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The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan
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Abstract
This paper reviews previous research on the physiological effects of Shinrin-yoku
(taking in the forest atmosphere or forest bathing), and presents new
results from field experiments conducted in 24 forests across Japan. The
term Shinrin-yoku was coined by the Japanese Ministry of
Agriculture, Forestry, and Fisheries in 1982, and can be defined as
making contact with and taking in the atmosphere of the forest. In order
to clarify the physiological effects of Shinrin-yoku, we
conducted field experiments in 24 forests across Japan. In each
experiment, 12 subjects (280 total; ages 21.7 ± 1.5 year) walked in and
viewed a forest or city area. On the first day, six subjects were sent
to a forest area, and the others to a city area. On the second day, each
group was sent to the other area as a cross-check. Salivary cortisol,
blood pressure, pulse rate, and heart rate variability were used as
indices. These indices were measured in the morning at the accommodation
facility before breakfast and also both before and after the walking
(for 16 ± 5 min) and viewing (for 14 ± 2 min). The R–R interval was also
measured during the walking and viewing periods. 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.
Keywords: Therapeutic effects of forest, Heart rate variability, Salivary cortisol, Blood pressure, Pulse rate
Introduction
The
growing interest in environmental stress has been accompanied by a
rapid accumulation of evidence indicating that environment can elicit
substantial stress in people living in urban environments [1]. Furthermore, it is broadly conceived that the natural environment can enhance human health [2].
There have been several questionnaire studies on the psychological
effects of forest environments. A previous study found an enhancement of
positive emotions among subjects who were shown pictures of natural
environments [3–6]. Moreover, other studies have also found that forest environments improve the psychological wellbeing of people [7–12].
The term Shinrin-yoku
(taking in the forest atmosphere or forest bathing) was coined by the
Japanese Ministry of Agriculture, Forestry, and Fisheries in 1982. It
can be defined as making contact with and taking in the atmosphere of
the forest: a process intended to improve an individual’s state of
mental and physical relaxation [13]. Shinrin-yoku is considered to be the most widespread activity associated with forest and human health.
Nowadays,
there is considerable interest in stress control and relaxation.
Further, the field of medical science has always favored evidence-based
medicine (EBM); this emphasizes the importance of scientific evidence in
medical practice. With improved measurement techniques, the relaxation
effect induced by forest settings can be clarified in a field test by
measuring the changes induced in physiological parameters such as
salivary cortisol, pulse rate, blood pressure, and heart rate
variability (HRV).
With this social background, the
Association of Therapeutic Effects of Forests was established in Japan
in 2004, with the purpose of conducting a Therapeutic Effects of Forests
project in Japan. At the European level, similar efforts were made
through COST Action E39 on forest and human health from 2004 to 2008 [14],
and on the global level, the International Union of Forest Research
Organizations (IUFRO) launched a new taskforce on forests and human
health in Finland in 2007 with the purpose of fostering
cross-disciplinary dialogue between the different researchers in this
field, especially forestry and health professionals.
As
part of this effort, the Japanese Society of Forest Medicine was
established in 2007 under the Japanese Society for Hygiene, with the
purpose of promoting research in the field of forest medicine, including
the effects of forest bathing trips and the therapeutic effects of
forests on human health. At the same time, several field studies on the
physiological effects of the natural environment were carried out [13, 15–19].
In this paper, we review selected field studies performed on the physiological effects of Shinrin-yoku
and a study dealing with the relationship between its psychological
effects and physical environmental factors. In addition, we report new
results from field experiments conducted in 24 forests across Japan.
Field methods
Subjects and study sites
We
conducted physiological experiments in 24 areas from 2005 to 2006 in
Japan. In each experiment, 12 normal male university students (280 in
total; ages 21.7 ± 1.5 years) participated as subjects; none reported a
history of physical or psychiatric disorders. The study was performed
under the regulations of the Institutional Ethical Committee of the
Forestry and Forest Products Research Institute in Japan. On the day
before the experiments, subjects were fully informed of the aims and
procedures of the experiment and their informed consent was obtained.
Physiological measurements
Seven physiological parameters were analyzed in the present study (Table 1).
