Environ Health Prev Med (2010) 15:27–37
DOI 10.1007/s12199-009-0091-z
SPECIAL FEATURE
The Trends on the Research of Forest Bathing in Japan,
Korea and in the World
Trends in research related to ‘‘Shinrin-yoku’’ (taking
in the forest atmosphere or forest bathing) in Japan
Yuko Tsunetsugu Æ Bum-Jin Park Æ Yoshifumi Miyazaki
Received: 18 July 2008 / Accepted: 27 May 2009 / Published online: 9 July 2009
Ó The Japanese Society for Hygiene 2009
Abstract ‘‘Shinrin-yoku’’, which can be defined as ‘‘tak-
ing in the forest atmosphere or forest bathing’’, has been
receiving increasing attention in Japan in recent years for its
capacity to provide relaxation and reduce stress. Since 2004,
the authors of this paper have been involved in an investi-
gation designed to ascertain the physiological effects of
‘‘Shinrin-yoku’’ within the framework of the ‘‘Therapeutic
Effects of Forests’’ project. We have conducted physiolog-
ical experiments, both in actual forests and in the laboratory,
to elucidate the physiological effects on individuals of
exposure to the total environment of forests or to only certain
elements of this environment, such as the odor of wood, the
sound of running stream water, and the scenery of the forest.
We have obtained physiological measurements of central
nervous activity, autonomic nervous activity, and bio-
markers reflecting stress response that can be applied in this
line of approach. Using these measurements, we have
summarized the separate elements of forests in terms of the
five senses. We have also reviewed a selection of field
studies and introduced a number of results from ongoing
projects as well as those from early studies. Future per-
spectives are also discussed.
Keywords Field study Á Forest bathing Á
Heart rate variability (HRV) Á
Near-infrared spectroscopy (NIRS) Á Salivary cortisol
Y. Tsunetsugu (&)
Forestry and Forest Products Research Institute,
1 Matsunosato, Tsukuba, Ibaraki 305-8687, Japan
e-mail: yukot@ffpri.affrc.go.jp
B.-J. Park Á Y. Miyazaki
Center for Environment, Health and Field Sciences,
Chiba University, Kashiwa, Japan
Introduction
In a survey conducted by the Ministry of Health, Labour
and Welfare of Japan [1], 54.2% of the respondents (32,000
Japanese individuals older than 12 years) rated their stress
level as ‘‘very high’’ or ‘‘relatively high.’’ This figure,
however, represents only an average of the assessments
made by all the respondents, and in certain age groups the
proportion of individuals who reported experiencing
excessive stress was as high as 60–70%. Furthermore, in
self-perceived health status evaluations, almost half
(42.3%) of the surveyed individuals rated their health status
as ‘‘mediocre’’ on a five-grade ‘‘poor’’-to-‘‘good’’ scale.
These data clearly indicate that modern societies are ‘‘high-
stress’’ societies and that many individuals, although not
actually affected by any particular disease, perceive
themselves as unhealthy.
In Japan, ‘‘Shinrin-yoku’’, which can be defined as
‘‘taking in the forest atmosphere or forest bathing’’ is
currently receiving increasing attention for its capacity to
provide relaxation and reduce stress. Humans empirically
recognize that getting in touch with nature provides a
feeling of comfort. In carrying out their investigations
on ‘‘Shinrin-yoku’’, the authors of this article have
assumed that the reason underlying this feeling of com-
fort with ‘‘nature’’ or ‘‘natural matters’’ is closely linked
with the human evolutionary process. Frumkin supports
this view and reports that a deep-seated connection
between the natural world and humans is unsurprising
from an evolutionary perspective [2]; based on this
description, we have also assumed that human physio-
logical functions have had to adapt to the natural envi-
ronment through the course of evolution. Consequently,
living in our modern ‘‘artificial’’ society is inherently
stressful. Given this background, it should be natural for
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humans to feel a sense of comfort or affinity with the
natural environment.
‘‘Shinrin-yoku’’ is considered to be one of the most
accessible ways to get in touch with the natural world and
to lower excessive stress to levels that are commensurate
with what our bodies are ‘‘expected’’ to cope with. In
Western societies, this approach has been incorporated into
the lives of individuals since the nineteenth century. The
best known example of this may be Kneipp therapy in
Germany. Kneipp therapy utilizes forests mainly as the
fields for exercise therapy, which is one of the five pillars
of the method. However, although Kneipp therapy some-
what realizes the concept of ‘‘Shinrin-yoku’’ in a practical
context, its precedents in the Western countries are based
mostly on empirical knowledge. The effects of ‘‘Shinrin-
yoku’’ have yet to be verified by scientific evidence.
The term ‘‘Shinrin-yoku’’ and its concept were intro-
duced in Japan by the Forest Agency of the Japanese
government in 1982. Over the 25 years that have passed
since then, interest in stress control or relaxation among
those living in our modern-day society has greatly
increased. In addition, in the fields of medicine and nurs-
ing, there has been a trend toward evidence-based medicine
or nursing, emphasizing the need for scientific evidence.
