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The Effects of Forest Therapy on Immune Function

International Journal of

Environmental Research

and Public Health

Systematic Review

The Effects of Forest Therapy on Immune Function

Youngran Chae 1, Sunhee Lee 2,*, Youngmi Jo 3, Soyean Kang 4, Suyoun Park 5 and Hyoyoung Kang 6

1 College of Nursing, Kangwon National University, Chuncheon 24341, Korea; yrchae@kangwon.ac.kr

2 Department of Nursing, Yeoju Institute of Technology, Yeoju-si 12653, Korea

3 Department of Nursing, Kangwon National University Hospital, Chuncheon 24289, Korea;

youngs905@hanmail.net

4 Department of Nursing, Daewon University College, Jecheon 27135, Korea; seizy@daewon.ac.kr

5 Department of Nursing, Kangwon National University, Chuncheon 24341, Korea; suyoun2419@gmail.com

6 Department of Nursing, Songho College, Heongseong 25242, Korea; sissy2@songho.ac.kr

* Correspondence: baezzang1224@yit.ac.kr

Citation: Chae, Y.; Lee, S.; Jo, Y.;

Kang, S.; Park, S.; Kang, H. The

Effects of Forest Therapy on Immune

Function. Int. J. Environ. Res. Public

Health 2021, 18, 8440. https://

doi.org/10.3390/ijerph18168440

Abstract: We conducted a systematic review of the effects of a forest therapy program on adults’

immune function. We used PICO-SD (participants, interventions, comparisons, outcomes, study

design) to identify key items. The participants were adults over the age of 18 and the intervention

was forest therapy. Our comparisons included studies that comparatively analyzed urban groups

or groups that did not participate in forest therapy intervention. Cases without control groups

were also included. Immunological outcome measures were included in measuring intervention

outcomes. All experimental studies, such as randomized controlled trials (RCTs), non-equivalent

control group designs (non-RCTs), and one-group pretest-posttest design were included in the study

design. A total of 13 studies were included for comparison. Forest therapy programs were divided

into lodging-type and session-type programs. The representative measures for evaluating the effects

of immune function were the number of NK cells, the cytotoxic activity of NK cells, and cytotoxic

effector molecules. Most studies reported improvement in these measures when comparing values

after intervention with values before the forest therapy intervention. Therefore, forest therapy has

been found to be effective in improving immune function. More RCT studies on the effects of forest

therapy on immune function are necessary.

Keywords: forest therapy; adults; immune function

Academic Editor: Paul B. Tchounwou

Received: 9 July 2021

Accepted: 6 August 2021

Published: 10 August 2021

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1. Introduction

Forest therapy creates a state of physical relaxation by exposing a participant to a

natural environment. It is thought to activate compromised immune function and improve

immune function for maintaining and promoting health [1]. Forest therapy makes use

of various elements of the forest environment to help individuals cope with stress and to

maintain and promote their health [1]. As awareness of forest therapy has increased, so

has the number of individuals participating in this therapy [2]. In addition, as stress levels

have escalated and public frustration caused by social distancing mandates during the

COVID-19 pandemic has increased, public interest in forest therapy has also increased.

An increasing number of visitors to forest areas have indicated that COVID-19 was the

motivation behind their forest visit [3].

In line with this trend, there has been a steady increase in studies investigating and

verifying the effects of forest therapy [4–10]. Numerous studies have reported that forest

therapy can have positive effects on physical and psychological health [4–6]. Forest therapy

has also been reported to improve depression [7]. Furthermore, forest therapy reportedly

reduces sympathetic nervous activity, increases parasympathetic nervous activity, and

regulates the balance of autonomic nerves, all of which lead to increased relaxation [8].

Along with its ability to reduce stress, forest therapy has been shown to improve

immune function [9,10]. Stress conditions affect immune function [11,12]. In particular,

Int. J. Environ. Res. Public Health 2021, 18, 8440. https://doi.org/10.3390/ijerph18168440

https://www.mdpi.com/journal/ijerph

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chronic stress suppresses immune responses and promotes pathological immune responses,

including inﬂammatory responses [13–15]. Thus, if forest therapy can reduce stress, it will

simultaneously enhance immune function. Moreover, some studies [16,17] have reported

that environmental factors have a greater impact on immune function than genetic factors.

This indicates that environmental characteristics, such as those provided by forest therapy,

can have a positive effect on immune function.

However, although studies of forest therapy have used various outcome measures

related to immune function [18–22], such as NK cells, T cells, B cells, perforin, granulysin,

granzymes, and interleukin-6, their results show inconsistently. Therefore, it is necessary to

identify the effective measures among the various outcome measures of immune function

and to analyze which outcomes of the immune function measurements show changes due

to the intervention of forest therapy.

