also revealed a decrease in levels of the stress hormone cortisol in test subjects after a walk in the forest, when compared with a control group of subjects who engaged in walks within a laboratory setting.

More recently, researchers have conducted investigations into the physiological changes that occur during a session of forest bathing and the ongoing benefits that surface as a result. A team from the Center for Environment, Health and Field Sciences at Chiba University have confirmed that spending time within a forest setting can result in numerous mental improvements. The most potent of these include reduced stress, decreased depressive symptoms, and lowered hostility. (Conversely, spending time in a polluted setting has been linked to violence and criminal behavior.)

It is no surprise that these shifts occur when we analyze the objective data derived from physical measurements. Examples of these can be found in a paper titled “The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan.” In the experiments documented by this paper, 12 subjects walked in and viewed either a forest or a city area. On the first day of each test, the group was halved, with 6 members being sent to the forest and the other 6 to a city area. On the second day each group was then sent to the opposing area by way of a cross-check.

Salivary cortisol, blood pressure, pulse rate, and heart rate variability were all measured in the morning before breakfast and also before and after the walking and viewing sessions. After correlating the data, the researchers noted: “The results show that forest environments promote lower concentrations of cortisol, lower pulse rate, lower blood pressure, greater parasympathetic nerve activity, and lower sympathetic nerve activity than do city environments. These results will contribute to the development of a research field dedicated to forest medicine, which may be used as a strategy for preventive medicine.”

As far as preventive medicine goes, forest bathing seems to significantly mitigate the root cause of a multitude of ailments: stress. Excess stress can play a role in headaches, high blood pressure, heart problems, diabetes, skin conditions, asthma, and arthritis, among many other ailments. As the results indicate, forest bathing catalyzes increased parasympathetic nervous system activity which prompts rest, conserves energy, and slows down the heart rate whilst increasing intestinal and gland activity.

Lower cortisol concentrations are also a signal that the body’s stress-response system is being triggered less. When this system is triggered, cortisol and other stress hormones are released into the body. Overexposure to these chemicals in response to chronic stress can increase the risk of anxiety, depression, heart disease, weight gain, and memory and concentration impairment.

Photo: The forest of Vincennes, Paris. Via: Elena Dijour | Shutterstock.

It seems certain that forest bathing acts as a simple yet profound preventive measure for stress-induced disease, as well as simply serving to take the edge off of the minor stressors that plague the day-to-day life of the average Westerner. But what other functions may this form of therapy provide?

Significantly, it can also aid the body in fighting disease through instigating changes in immune function. A scientific review titled, “Effect of forest bathing trips on human immune function,” by Qing Li of the Department of Hygiene and Public Health at Japan’s Nippon Medical School, details studies that show forest bathing enhances natural killer cell activity and the expression of anti-cancer proteins. Study participants, both male and female, experienced trips to forest areas that lasted a total of 3 days and 2 nights in duration. Blood and urine samples were taken on days 2 and 3, as well as days 7 and 30 after the trips. Among other parameters, the levels of natural killer cell activity, the numbers of natural killer cells, and the concentration of urinary adrenaline were measured. These same measurements were also taken on a normal working day before the trips, to serve as a baseline control measurement. The mean values of the concentration of urinary adrenaline, one of the key stress hormones, was dramatically lower on days when both male and female subjects participated in forest bathing.

The data also showed that natural killer cell activity and the number of natural killer cells were significantly increased after the trips and that this beneficial effect lasted for more than 30 days afterwards. In comparison, a trip to the city did not yield these results. The authors note that a forest bathing trip once a month would enable individuals to maintain a higher level of natural killer cell activity.

Natural killer cells are a type of white blood cell that play a crucial role in the body’s rejection of tumors and virally-infected cells, making them of paramount importance to our immune system. They are cytotoxic, meaning they are cell-killing. They achieve this using small granules within the cells cytoplasm, which contain proteins such as perforin alongside proteases known as granzymes. Perforin occurs within the natural killer cells of the immune system and serves as a type of weapon against undesired mutations and invaders such as tumors and viruses. This protein drills holes in cell membranes and causes the targeted cells to die off. Qing Li notes in the aforementioned paper that levels of cells expressing these proteins were also found to be increased after sessions of forest bathing.