Based on this social background, in 2005, the Forest
Agency instituted the ‘‘Therapeutic Effects of Forests
Plan,’’ which emphasizes the importance of scientific
investigation.
Since 2004, the authors have been involved in investi-
gations designed to ascertain the physiological effects of
‘‘Shinrin-yoku’’ within the context of the ‘‘Therapeutic
Effects of Forests’’ project. We have conducted physio-
logical experiments both in actual forests and in the labo-
ratory in order to elucidate the physiological effects of the
total forest environment or certain elements of the forest
environment, such as the odor of wood, the sound of run-
ning stream water, and the scenery of the forest. In this
paper, we introduce the physiological measurements of
central nervous activity, autonomic nervous activity, and
biomarkers reflecting stress response that can be applied in
this approach. Related studies that have been conducted in
Japan will also be reviewed; further, in the final section, we
will briefly discuss future perspectives.
Physiological evaluation of ‘‘Shinrin-yoku’’
In order to investigate the physiological effects of ‘‘Shinrin-
yoku’’, it is essential to conduct field experiments. However,
since it is more difficult to control the experimental condi-
tions in the field than in the laboratory, there are certain
practical requirements associated with measuring physio-
logical parameters under field conditions such as: (1) the
measurement device should be portable and able to with-
stand vibrations on being moved; (2) a simple measurement
process and short measurement time is preferable; (3)
measurement error due to environmental factors, such as
temperature, wind, light, and noise, should be avoided; (4)
battery-powered equipment is preferable. Until relatively
recently, few methods have been able to fulfill these
requirements, while simultaneously ensuring high-precision
measurements. However, the rapid progress of technology
has enabled certain physiological measurements to be car-
ried out during experiments in the field. In this section, the
physiological methods that can be applied in the laboratory
and field experiments are briefly described.
Measurement of central nervous activity
As an index of central nervous activity, the authors have
utilized cerebral hemodynamics estimated by near-infrared
spectroscopy (NIRS) [3, 4]. NIRS is able to measure con-
centration changes in oxygenated and deoxygenated
hemoglobin associated with brain activity with a 1-s time
resolution by utilizing the difference in the infrared
absorption properties between the two hemoglobin states.
The sensors can be attached to the subject by means of
double-sided tape without any difficulty and, therefore, less
stress is caused by the measurement itself compared to
other methods, such as electroencephalography. NIRS
equipment is portable and sufficiently robust to be suitable
for conducting measurements in the field. Nevertheless,
adequate light shielding should be provided since the
measurements are based on the principle of near-infrared
light transmission.
The most recent feature of this method that makes it
interesting is the possibility of measuring absolute hemo-
globin concentration by using the time-resolved spectros-
copy (TRS) technique [5, 6]. In earlier studies, NIRS could
only be applied to measure relative changes in hemoglobin
concentration from a certain baseline. However, with the
development of techniques that measure the actual optical
path length and solve a light diffusion equation using
picosecond pulses, it is now possible to obtain an absolute
value for hemoglobin concentration based on the assump-
tion of the existence of homogenous optical properties
beneath each optode. We have previously reported on the
changes in absolute hemoglobin concentrations induced by
walking or viewing scenery in forest and in urban envi-
ronments [7]. To date, this is the only example of applying
TRS in the field.
Measurement of autonomic nervous activity
Heart rate variability (HRV) [8] as well as blood pressure
and pulse rate are frequently employed to estimate changes
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in the autonomic nervous activity. In experiments carried
out by the authors, an ambulatory electrocardiogram
monitor (AC301A; GMS Corp.) was attached to the sub-
jects to store the data of the R–R interval for assessing
HRV before, during, and after the subjects walked around
or viewed the scenery in a forest area. The R–R interval
data were subsequently analyzed using the maximum
entropy method [9] (Memcalc; GMS Corp.). The power of
the low-frequency (LF; 0.04–0.15 Hz) and high-frequency
(HF; 0.15–0.4 Hz) components of the obtained heart rate
power spectrum for each minute are generally calculated.
HF power is considered to reflect parasympathetic nervous
activity, and either LF/HF or LF/(LF ? HF) is considered
to be an index of sympathetic nervous activity [8]. Our
results indicated that subjects in a forest environment had
more enhanced parasympathetic and lower sympathetic
nervous activity than those in an urban environment [10].
HRV analysis is thus a useful method for detecting the
sympathetic and parasympathetic nervous activity sepa-
rately on a minute-by-minute basis.
The newest technique for assessing autonomic nervous
activity involves the measurement of salivary amylase
activity. The secretion of salivary amylase is controlled by
the sympathetic–adrenal–medullary system, and it has
recently been determined that the secretion of salivary
amylase under sympathetic nerve regulation may reflect
mental stress, with increased secretion indicating higher
stress [11]. Equipment that evaluates amylase activity on-
site in 1 or 2 min has recently been developed [12] and is
currently being tested in various studies, including our
own. Since sampling of saliva causes considerably less
stress in subjects than blood or urine sampling, there are
high expectations for this method as a novel means of
evaluating mental stress expressed via sympathetic nervous
activity.