For the measurement of salivary cortisol concentration, saliva was
collected by holding two pieces of absorbent cotton in the mouth for
2 min and using a saliva collection tube (no. 51.1534, Sarstedt,
Numbrecht, Germany). On collection, the tube was sealed with tape and
immediately stored, refrigerated, and frozen; it was later analyzed for
cortisol concentration (SRL, Inc., Japan). Heart rate variability (HRV)
was analyzed for the periods between consecutive R waves in the
electrocardiogram (R–R intervals) measured by a portable
electrocardiograph (AC-301A, GMS Corporation). The power levels of the
high-frequency (HF; 0.15–0.4 Hz) and low-frequency components (LF;
0.04–0.15 Hz) were calculated [20] every minute by the maximum-entropy method (Mem-Calc, GMS Ltd. [21]). The HF power is considered to reflect parasympathetic nervous activity [22]. Furthermore, the power ratios HF/LF and LF/(LF + HF) were determined to reflect the sympathetic nervous activity [23].
Systolic blood pressure, diastolic blood pressure, and pulse rate were
measured by a digital blood pressure monitor using oscillometric methods
(HEM1000, Omron, Japan) on the right upper arm.
Psychological measurements
The Profile of Mood States (POMS) was used to gauge the psychological response [24].
The POMS consists of 30 adjectives rated on a 0–4 scale that can be
consolidated into the following six effective dimensions: T–A (tension
and anxiety), D (depression and dejection), A–H (anger and hostility), F
(fatigue), C (confusion), and V (vigor). Because of its responsiveness,
the POMS have been widely used in the assessment of mood changes
resulting from a variety of interventions. For the Japanese subjects,
the Japanese edition of the POMS was used.
Physical environmental factors
In
the physical experiment, the temperature and relative humidity, radiant
heat, wind speed, predicted mean vote (PMV), and predicted percentage
dissatisfied (PPD) were measured using a portable amenity meter (AM-101,
Kyoto Electronics Manufacturing Co. Ltd., Japan) at each study site. In
addition, atmospheric pressure (Kestrel 4000, Nielsen-Kellerman, Japan)
was also measured at some locations. Relative illumination was
calculated from photos of the sky captured by a digital camera (Coolpix
4500, Nikon, Japan) equipped with a fisheye lens (FC-E8, Nikon, Japan).
Experimental design
After
being given an orientation to the experiment on the day before the
first day of experimentation, the subjects visited and previewed the
forest and city study sites. Next, test measurements of all the
physiological indexes and subjective feelings were conducted at the
accommodation facility. In order to control the background environmental
conditions, identical, separate rooms were prepared as lodgings for
each subject and identical meals were served during the experiments.
The
subjects were randomly divided into two groups. On the first day of the
experiments, six subjects were sent to a forest site, and the other six
subjects to a city site. On the second day, the subjects were sent to
the other type of site as a cross-check. The first measurement was taken
in the early morning at the accommodations before breakfast. After the
first measurement, subjects were sent to either a forest or city site.
It took almost the same amount of time to reach both the forest and city
sites from the accommodations. As shown in Fig. 1,
upon arrival at the given site, the subjects were seated on chairs and
viewed the landscape (for 14 ± 2 min). They also walked around the given
site (for 16 ± 5 min). The second and third measurements were taken
before and after this walking. The fourth and fifth measurements were
taken before and after the viewing. These measurements were taken for
one person at a time. In addition to these five measurements, the R–R
interval was measured continuously during the walking and viewing
exercises at the given site. The HRV was calculated once a minute using
the R–R interval data. The exercise loads during the walking exercise in
the forest and city sites were estimated with an activity monitor
(AC-301A, GMS, Japan); there was no difference in exercise load between
walking in a forest site and walking in a city site.
The consumption of alcohol and tobacco was prohibited and caffeine consumption was controlled.
Review of field studies performed on the physiological effects of Shinrin-yoku in Japan
We
searched the major journals on medical science, physiological
anthropology, and environmental science for reports on field studies on
the physiological effects of Shinrin-yoku in Japan. Only articles presenting evidence of the relaxing effects related to Shinrin-yoku have been reviewed in this paper. Table 2 presents a summary of the reviewed papers.
An early study by Ohtsuka et al. [25]
showed that blood glucose levels in diabetic patients decrease when
they walk in a forest for 3 or 6 km, depending on their individual
physical ability. By the middle of the decade in which the
above-mentioned study was performed, research on the physiological
effects of Shinrin-yoku began in earnest, using improved
technologies for measuring physiological indicators. These studies used a
wide range of physiological indices such as salivary cortisol, pulse
rate, blood pressure, and HRV. Moreover, the experiments were designed
with full consideration for cross-checks and control stimuli. The
studies showed that viewing forest landscapes and walking in forest
settings leads to lower concentrations of cortisol, lower pulse rate,
lower blood pressure, enhanced HF component of the HRV, and lower LF/HF
[or LF/(LF + HF)]. In particular, Park et al. [13]
showed that forest environments can lower the absolute value of the
total hemoglobin concentration (t-Hb), an index of cerebral activity, in
the left prefrontal area of the brain. The absolute value of hemoglobin
concentration had never previously been measured in the field.