In addition, several researchers have presented a systematic review of studies on the

health effects of forest therapy [4,5,14,23], but few have presented a systematic review on

the effect of forest therapy on immune function speciﬁcally. Thus we present a system-

atic review of studies on the effects of forest therapy on the immune function of adults,

investigating the characteristics of forest therapy programs and analyzing their effects on

immune function measures.

2. Materials and Methods

2.1. Inclusion and Exclusion Criteria for Selection of Existing Studies

This study was conducted according to the guidelines of the Preferred Reporting

Items for Systematic Review and Meta-Analyses (PRISMA) [24]. The publication year of

the article was not limited. Key items of the criteria for selecting existing studies for this

study, were composed according to PICO-SD (participants, intervention, comparisons,

outcomes, and study design): (1) The participants were adults aged over 18 years; (2) The

intervention included forest therapy; (3) That the studies compared forest therapy groups,

with groups that did not participate in forest therapy intervention or urban groups, and

cases without control groups were also included; (4) Outcomes of the intervention included

immunological outcome measures, and (5) In terms of study design, all experimental

studies, such as randomized controlled trials (RCTs), non-equivalent control group designs

(Non-RCTs), and one-group pretest-posttest design were included. Studies without ex-

perimental designs, such as survey research and qualitative research, were excluded from

the analysis.

2.2. Searching for and Selecting Existing Studies

We included studies that could be retrieved in each database in our analysis following

a search conducted in the two weeks between 1 July and 14 July 2020. For domestic

databases, RISS, KISS, DBPia, and NDSL were used, and for international databases,

PubMed, Cochrane library, PsychoInFO, EMBASE, EBSCO, Web of Science, CINAHL,

and Scopus were used. The references of the searched articles were used to perform a

manual search in addition to the electronic search for collection. The search keywords

were (“shinrin-yoku” OR “forest bathing” OR “nature therapy” OR “forest therapy”)

AND (“Immunity” OR “Natural killer cell” OR “NK cell” OR “immune”). For domestic

keywords, Korean was used with the same meaning as the English keywords. Only studies

published in English or Korean were included.

Each database was searched following a discussion between the two researchers, and

one of the researchers deleted duplicate articles using a document management program.

The title and abstract were then reviewed according to the inclusion and exclusion criteria,

to screen the articles. Once an article passed the initial screening stage, its full text was

checked, and only those that met each of the selection criteria were selected. In the case of

any disagreement during this process, the two researchers had a discussion and reached a

mutual consensus regarding the ﬁnal selection of each article.

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2.3. Risk of Bias Assessment of Individual Studies

To assess the selected articles’ risk of bias, we used the Revised Cochrane Risk-of-Bias

tool (RoB 2) [25] for RCTs, while the Risk-of-Bias Assessment tool for Non-randomized

Study (RoBANS) [26] tool was used for Non-RCTs.

The risk-of-bias assessment tool for RCTs is composed of the following ﬁve domains:

randomization process, deviations from intended interventions, missing outcome data,

measurement of the outcome, and selection of the reported result. The risk of bias assess-

ment for these domains was performed using three categories: “low risk”, “some concerns”,

and “high risk”.

The RoBANS is composed of the following six items: selection of participants, con-

founding variables, measurement of intervention (exposure), blinding for outcome assess-

ment, incomplete outcome data, and selective outcome reporting. The risk was assessed

using “low risk”, “high risk”, and “uncertain risk” categories. “uncertain risk” means that

the study is judged to raise some concerns in at least one domain, but not to be at high

risk of bias for any domain. In this study, two researchers independently performed a

quality evaluation. When there was a disagreement between the two researchers, included

a third-party researcher in their discussion to help bring them to an agreeable conclusion.

2.4. Data Extraction

Items for data extraction included study information (author, publication year), partici-

pants (total number of participants, age, and diagnosis), study design and intervention program

characteristics, measurement tools, main outcome variables, and ethical considerations.

3. Results

3.1. Study Selection

We retrieved 870 articles from domestic databases and 1972 articles from international

databases for our analysis. Once duplicate articles were removed, 1782 articles remained.

The titles and abstracts of each of these articles were reviewed, and 1718 articles did not

comply with the inclusion and exclusion criteria. Finally, 64 articles remained after the

screening process. The full text of each of these articles was reviewed, and the following

50 articles were excluded in total: 40 articles that contained no report on immunological

outcomes, one article with non-experimental study, four articles that were not published in

English or Korean, four articles whose original version could not be retrieved, one article

with inadequate comparison, and one article on indirect forest therapy (aromatherapy).

Thus, only 13 peer-reviewed articles were selected for further analysis (Figure 1).