Biomarkers reflecting stress response
Salivary cortisol and immunoglobulin A (IgA) concen-
trations are established stress markers that are utilized in
various scientific fields; however, few investigations have
applied these indices to field studies involving a large
number of subjects over a specific period of time. In our
field studies, we have clarified that cortisol concentration
is significantly lower in subjects exposed to a forest
environment than in those exposed to an urban environ-
ment [10]. However, the results for IgA concentration
were not consistent. IgA concentration is known to
decrease under severe chronic stress [13], whereas it has
been reported to increase in response to acute stress [14].
Interestingly, however, it has also been demonstrated that
the levels of IgA in individuals engaged in stressful jobs
are significantly higher than those in individuals who are
not engaged in comparable activity [15]. It is thus
believed that the use of IgA concentration as a stress
marker should be re-evaluated by investigating the rela-
tionship between its change and the types and continuity
of emotional stress.
There are large individual differences in the baseline
values of salivary cortisol and salivary IgA concentrations.
We recently found that there was a significant negative
correlation between the baseline value and the amount of
change in both salivary cortisol and IgA levels when sub-
jects were walking and viewing scenery in both forest and
city environments [16]. This finding will contribute to the
interpretation of the individual differences in cortisol and
IgA responses.
Laboratory studies
In this section, studies aimed at elucidating the effects of
certain elements of the forest environment within the
laboratory setting will be surveyed by reviewing the
reports of previous studies. Prior to the ‘‘Therapeutic
Effects of Forests’’ project, there had been little field
research on ‘‘Shinrin-yoku.’’ It is clear, nonetheless, that
people have always recognized forests as familiar places
and have somehow felt that the forest atmosphere bene-
fitted human health. The number of studies targeting
‘‘Shinrin-yoku’’ gradually increased in the late 1990s
following the launch of the ‘‘Shinrin-yoku plan’’ in 1982.
Further, improvement in the technology of physiological
measurements provided the impetus for an increase in the
number of studies.
Forest environments affect humans via the five senses,
providing stimulation of various senses, such as vision
(scenery), olfaction (smell of wood), audition (sound of
running streams or the rustle of leaves), tactile sensation
(feel of the surfaces of trees and leaves). Sensory infor-
mation inputs via the five senses are processed in the
corresponding sensory areas of the brain and are further
transmitted through interaction among the various sensory
inputs. These signals subsequently reach the areas of the
brain that control emotions and physiological functions,
where they effect physiological changes. The objectives of
the laboratory experiments that were designed to elucidate
the physiological effects of each sensory input were: (1) to
obtain backup data for the field studies or to undertake a
detailed analysis of the results of these studies and (2) to
assess the effects of elements of nature when subjects are
introduced to an artificial environment (e.g., scenery shown
in the form of a poster or smell introduced in the form of
aroma therapy). Examples of laboratory-based studies that
have investigated the therapeutic effects of wood and forest
are summarized below.
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Visual stimulation
Suda et al. [17] investigated the physiological effects by
viewing scenery of ‘‘Sakura’’ and ‘‘Shinrin-yoku’’ on 70-
inch high-resolution displays. Sakura consisted of a pho-
tograph of a cherry tree in full bloom, and Shinrin-yoku
was a photograph of people taking a walk in the forest of
Vincennes, Paris. In the subjective evaluation, the photo-
graph of Shinrin-yoku was considered to be significantly
more ‘‘comfortable’’ and ‘‘soothing’’ than the gray screen
used as the control; in contrast, Sakura was evaluated as
relatively ‘‘awakening.’’ Subjects viewing Shinrin-yoku
had a significantly decreased blood pressure and prefrontal
activity, whereas those viewing Sakura had a significantly
increased pulse rate and blood pressure. Further, viewing
Sakura significantly increased activity in the prefrontal
area. Although in certain cases there was a considerable
correlation between the physiological responses and sub-
jective evaluations, in other cases no such correlation was
observed. In this particular study, however, there was a
considerable correlation between the two. This study
revealed that people experience elation on viewing cherry
blossoms. It would be interesting to learn whether or not
this physiological response is unique to Japanese.
Lee and Watanuki [18] investigated the differences
between the cardiovascular responses of Types A and B
subjects to visual stimulation of displeasure-evoking ima-
ges and nature-video clips. Significant differences were
observed in the heart rate and cardiac output between
Types A and B persons when they viewed displeasure-
evoking images, i.e., Type A individuals showed a smaller
decrease in heart rate and a significant increase in the
cardiac output. However, in the ‘‘post-stress session’’
where the subjects watched the nature-video clips, no
significant main effects of the Type A/B tendency or
interactions between Type A/B tendency and type of visual
stimulus were observed for any physiological recoveries.
The authors assumed that nature-video clips may be
effective in normalizing the stress-induced cardiovascular
responses in Type A individuals as there were no differ-
ences between Types A and B subjects with respect to the
physiological recovery responses.