Though
these studies focused on short-span exposures to stimuli (approximately
15 min of viewing and approximately 15 min of walking), the results
strongly supported that participating in Shinrin-yoku activity could effectively relax the human body.
Results of a physiological experiment in 24 forests across Japan
Figure 2
shows the average cortisol concentration in the saliva. Salivary
cortisol was significantly lower in the forest area (13.4% decrease
after the viewing; 15.8% decrease after the walking). Moreover, the
average pulse rate was significantly lower (Fig. 3) in the forest area (6.0% decrease after viewing; 3.9% decrease after walking). Figure 4
shows that the average systolic blood pressure was significantly lower
in the forest setting (1.7% decrease after viewing; 1.9% decrease after
walking). Figure 5
shows similar results for the average diastolic blood pressure (1.6%
decrease after viewing; 2.1% decrease after walking). The average power
of the HF components of the HRV, which is related to parasympathetic
nervous activity, increases when we feel relaxed. This value was
significantly enhanced in the forest settings (56.1% enhancement after
viewing; 102.0% enhancement after walking; Fig. 6).
The average LF/HF ratio of the HRV, which is related to sympathetic
nervous activity, increases when we feel stress. This value decreased
when the subjects were walking in or viewing a forest (18.0% decrease
after viewing; 19.4% decrease after walking; Fig. 7).
Change in salivary cortisol concentration after forest viewing and walking. Mean ± standard deviation (SD); ** p < 0.01; p-value by t test
Change in pulse rate after forest viewing and walking. Mean ± SD; ** p < 0.01; p value by t test
Change in systolic blood pressure after forest viewing and walking. Mean ± SD; ** p < 0.01; * p < 0.05; p value by t test
Change in diastolic blood pressure after forest viewing and walking. Mean ± SD; * p < 0.05; p value by t test
Change in HF power of HRV upon forest viewing and walking. Mean ± SE; ** p < 0.01; * p < 0.05; p value by t test
Change in LF/HF of HRV upon forest viewing and walking. Mean ± SE; ** p < 0.01; * p < 0.05; p value by t test
Overall,
the results show that viewing forest landscapes leads to lower
concentrations of cortisol, lower pulse rate, lower blood pressure,
enhanced HF components of HRV, and lower LF/HF. These results strongly
support the findings of indoor research using heart rate and blood
pressure on the effects of viewing a forest scene on recovery from
stress [1, 26–28].
The effect of walking in a forest setting is the same as that of
viewing a forest setting. This result corroborates Hartig et al.’s
finding [27]
that walking in a nature reserve initially fosters blood pressure
changes that indicate greater stress reduction than that afforded by
walking in city surroundings.
From the perspective of
physiological anthropology, human beings have lived in the natural
environment for most of the 5 million years of their existence.
Therefore, their physiological functions are most suited to natural
settings [29].
This is the reason why the natural environment can enhance relaxation.
The results of the physiological experiments conducted in this study
yield convincing answers explaining the relationship between the natural
environment and the relaxation effects in a human being (e.g., decrease
in blood pressure and pulse rates, inhibition of sympathetic nervous
activity, enhancement of parasympathetic nervous activity, and decrease
in cortisol concentration levels in human beings).
The
endocrine stress system comprises two broad components with considerable
central anatomic interconnection, namely, the sympathetic
adrenal-medullary (SAM) axis and the hypothalamic-pituitary-adrenal
(HPA) axis [30].
The SAM axis is involved in immediate sympathetic activation preparing
an individual to deal with a stressor, resulting in changes such as
increased heart rate (HR) and blood pressure (BP) [31]. Cortisol is released by the HPA axis in response to stress [32].
While subjects viewed forest landscapes or walked around forest
environments, their pulse rate, blood pressure, and cortisol
concentration decreased. This suggests that both the main components of
the endocrine stress system reacted in response to Shinrin-yoku.
In particular, high cortisol levels can correspond to a low value of natural killer (NK) activity [33].
Further, cortisol concentration also holds great significance in terms
of human immunological activity. Furthermore, the study of Li et al. [34–36]
reported that forest surroundings could aid in the recovery of the
human immune system, as determined from the perspective of NK activity.
For
this reason, it can be suggested that not only forest environments but
also other natural settings such as watersides or grasslands could
promote relaxation in human beings. No evidence from field experiments
conducted on other natural environments are available; however, Laumann
et al. [28]
have reported that, when subjects viewed natural environment through
videos, including those of waterside scenes, they had a longer cardiac
interbeat interval and lower heart rate, measured as the difference from
the baseline, compared with subjects who viewed urban environment
through videos.