Figure 1. PRISMA ﬂow chart.

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3.2. Characteristics of the Existing Studies

In terms of the selected existing studies’ publication year, there were no reports

published before 2006, while there were three articles published between 2007 and

2010 [18–20], four articles between 2011 and 2015 [21,22,27,28], and six articles from 2016 to

present [29–34]. With regard to participants, six of the studies used healthy adults as

participants [18–21,27,34], while ﬁve had participants who were adults with health prob-

lems [22,28–31], and two did not report as to their participants’ health status [32,33]. In

terms of countries/regions where the study was conducted, four studies were conducted in

Korea [27,28,30,32], China [21,29,31,34], and Japan [18–20,22], respectively, and one study

was conducted in Taiwan [33]. As for the study design, three studies were RCTs [21,29,31],

ﬁve studies used non-equivalent control group pre-posttest design [20,27,30,32,34], and

ﬁve studies used a one-group pretest-posttest design [18,19,22,28,33]. In terms of sample

size, there were seven studies with a sample size of 20 or less [18,19,21,28,29,32,33], four

studies with a sample size between 21 and 50 [20,22,27,31], and two studies with a sample

size of 51 or larger [30,34]. With regard to ethical considerations, apart from one study with

no description of Institutional Review Board (IRB) approval status [32], the other 12 studies

were all approved by the IRB (Table 1).

Table 1. General characteristics of included studies (n = 13).

Characteristic

Publication year

Participants

Country/Regions

Study Design

Sample size

Statement of ethical consideration

Categories

≤2006

2007–2010

2011–2015

≥2016

Healthy adults

Adults with health problems

Not reported

China

Korea

Japan

Taiwan

Randomized control group

Nonequivalent control group

pre-posttest design

One group pre-post test

design

≤20

21–50

≥51

Yes

N (%)

0 (0)

3 (23.1)

4 (30.8)

6 (46.2)

5 (38.5)

2 (15.4)

4 (30.8)

1 (7.7)

3 (23.1)

5 (38.5)

7 (53.8)

4 (30.8)

2 (15.4)

12 (92.3)

1 (7.7)

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3.3. Characteristics of the Forest Therapy Program

We found that “forest bathing” was the most commonly used term in ﬁve articles,

followed by “forest therapy”, used in four articles, while “visiting forest”, “green space”,

“forest environment”, and “forest walking” were each used in one article, respectively.

The forest therapy programs were classiﬁed into lodging-type and session-type pro-

grams. For lodging-type programs, seven studies lasted for 2–3 days [18–21,27,30,34], three

studies that lasted 4–5 days [29,31,33], and one study [28] that lasted for 14 days. For

session-type programs, two studies were conducted for 12 weeks [22,32], one of which held

a session weekly [22] while the other held sessions three times weekly [32]. There were eight

studies in which the intervention only included walking in the forest [18–21,29,31–33], while

four studies used meditation, horticultural therapy, yoga meditation, support group ther-

apy, music, cognitive-behavioral therapy, and relaxation therapy as interventions, alongside

walking in the forest [22,27,28,30]. One study [34] provided no speciﬁc description of the

intervention but simply mentioned forest exposure.

There were eight studies in which the participants were healthy adults and ﬁve

studies in which the participants were adults with health problems. The healthy adult

participants included college students [19,21,34], middle-aged women or men [18,33,34],

nurses [19], and workers in the healthcare and counseling service industries [27]. As

for the participants who were adults with health problems, two studies included cancer

patients [22,28], one included patients with chronic obstructive pulmonary disease [29],

one included chronic pain patients [30], and one study’s participants were patients with

congestive heart failure [31] (Table 2).

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Authors

(Years)

Li et al.

(2007) [18]

Li et al.

(2008) [19]

Li et al.

(2008) [20]

Table 2. Summary of studies included in the systematic review (k = 13).

Study Design

Participants (n)

One group pre-posttest

design

Healthy male, aged

37~55 years (12)

Mean age: 43.1 ± 6.1

One group pre-posttest

design

Healthy female nurses,

aged 25~43 years (13)

Mean age: 28.8 ± 4.6

None-equivalent control

group pre-posttest

design

Healthy male, aged

35~56 years (E:12, C:11)

Mean age: 45.1 ± 6.7

Intervention

Control

Outcome (Measurements)