Tsunetsugu et al. [19, 20] investigated the visual effects
of wooden room interiors by evaluating physiological
responses in mock-up living rooms. The objective of the
study was to determine whether the wood ratio (i.e., the ratio
of the area covered with wooden material to the whole area
of the ceiling, walls, and floor) affected physiological
responses. In this study, the authors attempt to focus atten-
tion solely on the visual effects; thus, they used veneer on the
floor and walls of the rooms, and they ventilated the rooms
appropriately. Subjects in a room with a 30% wood ratio,
which is the standard type of living room commercially
available in Japan [21] had a significantly decreased pulse
rate and diastolic blood pressure, indicating the extent of the
relaxing effect of this type of interior. A 45% wood ratio
room tended to be evaluated as the most comfortable; fur-
ther, when this wood ratio was utilized, the highest scores in
the ‘‘vigorous’’ feeling in the mood test were generated.
Pulse rate significantly increased in the 45% wood ratio
room, which was considered to be in agreement with the
results of the mood evaluation. A 90% wood ratio room
initially caused a significant and large decrease in both
systolic and diastolic blood pressure; however, this wood
ratio subsequently led to a rapid decrease in prefrontal
activity and an increase in pulse rate. In the subjective
evaluation, all of the wooden interiors were rated as ‘‘com-
fortable’’; however, physiological measurements revealed
that an excessive amount of wood caused an awakened
response. Consequently, the researchers hypothesized that
there is an appropriate wood quantity (wood to room surface
area ratio) in the presence of which people experienced
physiological relaxation.
Olfactory stimulation
Phytoncides [derived from ‘‘phyto’’ (referring to plant) and
‘‘cide’’ (meaning ‘‘to kill’’)] are defined as volatile or
nonvolatile substances produced by all types of plants that
have an influence on other organisms [22]. The word
phytoncide has become so popular that there is a persistent
misunderstanding that the effects of ‘‘Shinrin-yoku’’ are
solely attributable to phytoncide activity. However, phy-
toncide is a general name for a range of substances. These
chemicals tend to differ from forest to forest. The atmo-
spheric concentrations of phytoncides in forests are very
low and vary depending on many factors, such as season,
climate, and forest composition. This leads to the reason-
able assumption that the effect of ‘‘Shinrin-yoku’’ is
complex and comprises all of the elements associated with
the senses, including phytoncides.
Phytoncides are olfaction-related elements of the forest
environment. Smell has commonly been considered to be
associated with instinct, emotion, and preference, and to
have a greater influence on physiological change than
stimuli for other senses [23].
Trees have scents that are species-specific. Japanese
cedar (Cryptomeria japonica) is a representative tree spe-
cies in Japan, and its smell is one of the most familiar to
the Japanese population since cedar wood is frequently
employed in the construction of houses. Miyazaki et al.
[24] revealed that the smell of Japanese cedar chips sig-
nificantly decreased systolic blood pressure, particularly at
40–60 s following the commencement of inhalation. The
prefrontal area also exhibited a rapid decrease in activity
with the inhalation of the smell of cedar chips, which
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became significant at 70–90 s following the first inhalation.
The subjects evaluated the smell as ‘‘preferable.’’ The
smell of Japanese cedar wood improved subjective comfort
and caused a relaxed physiological state.
There are large individual differences in odor prefer-
ences. In the experiment just described, although Japanese
cedar was evaluated as ‘‘preferable’’ on average, there were
some subjects who disliked the smell. However, contrary to
expectation, there was no rise in the blood pressure of those
subjects with an aversion to the smell. The authors assumed
that human physiology has adapted to the natural envi-
ronment and that the smell of natural matter does not lead
to stress despite the fact that it is perceived as being
unpleasant.
Hiba (Thujopsis dolabrata) is also a commonly used
species in Japan that possesses a characteristic smell. Itai
et al. [25] examined the effects of aromatherapy on the
mood by exposing subjects to hiba oil and demonstrated
that the smell of hiba oil significantly mitigated depression
and anxiety in chronic hemodialysis patients. Hiruma et al.
[26] investigated the effects of the odor of hiba on con-
tingent negative variation (CNV) and mismatch negativity
(MNN). The amplitude of the CNV components was sig-
nificantly larger under the odor condition than under the
no-odor condition, while the amplitude of the MNN
showed no difference whether or not the odor existed. The
authors interpreted the results to be attributable to the
nervous system being roused by the odor of hiba without
affecting the autonomic information process.
Miyazaki et al. [27] reported the effects of the smell of
Taiwan cypress (Chamaecyparis taiwanensis) on subjects.
The inhalation of Taiwan cypress essential oil decreased
the maximal blood pressure. In a work efficiency test in
which the subjects deleted a specific character from a
random character string, the odor of Taiwan cypress tended
to improve task performance. The inhalation of Taiwan
cypress essential oil appeared to relax the subjects, possibly
enabling them to concentrate on the task.