All the indices were
generally in excellent agreement with each other, implying that the
forest environment possessed relaxing and stress-relieving effects. Our
results also corroborate widely held beliefs that forest surroundings
aid the physical relaxation of urban dwellers. In addition, these
results suggest that physiological responses—pulse rate, blood pressure,
salivary cortisol concentration, and HRV—can reflect the relaxing
effects of forest environments.
Psychological effects and relationship between psychological effects and physical environmental factors in ten forests across Japan
The changes in the average POMS subscale scores after the viewing are presented in Fig. 8.
Significant differences are seen between the changes resulting from
viewing a forest landscape and those from viewing a city landscape. When
subjects viewed a forest landscape, the POMS tension subscale score
changed by –1.1 points, which is significantly lower than the change
(3.5 points) after viewing a city landscape. The change in the POMS
depression subscale score (–0.3 points) on viewing a forest landscape is
also significantly lower than the score (0.1 points) on viewing a city
landscape. There is a significant difference in the change in the POMS
anger subscale score between viewing forest (–0.2 points) and city
landscapes (1.0 points). The change in the POMS fatigue subscale score
(–3.1 points) on viewing a forest landscape is significantly lower than
the score (1.8 points) on viewing a city landscape. The change in the
POMS confusion subscale score (–1.0 points) on viewing a forest
landscape is also significantly lower than that (1.8 points) on viewing a
city landscape. However, the change in the POMS vigor subscale score
(1.9 points) on viewing a forest landscape is significantly higher than
that (–1.9 points) on viewing a city landscape.
Change in average POMS scores upon forest viewing. Mean ± SD; n = 116; ** p < 0.01; * p < 0.05; p value by Wilcoxon signed-rank test. T–A tension ...
The changes in the average POMS subscale scores after walking are shown in Fig. 9.
The results are the same as those for viewing. When walking, the
changes in the average POMS subscales of tension (forest:–1.1 points,
city: 3.2 points), depression (forest: –0.2 points, city: 0.8 points),
anger (forest: –0.2 points, city: 0.8 points), fatigue (forest: –2.1
points, city: 1.3 points), and confusion (forest: –1.1 points, city: 1.1
points) are significantly different in the forest and city areas. And
the change in the POMS vigor subscale score (4.2 points) on walking in
forest settings is significantly higher than that (–0.2 points) on
walking in city settings.
Change in average POMS scores upon forest walking. Mean ± SD; n = 78; ** p < 0.01; p value by Wilcoxon signed-rank test. T–A tension and anxiety, D depression and dejection, ...
The
POMS measurements show that forest environments can relieve human
psychological tension, depression, anger, fatigue, and confusion, and
moreover, that they can enhance human psychological vigor. Furthermore,
from the viewpoint of attention restorative theory (ART) [37], these results strongly support that the forest is a good restorative environment for human beings.
Kasetani et al. [38] reported that a relationship exists between the POMS score and the physical environmental factors (Fig. 10). The POMS anger subscale score and relative illumination had a significant correlation coefficient (R = 0.66)
in the forest areas. Moreover, the POMS fatigue subscale score and
relative humidity had a significant correlation coefficient (R = 0.70). Finally, the POMS depression subscale score and atmospheric pressure had a significant correlation coefficient (R = 0.63).
When
viewing a forest landscape, the low relative illumination reduces
anger, and the low relative humidity lowers fatigue. Forests located at
high elevations with low atmospheric pressure can reduce depression. We
hope that these results can be used as basic guidelines in the design of
therapeutic forest environments [39].
Conclusion
The results of studies performed on the physiological effects of Shinrin-yoku
show that forest environments could lower concentrations of cortisol,
lower pulse rate, lower blood pressure, increase parasympathetic nerve
activity, and lower sympathetic nerve activity compared with city
settings. The results of the physiological measurements suggest that Shinrin-yoku
can aid in effectively relaxing the human body, and the psychological
effects of forest areas have been correlated with the various physical
environmental factors of forest. The studies of Shinrin-yoku provide valuable insights into the relationship between forests and human health.
These results of Shinrin-yoku
studies will contribute to support the development of a research field
dedicated to forest medicine, which may be used to develop new
strategies in preventive medicine. The results of the field experiments
also provide a platform for interested enterprises, universities, and
local governments to promote the effective use of forest resources in
stress management, health promotion, rehabilitation, and the prevention
of disease.
Acknowledgments
This
study was partly supported by a Grant-in-Aid for Scientific Research
(S: 16107007) from The Ministry of Education, Culture, Sports, Science,
and Technology (MEXT).
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