A three-day/two-night trip

First day: walked for two hours in the

afternoon in a forest ﬁeld

Second day: walked for two hours

each in the morning and afternoon in

two different forest ﬁeld

On day 3: ﬁnished the trip

A three-day/two-night trip

First day: walked for two hours in the

afternoon in a forest ﬁeld

Second day: walked for two hours

each in the morning and afternoon in

two different forest ﬁeld

On day 3: ﬁnished the trip

A three-day/two-night forest bathing

program

First day: walked for two hours in the

afternoon in a forest ﬁeld

Second day: walked for two hours

each in the morning and afternoon in

two different forest ﬁeld

On day 3: ﬁnished the trip

None

A three-day/two-night

city trip

First day: walked for two

hours in the afternoon in

an old-style district in the

city

Second day: walked for

two hours around baseball

Dome in the morning and

2 h around/in airport in

the afternoon

On day 3: ﬁnished the trip

· NK cells **↑, NK activity **↑,

· T cells

· granulysin **↑, perforin **↑,

granzymes A **↑/B **↑

· WBC, (Lymphocytes *↑,

Monocytes*↑,

Granulocytes **↓)

· T-A *↓, D *↓, A-H, F, C, V **↑(POMS)

· the hours of sleep

· NK cells **↑, NK activity **↑

· T cells *↓

· granulysin **↑, perforin **↑,

granzymes A **↑/B **↑

· Adrenalin **↓, noradrenaline (Urine)

**↓

· Estradiol, progesterone

· T-A, D **↓, A-H, F, C, V (POMS)

· NK cells *↑, NK activity *↑

· T cells

· granulysin **↑, perforin **↑,

and granzymes A **↑/B **↑

· WBC

· Adrenaline (Urine) *↓

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Authors

(Years)

Mao et al.

(2012) [21]

Nakau et al.

(2013) [22]

Jung et al.

(2015) [27]

Kim et al.

(2015) [28]

Table 2. Cont.

Study Design

Participants (n)

Intervention

Control

Outcome (Measurements)

RCT

One group pre-posttest

design

None-equivalent control

group pre-posttest

design

One group pre-posttest

design

Healthy male university

students

(E:10, C:10)

Mean age: 20.79 ± 0.54

Breast cancer or lung

cancer (22)

Mean age: 58.1 ± 10.8

Workers in the

healthcare and

counseling service

industries (E:19, C:20)

Mean age

E: 29.42 ± 8.92,

C:36.45 ± 12.23

Stage 3 Breast cancer

patient, aged 25~60

years (11)

Mean age: 56 ± 5.12 (11)

two-night trip

walked for 1.5 h each in the morning

and afternoon in a forest ﬁeld

Walking in the forest, Horticultural

therapy, yoga meditation, and

support group therapy, and sessions

were conducted once a week for 12

weeks

3 days, 2 nights, Walking and

meditation in the forest, and exposure

to a psychological program using

music and cognitive-behavioral

therapy.

Stay in the forest for 14 days, Forest

therapy program (Walk, forest life)

walked for 1.5 h each in

the morning and afternoon

in a city site

None

frequently use the

environment without

participating in the

program

None

· NK cells

· T cell, B cell *↑, T-helper cells.

suppressor cells, natural killer cells

· IL-6*↓, TNF-α *↓

· T-SOD, MDA **↓

· ET-1 **↓, Platelet activation

· Cortisol *↓, Testosterone

· T-A *↓, D *↓, A-H *↓, F *↓, C, V

*↑(POMS)

· NK activity **↑

· Well-being of functional and

spiritual (FACIT-Sp): GP, GS, GE, GF

*↑, Sp *↑

· Quality of life (SF-36): PF *↑, RP *↑,

BP,

GH *↑, VT **↑, SF, RE *↑, MH *↑

· Fatigue (CFS) **↓

· T-A *↓, D, A-H, F, C **↓, V (POMS)

· STAI: State anxiety **↓, Trait anxiety

*↓

· NK activity

· Cortisol *↑

· HRV

· MBI-GS **↓, WRSI *↓, REQ **↑

· NK cells **↑

· Perforin *↑, Granzyme B *↑

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Authors

(Years)

Jia et al.

(2016) [29]

Han et al.

(2016) [30]

Mao et al.

(2017) [31]

Lee et al.

(2017) [32]

Table 2. Cont.

Study Design

Participants (n)

Intervention

Control

Outcome (Measurements)

RCT

None-equivalent control

group pre-posttest

design

RCT

None-equivalent control

group pre-posttest

design

elderly patients

with chronic obstructive

pulmonary disease (E:

10, C: 8)

Mean age

E: 70.1 C: 70

Full-time employees

with chronic widespread

pain for more than three

months, aged 25~49

years

(E: 33, C: 28)

Mean age

E: 41.6 ± 6.5, C:

37.5 ± 8.4

Elderly patients with

chronic heart failure,

Aged from 65 to 80 years

(E: 23, C: 10)

Mean age

73.86 ± 5.85 years old

Women in their 50 s (E: 9,

C: 9)

Mean age

E: 53.9 ± 2.69, C:

55.5 ± 1.84

4 days, walking in the forest (total 3 h

walk/day)

2 days forest therapy program

Walking and therapeutic activities in

the forest activities (music therapy,

psycho-education: coping with pain

and stress, bodily exercises,

mindfulness-based meditation)

4 days, Walking in the forest (total 3 h

walk/day)

Forest walking exercise for 12 weeks

(3 times/week, 110 min/day)

4 days, walking in the city

(total 3 h walk/day)

The control group was

instructed not to conduct

either heavy loads of

domestic or occupational

work during the

enrollment in this study.