One of the major components of wood scent, a-pinene,
is generally detected in the air of coniferous forests. Tsu-
netsugu et al. [28] investigated the effects of the concen-
tration of a-pinene on human physiological responses.
Dilutions were made to obtain 10, 100, and 500 lL of a-
pinene in 30 L of air. The odor was emitted at a rate of 3 L/
min from a position approximately 15 cm beneath the nose
of the subjects. The 10 lL/30 L and 100 lL/30 L con-
centrations, which were rated as ‘‘slight odor’’ and expe-
rienced to be ‘‘slightly comfortable’’ in the subjective
evaluation, caused decreased systolic blood pressure.
However, the 500 lL/30 L concentration, which was rated
as a ‘‘strong odor’’ and ‘‘slightly uncomfortable,’’ did not
cause a decrease in systolic blood pressure although it did
increase the pulse rate. The 500 lL/30 L concentration that
caused the slightly uncomfortable experience was consid-
ered to induce dominant sympathetic nervous activity. It
was also observed that the degree of increase in prefrontal
activity was enhanced concomitant with the increase in the
concentration of a-pinene. Collectively, these results indi-
cate that a weak smell of a-pinene induces a relaxed
physiological state, whereas a relatively strong smell of a-
pinene induces a stress state. Although a-pinene is a natural
product, human beings have likely never been exposed to
relatively high concentrations of a-pinene in the natural
environment during evolution. Thus, it caused a stress state
that was experienced as ‘‘uncomfortable.’’
Limonene is another common phytoncide of wood. It can
also be found in citrus peel and has a lemony, grapefruit-
like smell. Tsunetsugu et al. [28] demonstrated that limo-
nene at a concentration of 10 lL/30 L emitted at a rate of
3 L/min from a position approximately 15 cm beneath the
nose of subjects was evaluated as being ‘‘slightly comfort-
able’’ and ‘‘slightly soothing.’’ Blood pressure started to
decrease after 20 s from the commencement of inhalation;
this decrease reached a significant level at 33–44 s. These
researchers concluded that the inhalation of limonene was
experienced as comfortable and soothing and that it sup-
pressed sympathetic nervous activity.
Dayawansa et al. [29] tested the effects of cedrol, which
is a compound that occurs in cedar extract, on cardiovas-
cular and respiratory functions and observed that cederol
inhalation significantly decreased the heart rate, systolic
blood pressure, diastolic pressure, and respiratory fre-
quency. The authors assumed that cedrol inhalation
reduced respiratory frequency, which in turn, decreased the
blood pressure via an enhancement of the baroreceptor
sensitivity that has been reported to be inversely related to
sympathetic activity.
Despite the large number of studies, it remains unclear
to date whether odor affects human physiology through the
olfactory pathway or via the blood-borne route in the form
of chemical compounds. Buchbauer et al. [30] reported that
odorous compounds were observed in the serum of mice,
though in low concentrations, after an inhalation of seda-
tive odors. Dayawansa et al. [29] referred to both possi-
bilities in their above-mentioned report that cedrol could
act via the central olfactory and limbic systems, and/or via
the blood-borne route or peripheral afferent fibers of the
vagal nerve innervating the respiratory system. The most
recent human study demonstrated that cedrol directly
inhaled through the lower airway, but not introduced to the
upper airway, causes a suppression of sympathetic outflow
and an increase in parasympathetic outflow [31]. The
results of various studies on other scents, such as lavender,
lemon, valerian, and others, showed considerable agree-
ment with respect to the fact that the effectiveness of odors
in prolonging pentobarbital sleep time [32, 33], lowering
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blood pressure [34], and suppressing renal sympathetic
nerve activity [34] disappeared in anosmic mice or rats,
thus indicating the mediation of the olfactory system in the
whole process. The authors consider that odors affect
human physiology mainly through the olfactory processing
pathway; however, in the case of laboratory experiments,
the possibility that odorous compounds act directly via the
blood-borne pathway could not be eliminated since the
concentration of inhaled compounds is relatively high. In
the case of ‘‘Shinrin-yoku’’ performed in the field, where
the odorous compounds (phytoncides) exist in very low
atmospheric concentrations, it is likely that phytoncides
exert their effects via the olfactory processing pathway, and
not via the blood-borne route.
Tactile sensation
Among the elements of the forest environments, interior
wood is considered to have the strongest association with
tactile sensation. Sakuragawa et al. [35] attempted to
clarify the effects of contact with wood used as an interior
material. They compared the physiological responses
shown by the subjects to a metal plate and an oak plate.
The effect of the temperature of the materials was con-
trolled by warming the metal plate and cooling the oak
plate. The blood pressure of subjects was observed to
increase when they touched the metal at room temperature;
however, this increase was suppressed when the plate was
warmed. The expectation was that the cold wood panel
would cause an increase in blood pressure; however, it did
not. In the subjective evaluation, the cold oak was felt to be
uncomfortable but natural; this probably explains the sup-
pression of a blood pressure increase.
Miyazaki et al. [36] reported on the effects of paint.
