In 2021, at the request of the Société des établissements de plein air du Québec (Sépaq), Prevention Watch produced a report (in French) on the benefits of nature on overall health. In this report, we reviewed the scientific literature and assessed the level of proof obtained by all the studies and meta-analyses published on the effect of exposure to the forest, most often through a “forest bath” (shinrin-yoku), a guided relaxation activity in nature. The term Shinrin-yoku (森林浴), coined by the Japanese Ministry of Agriculture, Forestry and Fisheries in 1982, describes an unconventional therapeutic practice based on the idea that walking or staying in the forest or near trees would have beneficial effects on physical and mental health. It is a meditative practice that involves walking (generally 2 to 4 hours, sometimes less) in the forest at a regular pace and taking breaks to rest, do breathing exercises, and contemplate the natural environment.
New work on the health benefits of nature has since been published and an umbrella review (overall review of existing systematic reviews on a subject) was carried out by a group of Italian researchers in 2022. The main findings of this study are summarized here.
The study authors selected 16 systematic reviews that met the inclusion criteria. Overall, the best evidence indicates that forest bathing has proven effects on psychological well-being, but it is unclear whether the practice has therapeutic effects on diseases or physiological disorders. Depending on the level of scientific evidence, the researchers used an evaluation grid containing 5 grades of recommendations: strong recommendation (grade 1), scientific presumption (2A), possible (2B), limited (2C), and insufficient evidence (2C). These grades have been grouped into two main categories: major recommendations (grades 1 to 2A) and minor recommendations (grades 2B to 2C).
The major recommendations (grade 1 and 2A) for the practice of forest bathing to promote well-being are:
- Psychological relaxation and stress relief (grade 1);
- Reduction of anxiety (grade 2A) and improvement of mood (grade 2A).
Minor recommendations for practising forest bathing to promote well-being are:
- Improved sleep quality (grade 2C);
- Improvement in mood in people suffering from alcoholism (grade 2B);
- Decreased stress and increased sense of well-being in people suffering from war-related post-traumatic stress syndrome (grade 2C);
- Reduction of symptoms in people with attention deficit disorder with or without hyperactivity (ADHD, grade 2C);
- Improvement in cognitive function in healthy people (grade 2B);
- Short-term reduction in heart rate and blood pressure, reduction in inflammation and improvement in mood in people with heart failure, hypertension or chronic coronary artery disease (grade 2B);
- Decreased inflammation and improved mood in people with chronic obstructive pulmonary disease (grade 2C);
- Improved blood sugar control, reduction in stress hormones, increased levels of adiponectins in diabetics (grade 2C);
- Reduction of symptoms in people with atypical dermatitis (grade 2C);
- Increased number of natural killer (NK) lymphocytes, decreased biomarkers of inflammation and stress hormones in healthy people (grade 2B).
Optimal exposure time in the forest
Based on the evidence and critical reviews, the authors of the umbrella review conclude that walking in the forest for at least 2 hours/week is an activity that promotes good health. If it is not possible to achieve this exposure time in nature, even a single monthly session can be useful, in combination with other healthy lifestyle habits, to improve well-being.
The components of forest bathing (slightly adapted from Antonelli et al., 2022, pp. 1855-1857).
Stimulating the five senses through physical interaction with the natural environment is a fundamental element of forest bathing. Even though the visual component seems to be very relevant, especially with regard to the short-term relaxing effects (see here and here), all sensory elements seem to play an integrated role in the overall result of this practice on individual well-being (Figure 1).
Figure 1. Sensory components of forest bathing and their effects on individual well-being. From Antonelli et al., 2022.
During any forest bathing session, each of the five senses is generally stimulated as follows (from Antonelli et al., 2022, pp. 1855-1857):
- Sight: It is well documented that the sight of a forest, even a virtual one, can exert beneficial anti-stress effects, thus promoting psychophysical relaxation and improving individual stress resilience (see here, here, here and here). The prevalence of colours like green, brown and blue in any forest environment, along with the contemplation of fractal patterns and shapes (e.g., branched trees, bushes, rivers, etc.) may convey specific visual stimulations to the central nervous system that humans have been used to perceiving as “familiar” since ancient times.
- Smell: Forest trees tend to release specific volatile organic compounds (VOCs) such as limonene, alpha- and beta-pinene, but also beta-myrcene and camphene, which are responsible for antiphlogistic and antioxidant effects on the respiratory airways as well as for a general relaxing, calming and anxiolytic action on the central nervous system after their inhalation and systemic absorption (see here, here and here). The concentration of these compounds in the forest air tends to follow cyclic diurnal modifications and it depends on many factors, including tree species, seasons, temperatures and weather conditions (see here, here and here).
- Sound: Forest-derived auditory stimulation, such as that produced by the wind blowing through the trees, birds singing or flowing water in the natural environment, can contribute to psychophysical relaxation and aid recovery from stress, as it is plausible that, evolutionarily speaking, we are naturally more “attuned” to these cues than to urban noises (see here, here, here and here).
- Touch: It has been shown that touching wood with the hands can be relaxing and can stimulate parasympathetic nervous activity more than touching other materials. Touching plant foliage is also associated with an unconscious calming reaction in healthy adults. Overall, tactile stimulation arising from touching living forest plants and their different parts (roots, bark, leaves, fruits) may play its part in the anti-stress effect of forest bathing.
- Taste: Although not necessarily a part of shinrin-yoku, a visit to a forest sometimes includes tasting edible berries, roots, herbs, mushrooms and other natural products collected during the trip. Some studies have shown that natural organic food can be instinctively perceived as healthier by consumers and that, on the contrary, a high intake of processed food can be associated with a higher prevalence of both physical and mental diseases (see here, here and here). In addition to the nutritional aspects, tasting natural products found in the forest may help create a deeper connection with the natural environment (in this regard, it is important to be able to thoroughly distinguish edible natural foods from poisonous plants and mushrooms).
Forest bathing and the mind
From a psychological perspective, nature-based therapies and, more generally, any therapeutic interaction with natural environments are considered to have three major components (emotional, cognitive and behavioural), which, taken together, can form a theoretical model composed of six elements:
- Stimulation of the five senses by the forest.
- Acceptance, i.e., feeling “welcomed” in the natural environment.
- Purification, which occurs when negative thoughts and bad feelings dissolve.
- Reflection, or a moment of introspection and meditation.
- Recharging with positive energy (hope, courage and confidence).
- Final “change”: participants feel that they have experienced a change in their body and mind.
Conclusions and limitations
Overall, the strongest available evidence supports the use of forest bathing as a complementary practice for improving mental well-being (stress, anxiety, emotional imbalance, mild mood disorders), whereas the evidence supporting its prescription for specific diseases remains to be established. The positive impact of forest bathing on individual quality of life, as well as its favourable cost-effectiveness profile, justify its possible adoption for the promotion of well-being in preventive medicine. However, despite the short-term improvements noted in several health biomarkers, psychological state and well-being, there is no strong evidence for the long-term effects on cardiovascular health, and some studies show less conclusive results, even in the short term. It is possible that greater long-term effects may require a higher level of physical activity than is usually achieved in forest bathing sessions. Future studies are needed to better understand the causal links and physiological mechanisms involved. This knowledge would make it possible to more effectively prescribe exposure sessions in nature while respecting the differences and needs of each person.