4 days, Walking in the city

(total 3 h walk/day)

Ground walking exercise

for 12 weeks (3

times/week, 110 min/day)

· NK cells, NKT-like cell, CD8+ T cells

· Perforin **↓, Granzyme B

· IL-6 *↓, IL-8 **↓, interleukin-1β *↓,

TNF-α

interferon-γ *↓, CRP *↓

· Cortisol *↓, Epinephrine *↓

· RARC/CCL-18 **↓, TIMP-1 *↓, SP-D

*↓

· T-A **↓, D *↓, A-H *↓, F, C, V

(POMS)

· NK activity **↑

· Heart rate variability (HRV) **↑

· Self-reported pain **↓ (VAS)

· Depression **↓ (BDI)

· Health-related quality of life **↑

(EQ-VAS)

· IL-6 *↓, TNF-α, CRP

· BNP **↓, N-terminal pro BNP

· ET-1 **↓, Renin, AGT, Ang II, AT1,

AT2 **↑

· T-SOD *↑, MDA *↓

· T-A *↓, D **↓, A-H **↓, F, C **↓, V

(POMS)

· NK cells *↑

· Melatonin *↑

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Table 2. Cont.

Authors

(Years)

Study Design

Participants (n)

Intervention

Control

Outcome (Measurements)

Tsao et al.

(2018) [33]

One group pre-post test

design

Middle-aged subjects

(11)

Mean age: 60.4

A ﬁve-day/four-night trip (maintain

dietary control and walking exercise)

First day: walked for 1.5 h in the

afternoon in a forest ﬁeld

Next three days: walked for 1.5 h

each in the morning and afternoon in

two different forest ﬁeld

Fifth day: ﬁnished the trip

None

· NK cells, NK activity **↑

Lyu et al.

(2019) [34]

None-equivalent control

group pre-posttest

design

Male College Students

(E: 45, C: 15)

Mean age

E: 20.9 ± 0.24, C:

21.3 ± 0.45

Bamboo forest site exposure for three

days

City site exposure for three

days

· NK cells *↑. NK activity *↑,

· granulysin, perforin *↑,

granzymes A *↑/B *↑,

· corticosterone *↓

· SBP *↓, DBP, HR, SpO2

· T-A *↓, D *↓, A-H *↓, F *↓, C *↓, V *↑

(POMS)

*: p < 0.05; **: p < 0.01; ↑: indicators rise; ↓: indicators decline; E = Experimental group; C = Control group; POMS = Proﬁle of mood states; T-A = Tension-anxiety; D = Depression: A-H = Anger-hostility,; F = fatigue;

C = Confusion; V = Vigor; WBC = hite blood cell; IL-6 = Interleukin-6; TNF-α = Tumor necrosis factors α; T-SOD = Total superoxide dismutase; MDA = Activity and malondialdehyde; ET-1 = Endothelin-1;

FACIT-Sp = Functional assessment of chronic illness therapy-spiritual; GP = Physical well-being; GS = Social/family well-being; GE = Emotional well-being; GF = Functional well-being; Sp = Spiritual well-being;

CFS = Cancer fatigue scale; SF-36 = 36-Item short-form health survey; PF = Physical functioning; RP = Role-physica; BP = Bodily pain; GH = General health perception; VT = Vitality; SF = social functioning;

RE = Role emotional; MH = Mental health; STAI = State-trait anxiety inventory; HRV = Heart rate variability; MBI-GS = Maslach Burnout Inventory-General Survey; WRSI = Worker’s Stress Response Inventory;

REQ = Recovery Experience Questionnaire; CRP = C-peptide protein; RARC/CCL-18= Pulmonary and activation-regulated chemokine, TIMP-1= Tissue inhibitor of metalloproteinase-1; SP-D = Surfactant

protein D; VAS: Visual analog scale; BDI = Beck Depression Inventory; EQ-VAS= EuroQol Visual Analog Scale; BNP = Brain natriureti; AGT = Angiotensinogen; Ang II = Angiotensin II; AT1 = Angiotensin II type

1 receptor; AT2 = Angiotensin II type 2 receptor; SBP = Systolic blood pressure; DBP = Diastolic blood pressure; HR = Heart rate.