They compared changes in systolic blood pressure caused
by touching Japanese cedar panels that had not been
painted, that had been painted with a thin layer (oil finish),
and that had been painted with a thick layer (polyurethane
paint). A metal plate was also prepared as a reference. In
the case of no paint and thin paint, systolic blood pressure
rose transiently and then rapidly fell back to baseline lev-
els, whereas it remained high when the subjects touched
the thick-painted panel and metal. These results indicate
that cedar wood can be as much a stressor as metal when
coated with a thick layer of paint. Wood is usually coated
with paint for maintenance reasons; however, this tends to
negate the comparative advantage of the material in terms
of human-friendliness.
Auditory stimulation
Mishima et al. [37] compared the changes in brain activity
and systolic blood pressure caused while listening to the
sound of a turbine (dental implement), representing an
artificial sound, or to a stream, representing a natural
sound. No sound was introduced in the control session, i.e.,
the subjects heard no sound. Systolic blood pressure was
significantly increased with the sound of the turbine;
however, it was not changed by the sound of a stream.
Prefrontal activity estimated using the NIRS method was
considerably reduced when the subject was listening to the
turbine, while it showed a slight but significant decrease
when he/she was listening to the stream. Brain activity is
frequently enhanced when stress is perceived; however, in
this case, the sound of a turbine was so unpleasant that the
flight reaction may have been induced. In the ‘‘Null’’ ses-
sion, systolic blood pressure and cerebral activity did not
show significant changes. The study was conceived with a
view to masking the alarming sound of a turbine with the
gentle sound of a stream; however, further studies on
individual differences in physiological responses in terms
of age, dental experiences and other factors would be
required in order to be able to implement this idea into
practice.
Field studies on ‘‘Shinrin-yoku’’
Prior to the ‘‘Therapeutic Effects of Forests’’ project,
methods for performing physiological measurements in the
field had not been established, which is one of the reasons
for the predominance of laboratory-based studies. How-
ever, as already mentioned, the rapid progress of technol-
ogy has allowed researchers to apply certain physiological
measurements to field studies. In this section, we first give
an overview of the early field studies reported prior to the
launching of the ‘‘Therapeutic Effects of Forests’’ project.
A further problem associated with field studies is that of
reproducibility. Experimental conditions such as weather,
temperature, among others are not controllable under field
conditions. Consequently, large sample sizes are required
to improve the signal-to-noise ratio. Given the amount of
research funds and the number of staff required, the
objective was to conduct a study on ‘‘Shinrin-yoku’’ on a
national scale. Various elements, such as advances in the
development of measurement techniques, increasing con-
cern about stress and healing, and a new policy framework
from the Forestry Agency to harness the multiple functions
of forests, were well integrated to provide a firm foundation
for the launch of the ‘‘Therapeutic effects of forests’’
project on a national scale.
The authors started to accumulate data from field exper-
iments when a large project called the ‘‘Physiological Effects
of Forest Environmental Components on Humans’’ was
launched in 2004, supported by the Ministry of Agriculture,
Forestry and Fisheries of Japan. The field experiments were
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continued in the ‘‘Therapeutic Effects of Forests’’ project
that was initiated in 2005. A selection of the results that we
obtained during the course of conducting the ‘‘Therapeutic
Effects of Forests’’ project as well as other emerging studies
will be introduced later in this section. Physical and chemical
measurements conducted in addition to our physiological
investigation will also be described.
Past field studies on ‘‘Shinrin-yoku’’
Although limited, some field studies have been performed
on ‘‘Shinrin-yoku’’ in the past.
Miyazaki et al. [38] conducted a study on Yakushima
Island involving five subjects. This probably represents the
first investigation of the physiological effects of ‘‘Shinrin-
yoku’’. In this study, the subjects entered the forest of
Yakusugi, famed for its ancient cedar trees, and walked for
40 min in both the morning and the afternoon. They also
walked for 40 min in a laboratory where temperature and
humidity were set at the same levels as within the forest.
The forest was perceived as ‘‘comfortable,’’ ‘‘natural,’’ and
‘‘relaxing.’’ In the profile of mood states (POMS) test, the
scores in the negative subscales, such as tension, depres-
sion, anger, fatigue, and confusion, were lowered; further,
the score for vigor was increased. Salivary cortisol con-
centration, which is known to increase in response to stress,
was lower in the forest than in the laboratory. Thus, the
stress-reducing effect of ‘‘Shinrin-yoku’’ was revealed for
the first time.
Ohtsuka et al. [39] examined the effect of ‘‘Shinrin-
yoku’’ on the blood glucose levels of 87 noninsulin-
dependent diabetes mellitus patients (29 males and 58
females; average age 61 years). The patients were exposed
to ‘‘Shinrin-yoku’’ (a 3- or 6-km walk depending on their
physical ability and/or the existence of diabetic complica-
tions) nine times in 6 years. The number of occasions on
which each subject participated in these sessions varied
from one to nine, amounting to a total of 237 participant-
events. Blood glucose levels measured after ‘‘Shinrin-
yoku’’ were significantly lower than those measured in the
morning prior to ‘‘Shinrin-yoku’’ in all nine sessions; on
average, it significantly decreased from 179 to 108 mg/dL.