It is both tremendously exciting and beneficial that a simple walk within a forest can stimulate such a potent defense within our bodily systems. But how exactly is such an immune reaction stimulated? It could very well be the result of tree essential oils present in the forest air, in essence a form of aromatherapy. These oils contain antimicrobial volatile organic compounds known as phytoncides, which have been tested in vitro to determine how they effect an incubated type of human natural killer cell (NK-92MI) activity. In his paper, Qing Li states that phytoncides such as α-pinene and d-limonene, and essential oils extracted from trees including Japanese Cedar, significantly increased the cell-killing capabilities of NK-92MI cells in a dose dependent manner, as well as the levels of intracellular perforin within them.

The Department of Psychiatry of Mie University School of Medicine in Japan also demonstrated that the citrus fragrance found in forests normalized neuroendocrine hormone levels and immune function and was more effective than antidepressants in depressive patients. Neuroendocrine hormones transmit signals between the human nervous system and the endocrine system, a network of glands that produce hormones that regulate metabolism, growth, sexual function, sleep, and more.  One such hormone, also serving as a neurotransmitter, is oxytocin, which is known as the “love hormone” and influences social behavior and emotion. The correct balance of these hormones is therefore essential to promote feelings of well-being.

It would seem that even a brief sojourn in the nourishing womb of nature is clearly a worthwhile endeavor for any city dweller or suburban settler. That said, not everybody is fortunate enough to be able to make frequent trips out into nature, yet alone live in a rural setting suffused with trees and decorated with foliage. However, if you live in an urban environment, you can still reap the benefits of a more indirect form of forest therapy. Interestingly, research has indicated that attempts to mimic a forest environment can still have positive physical and psychological effects. An article entitled “Trends in research related to ‘Shinrin-yoku’ (taking in the forest atmosphere or forest bathing) in Japan” suggests that visual stimulation in the form of natural images are perceived as more “comfortable” and “soothing” when compared to a gray screen control. Subjects who viewed “Shinrin-yoku images” — specifically a photograph of people taking a walk in the forest of Vincennes in Paris — had significantly decreased blood pressure and prefrontal activity. Such reduced activity in the prefrontal cortex has been associated with a decreased risk of relapsing back into depression. It is thought that this is due to the prefrontal brain’s ability to deeply analyze sadness, which can result in unhealthy thought patterns, whereas the visual areas of the brain are associated with acceptance and non-judgement.

Photo: Wood in the interiors of homes also promotes wellbeing. Image: Mr. Interior | Shutterstock.

Additionally, research has been performed that measures physiological responses to differing ratios of wooden room interiors. Subjects in a room with a 30 percent wood ratio experienced significant decreases in pulse rate and diastolic blood pressure, indicating a relaxed response to such an environment. At a ratio of 45 percent wood, the subjects pulse rate actually increased, yet this was linked to high scores in the “vigorous” feeling that was also observed when it came to mood evaluation. The final test was done with a 90 percent wood ratio. At this high percentage there was a large decrease in both systolic and diastolic blood pressure, along with a rapid decrease in prefrontal activity and an elevated pulse rate. The researchers concluded that a wood ratio of 45 percent was the most comfortable for interiors.

There are therefore manageable options available for urban dwellers who may prefer to enjoy such stimulation from the comfort of their own home. If you layer the scent of one or two essential oils to mimic the smell of trees, you can create a simulated version of the  stress-reducing and natural killer cell-promoting environment of a forest.

Are you well-grounded? Photo: Erlo Brown | Shutterstock

Meanwhile, those living in more favorable regions and climates, or with a high degree of dedication, may wish to take the practice of forest bathing one step further. The practice of earthing or grounding involves bare skin contact with the Earth, usually through the simple act of barefoot walking. Research has demonstrated that earthing can produce a stable “internal bioelectrical environment,” which aids the function of all bodily systems.

Researchers posit that the benefits of grounding include improved sleep, muscle relaxation, decreased levels of chronic back and joint pain, and general improved health. It’s hypothesized that this is due to the introduction of antioxidant electrons, that help reduce  inflammation, and which enter the body upon bare contact with the Earth. For those wishing to add this benefit to their indoor simulated experience, grounding mats can be purchased that perform a similar function.

The research discussed here raises numerous questions: Are our minds and bodies really cut out for city life and the torrents of stressors associated with it? Or do the stress-reducing and immune boosting reactions of a our bodies in response to a forest environment signal that we have perhaps strayed too far from our natural lifestyles?