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3.4. Effects of Forest Therapy on Immune Function

With regard to immune function measures, the number of NK cells or NK cell activity

was the most frequent measure used in the studies we reviewed. The number of NK cells

was reported in nine studies [18–21,28,29,32–34]; a signiﬁcant increase in NK cell count

was reported in six studies [18–20,28,32,34], and no signiﬁcant change in the number of

NK cells was reported in three studies [21,29,33].

NK cell activity was measured in eight studies [18–20,22,27,30,33,34], and a signiﬁcant

change was reported in seven studies. The study that reported no signiﬁcant change in

the activity of NK cells [27] applied a combination of interventions, including walking,

meditation, counseling, cognitive-behavioral therapy, and music therapy for three days

and two nights in the forest, for workers in the healthcare and counseling service industries.

In a study with female nurses as participants, which used a walking intervention of two

hours in the morning and afternoon, respectively, for three days and two nights [19], a

signiﬁcant increase in both the number and activity of NK cells was reported, for up to

7 days after returning from the forest. In addition, in a study with healthy male adults as

participants which used a walking intervention of two hours each morning and afternoon,

respectively, for three days and two nights [20], a signiﬁcant increase in the number and

activity of NK cells was reported for not only up to 7 days after the intervention, but also

30 days after returning from the forest.

T cells were measured in four studies [18–21] and showed no signiﬁcant changes

in three of these studies [18,20,21]. In a study with healthy male university students

as participants, where the intervention included 90 min of walking in the morning and

afternoon, respectively, for two days [21], B cells, T-helper cells, suppressor cells, and NK

cells were measured; a signiﬁcant change was observed only in B cells. In a study in which

patients with chronic obstructive pulmonary disease participated, that included 90 min

of walking intervention in the morning and afternoon, respectively, for four days [29], no

signiﬁcant changes were seen in the Natural Killer T(NKT)-like cells or CD8+ T cells.

Six studies [18–20,28,29,34] measured granulysin, perforin, and granzymes A and B,

which are cytotoxic effector molecules. In a study with male college students as partici-

pants [34], there was no signiﬁcant change in granulysin, and in a study with chronic

obstructive pulmonary disease patients as participants [29], there was no signiﬁcant

change in granzyme B, while signiﬁcant changes in these measures were reported in four

other studies.

Three studies [21,29,31] measured the levels of proinﬂammatory cytokines. In a study

that used a walking intervention for healthy male university students [21], IL-6 and TNF-α

were measured, and signiﬁcant changes were reported in both. In a study that used 90 min

of walking intervention for patients with chronic obstructive pulmonary disease, in the

morning and afternoon, respectively, for four days in the forest [29], IL-6, IL-8, IFN-γ, IL-1β,

and TNF-α were measured; signiﬁcant changes were reported in each of these outcome

measures apart from TNF-α. In a study that used a walking intervention for chronic heart

failure patients for four days in the forest [31], there was a signiﬁcant change seen in IL-6

levels, but no signiﬁcant change was reported in TNF-α levels (Table 3).

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Table 3. Summary of the effects of forest therapy on immune function.

Outcome

Number of NK cells

NK cells cytotoxic activity

T cells

B cells

T suppressor cells

T-helper cells

Natural killer T(NKT) like cells

Cytotoxic T cells

Cytotoxic effector

molecules

granulysin

perforin

granzymes A

granzymes B

Proinﬂammatory

cytokines

interleukin-6 (IL-6)

interleukin-8 (IL-8)

interferon-γ (IFN-γ)

interleukin-1β (IL-1β)

tumor necrosis factor α

(TNF-α)

Article Number

Signiﬁcant

Not Signiﬁcant

18, 19, 20, 28, 32, 34

18, 19, 20, 22, 30, 33, 34

21, 29, 33

18, 20, 21

18, 19, 20

18, 19, 20, 28, 29, 34

18, 19, 20, 34

18, 19, 20, 28. 34

21, 29, 31

29, 31

3.5. Risk of Bias Assessment

The results of the risk of bias assessment we conducted on the 13 articles reviewed in

this study are presented in Figure 2.

Figure 2. Cont.

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Figure 2. Risk of bias in included studies.

With regard to the three RCTs, none of the three articles included a detailed description

of the randomization process and thus were assessed as having “some concerns” in the

category of Randomization Process. In terms of Deviations from Intended Interventions, no

information on dropouts was presented and the dropouts were not included in the analysis

of two of the RCTs; thus, these two studies were assessed as having “some concerns,”

while the other RCT was assessed as “low risk.” In terms of Missing Outcome Data,

Measurement of the Outcome, and Selective Outcome Reporting, all three studies were

assessed as “low risk.”