The effect was found to be independent of the distance
walked. The authors concluded that ‘‘Shinrin-yoku’’ was
useful in the treatment of diabetes mellitus.
Ohira et al. [40] assessed the physiological effects of
‘‘Shinrin-yoku’’ on 20 student subjects. After exposure to a
forest for 8 h, natural killer (NK) cell activity and immu-
noglobulin A, G, and M levels were significantly increased;
however, no changes were observed in other physiological
parameters of the endocrine system, electroencephalogra-
phy (EEG) and electrocardiography (ECG), or in psycho-
logical parameters. The authors speculated that the
ambiguous results were due to the low temperatures in the
forest attributable to bad weather.
Psychological effects of ‘‘Shinrin-yoku’’
Morita et al. [41] investigated the psychological effects of
forest environments with 498 respondents who visited a
forest during 4 days of the survey. The survey revealed that
the mood of the respondents (hostility, depression, liveli-
ness, and three other positive and negative mood subscales)
were significantly improved on the day of the forest visit
compared to the control day, when the respondents did not
visit a forested area. The authors also analyzed factors
possibly related to the magnitude of ‘‘Shinrin-yoku’’ and
found that the duration of the stay or other conditions
associated with the forest visit were not linked to the
degree of the effect; in contrast, based on self-rated psy-
chological stress levels were linked to the degree of the
effect, with those individuals who were chronically stres-
sed experiencing greater beneficial effects.
‘‘Shinrin-yoku’’ in Seiwa Prefectural Forest Park,
Chiba Prefecture [7]
This study was conducted in a forest located within a
reasonable distance of a populated area, namely the Seiwa
Prefectural Forest Park, and also in the area around Chiba
Station, which was used as a control. The predominant tree
species in this forest is oak.
Subjects were asked to walk for 20 min in each area, and
the absolute hemoglobin concentration, determined using
the TRS technique, was found to be significantly lower in
subjects walking in the forest area than in those walking in
the urban area. It also tended to be lower in subjects in the
forest area prior to viewing scenery, which may be attributed
to the fact that the subjects had remained in the respective
area since approximately 10:00 a.m. although measure-
ments were performed between 2:10 p.m. and 3:00 p.m.
These results indicate that the activity in the prefrontal area
was reduced in subjects exposed to the forest area (Fig. 1).
The salivary cortisol concentration, which is an indicator of
stress levels, was significantly lower in subjects in the forest
area than in subjects in the city area, both prior to and fol-
lowing viewing scenery in each area. Thus, the results of this
study demonstrated that prefrontal activity was reduced and
the concentration of the stress hormone was lowered in
subjects exposed to a forest environment.
‘‘Shinrin-yoku’’ in an old-growth broadleaf
forest in Yamagata Prefecture [42]
Oguni Town in Yamagata Prefecture is situated on the
border between Yamagata and Niigata, with forests
123
34
100
80
Forest area
City area
p=0.051
60
p<0.05
p<0.05
Environ Health Prev Med (2010) 15:27–37
1.2
Forest area
*
City area
**
0.8
* p<0.059
0.4
40
Morning
Post-Walk
Post-Watch
Pre-Walk
Pre-Watch
Fig. 1 Change in the average value of the absolute concentration of
total hemoglobin (tHb) in the left prefrontal area in subjects walking
for 20 min in a forest and city area [7]. n = 5–12, mean ± SD,
p value by paired or unpaired t test (one-side test)
covering 95% of the local area. This study was conducted
in a deciduous broadleaf forest that mainly consisted of
old-growth beech, with an area around Niigata Station used
for comparison.
Systolic blood pressure was significantly lowered in
subjects in the forest area prior to walking, and prior to and
following viewing scenery compared with subjects in the
urban area. Diastolic blood pressure was also lowered in
subjects in the forest area prior to walking and following
viewing scenery. The pulse rate was significantly lowered
prior to walking and tended to be lowered following
walking. There was a distinct difference in the cortisol
concentrations of subjects between the two areas in that it
was lower in the forest area than in the city area at all
measurement times (Fig. 2). There was also a significant
difference in cortisol levels between the two sites prior to
and following walking, and following viewing of the
scenery. The p value was less than 0.06 prior to viewing.
The results of the HRV analysis revealed that parasympa-
thetic nervous activity was significantly higher and sym-
pathetic nervous activity was significantly lower in subjects
exposed to the forest environment. Thus, this study clearly
demonstrated the relaxing and stress-relieving effects of
‘‘Shinrin-yoku.’’