Of the 10 non-RCT studies, two were assessed as of “uncertain risk” because the

recruitment criteria of the patient group and control group were not consistent in terms of

Selection of Participants, and the rest were assessed as “low risk.” In terms of Confounding

Variables, one study was assessed as “high risk”, because there was no clear description

of the management of NK cells, and multiple items of subjective quality assessment

were related to the intervention and determined as the factors affecting the outcomes.

Further, four studies were assessed as being of “uncertain risk” because they gave no clear

description of the control of variables nor any clear information on the exclusion of the

time elapsed, considering the intervention. The remaining studies were assessed as “low

risk.” In terms of Measurement of Intervention (Exposure), one study was assessed as

being of “uncertain risk” because it did not give a sufﬁciently detailed description of the

intervention (exposure) method other than naming the place of intervention, while the

other nine studies were assessed as “low risk.”

In terms of the Blinding for Outcome Assessment, ﬁve studies were assessed as being

of “uncertain risk” because it was unclear whether the blinding status would affect outcome

measurements, while the other studies were assessed as “low risk.” In terms of Incomplete

Outcome Data, two studies were assessed as being of “uncertain risk”, because they did

not have sufﬁcient information on missing data, and the other studies were assessed as

“low risk.” In terms of Selective Outcome Reporting, all 10 studies included all expected

outcomes, and they were assessed as “low risk”.

4. Discussion

This systematic review of studies on the effects of forest therapy on immune function

aims to identify the characteristics of forest therapy programs and to analyze the effects of

forest therapy on immune function outcome measures.

Since 2006, all studies have investigated the effects of forest therapy on immune

function. An increasing number of studies have been conducted within the last ﬁve years,

indicating a recent surge of interest in the effects of forest therapy on immune function.

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However, among the studies published so far, one group of pre-post test design studies

accounted for a high proportion (38.5%), and the sample size of each of the studies we

reviewed was small. This indicates that a low level of evidence is provided by these studies.

In order to produce more systematic and scientiﬁc results, a more stringently controlled

study design will be required in future research.

Examining the details of the forest programs reveals that 61.5% of the studies only used

a forest walking intervention and no other interventions. In fact, the majority of studies

used a walking intervention, meaning that the main component of forest therapy was

walking. This ﬁnding is consistent with the results of a previous forest therapy study [35],

in which walking accounted for a major part of the forest therapy program’s composition.

In terms of the intervention period, programs were classiﬁed into lodging-type (lasting

anywhere from three days and two nights to 14 days) and session-type programs. No

program operated as a short-term one-off type, as reported by Chae et al. [6], indicating

that an intervention period of a minimum of two nights and three days is required for the

improvement of immune function and maintenance of improved outcomes.

To date, there have been few standardized forest therapy programs and insufﬁcient

individual forest therapy studies, posing difﬁculties in clearly identifying the most effective

intervention method [36]. Only 13 articles were included in this systematic review and

the studies presented were highly heterogeneous and thus unsuitable for meta-analysis.

Consequently, we were only able to conduct a systematic review of these existing studies.

If more individual studies are accumulated in the future, we will perform a meta-analysis

according to intervention type and duration, which will enable a more objective evaluation

of the effects of forest therapy on immune function.

We considered the number of NK cells, NK activity, and cytotoxic effector molecules

as measures to evaluate forest therapy’s effects on immune function. In 12 of the reviewed

studies, the number of NK cells and/or NK activity was reported. NK cells are capable

of attacking and killing virus-infected cells or tumor cells and play an important role in

the human endocrine and immune systems [37]. It is thought that NK cells were often

measured in these studies because, in the forest environment, the activity of NK cells

is enhanced by an increase in the number of NK cells and cytotoxic effector molecules,

leading to enhanced immune function [18,38]. In general, NK cells are highly important

lymphocytes [39] that serve as a ﬁrst-line defense against virus-infected cells. They rapidly

proliferate in the stress of transient acute exercise but are vulnerable to chronic stress.

Among the lymphocyte subtypes (T cells, B cells, and NK cells), NK cells are known to be

most responsive to exercise intensity [40].

In this study, six of the nine studies that reported the number of NK cells reported a

signiﬁcant increase in the number following forest therapy intervention, and out of the eight

studies that reported the activity of NK cells, seven reported a signiﬁcant increase in NK

cell activity following forest therapy intervention. In particular, two studies with healthy

adults [19,20] revealed that the number and activity of NK cells continued to increase

signiﬁcantly, for up to seven days or longer after returning to the urban environment, while

a study [20] with healthy male adults showed a signiﬁcant increase in NK cell activity up

to 30 days after the intervention. In other words, the results indicate that forest therapy can

have a long-term effect on the number and activity of NK cells in the human body. The

study [28] on an urban woman with breast cancer who had received anti-cancer treatment

suggested the potential of forest therapy as adjuvant anti-cancer therapy after standard

treatments. However, since both healthy adults and adults with health problems were

included in the studies that showed a signiﬁcant change and the studies that did not

show a signiﬁcant change, and because the type of forest therapy was different in different

studies, our investigation of the difference in the effects according to the characteristics of

participants and the types of forest therapy programs was limited.