The effects of ‘‘Shinrin-yoku’’ on immunological
function
Li et al. are actively researching on the effects of forest
environments on immunological function. They first eluci-
dated, by conducting an in vitro experiment, that essential
oil(s) or odorous components of wood, such as cypress
(hinoki) stem oil, a-pinene, and 1,8-cineole, significantly
0
Morning
Post-Walk
Post-Watch
Pre-Walk
Pre-Watch
Evening
Fig. 2 Changes in salivary cortisol concentration at six measurement
times in subjects a forest area and in a city area [42]. n = 9 at ‘‘before
walking’’ and ‘‘after walking’’, n = 11 at the other times. *p \\ 0.05
by the paired t test
increased the NK activity, at least partially, via an increase in
the intracellular levels of three cytolytic molecules, namely,
perforin, granzyme A, and granulysin [43]. They subsequently
conducted a field study with healthy male office-workers [44]
and found that the NK activity of these office-workers
significantly increased after they had experienced a 3-day/
2-night stay in the forests. This enhancement in NK
activity was at least partially mediated by an increase in the
expression of the three cytolytic molecules mentioned
above. These researchers also found that the effect of the
forest environments on the NK activity lasted for more than
7 days following the trip, whereas a corresponding trip to
an urban area did not cause any changes in NK activity
[45]. Based on their most recent study, they report that
similar results of increased and lasting NK activity after
‘‘Shinrin-yoku’’ were confirmed in female subjects [46].
Physical and chemical measurements
In the ‘‘Therapeutic Effects of Forests’’ project, measure-
ments are also being carried out on the thermal and light
environments, and volatile components (phytoncides) in
the air are being evaluated. Takayama et al. [47], for
example, calculated the predicted mean vote (PMV) value
in forest and urban areas in the Chiba Prefecture study and
reported that throughout the day, the forest environment
provided more comfortable thermal conditions than the
urban environment. The subjects evaluated the forested
area to be as bright as the urban area, even though the
illuminance in the forest was less than 5% of that in the
city. The authors concluded that perceived brightness is not
necessarily correlated with actual illuminance.
Ohira et al. [48] reported that among the volatile matters
detected in coniferous forests, a-pinene had the highest
123
Environ Health Prev Med (2010) 15:27–37
35
concentration. The detectable amounts of a-pinene were
high in cypress forests. Matsui et al. [49] described that
large amounts of isoprene were detected in the air of
broadleaf forests. For example, there were extremely large
amounts of isoprene in the beech forest in the town of
Oguni. The second most abundant substance was a-pinene,
although levels of this volatile were found to be less than
10% of the those of isoprene. These researchers as well as
others have pointed out that there is a diurnal variation in
the concentrations of these volatile matters and that
weather may influence volatilization. It is expected that
considerable advances in our understanding of phytoncides
will be gained by adopting a large-scale approach as
planned in ‘‘Therapeutic Effects of Forests’’ project.
The reliability of this index could be further enhanced by
continuing fundamental analysis of diurnal variation and/or
individual difference in baseline concentrations.
Although the aim of our review was to focus on research
that has been carried out in Japan or is currently being
carried out, an increasing number of studies are now
emerging in various countries [52–58]. Thus far, these have
been mostly psychological investigations; however, some
of them refer to the importance of physiological investi-
gations. The effects of forest environments on human
health are expected to be verified by international efforts.
Acknowledgments This review was partly supported by Grant-in-
Aid for Scientific Research (S:16107007) from The Ministry of
Education, Culture, Sports, Science and Technology (MEXT).
Future perspectives
In order to firmly anchor ‘‘Shinrin-yoku’’ in our society and
to fully exploit the therapeutic effects of forests by
enjoying the benefits of ‘‘Shinrin-yoku’’ on a daily basis,
the accumulation of data based on scientific evidence
should be continued. A desirable step would be to increase
the variety of types of forest settings appropriate for
‘‘Shinrin-yoku’’. This should be linked to a requirement for
‘‘Shinrin-yoku’’ programs that are customized for individ-
uals by experts and for simple physiological indices that
enable on-the-spot evaluations of the effects of ‘‘Shinrin-
yoku.’’ Future studies will focus on individual differences
and the development of novel systems used to evaluate the
effects of ‘‘Shinrin-yoku.’’
Research on the effects of individual differences, such
as age, physical strength, personal preference, physiologi-
cal character, and personality, are required to enable
experts to devise individually tailored ‘‘Shinrin-yoku’’
programs. While individual differences are a matter of
continuing concern in human studies, data have typically
been treated as ‘‘an average value and outlier values’’
owing to methodological difficulties. Research that scien-
tifically treats individual differences based on the view that
‘‘within any group of people certain physiological sub-
groups will exist,’’ will be warranted in the future.
In addition to blood pressure and pulse rate, biomarkers
in saliva are considered to be potential candidates as simple
indices by which to assess the effects of ‘‘Shinrin-yoku.’’
As a consequence of recent improvements in measurement
accuracy, other indices, such as cortisol and immuno-
globulin A, can be currently used at saliva biomarkers.
Further, indices such as salivary amylase activity [12] and
chromogranin A [50, 51] are considered to be potential new
stress markers. We are currently accumulating data with a
simple instrument that estimates the amylase activity in
saliva for 1 or 2 min using a recently developed technique.
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