Of the seven studies that analyzed cytotoxic effector molecules such as perforin,

the majority reported signiﬁcant changes in the outcomes. This could be because NK

cells secrete perforin and granzymes [41,42], and granulysin [43,44] through the granule

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exocytosis pathway, which leads to the destruction of tumor cells or virus-infected cells.

Activation of NK cells via the release of perforin and granzymes is important for inducing

natural cytotoxicity [45]. Studies in perforin-deﬁcient mice indicated that NK cell-mediated

cytotoxicity is greatly impaired in such mice [46]. The NK cells of mice with a deﬁciency in

the granzyme B cluster, induce apoptosis in target cells more slowly than wild-type NK

cells [47].

A number of studies [1,7,8,38,48,49] reported that volatile substances (phytoncides)

extracted from trees have a positive effect on immune function, supporting the idea that a

forest’s environmental factors play an instrumental role in improving immune function.

An in vitro study indicated that certain volatile tree chemicals, called phytoncides, increase

the activation of NK cells and intracellular anti-cancer molecules [50]. This assertion is

supported by several studies in animals [51,52] and humans [53], suggesting that fragrances

from trees can reverse stress-induced immunosuppression, and normalize immune function

and neuroendocrine hormone levels.

Stress increases sympathetic nervous system activity and hypothalamus-pituitary-

adrenal system activity to increase cortisol secretion. However, phytoncides have positive

effects on stress reduction, cortisol level reduction, blood pressure reduction, immune sys-

tem enhancement, autonomic nervous system, and chronic fatigue, without side effects [54].

It has also been reported that inhalation of phytoncide through breathing during forest

bathing, or smell, can enhance the cytotoxic activity of NK cells [55]. In addition, NK cells

are increased by the decreased production of stress hormones and the increased production

of anticancer proteins caused by phytoncides [49].

Several studies have analyzed cytokines such as interferon-γ, interleukin-1β, and

tumor necrosis factor α or lymphocyte subtypes, but it was difﬁcult to determine outcomes

from their results due to their small number. However, in the case of T cells, previous re-

search suggests that careful consideration is required while selecting T cells for veriﬁcation

of forest therapy effects in the future, as three out of four studies included in this review

reported that there were no signiﬁcant changes in T cells.

One limitation of this study is that it analyzed only articles written in English and

Korean, and included non-RCT studies as well as RCT studies that have a high level

of evidence. In addition, since only a small number of studies were included, another

limitation was found in presenting the effect size according to the characteristics of the

participants or the program used. However, this systematic review is signiﬁcant in that it

suggests that there is evidence to support the theory that forest therapy can have a positive

effect on immune function.

5. Conclusions

The results of this review recommend the use of the number or activity of NK cells

for evaluating the effects of forest therapy on immune function, and cytotoxic effector

molecules are also thought to serve as effective outcome measures. Forest therapy pro-

grams, including walking in the forest, may contribute to the improvement of immune

function, and forest therapy is expected to be utilized for the enhancement of immune

function in the future.

More RCT studies on the effects of forest therapy on immune function are necessary,

to strengthen the body of evidence to support the use of forest therapy for improving

immune function.

Author Contributions: Conceptualization, Y.C. and S.L.; methodology, Y.C. and S.L.; software, S.L.,

S.P. and H.K.; validation, Y.C., S.L., Y.J., S.K., S.P. and H.K.; formal analysis, S.L.; investigation, Y.C.,

S.L., Y.J., S.K., S.P. and H.K.; data curation, S.L. and S.K.; writing—original draft preparation, S.L.;

writing—review and editing, Y.C.; visualization, S.L.; supervision, Y.C.; project administration, Y.C.;

funding acquisition, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding: This work was supported by the National Research Foundation of Korea (NRF) grant

funded by the Korean government (MSIT) (NRF-2019R1F1A1060253).

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Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: No new data were created or analyzed in this study. Data sharing is

not applicable to this article.

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Song, C.; Ikei, H.; Miyazaki, Y. Physiological effects of nature therapy: A review of the research in japan. Int. J. Environ. Res.

Public Health 2016, 13, 781. [CrossRef]

2. Park, J.S.; Yeon, P.S. A network analysis on the forest healing issues using big data—Focused on Korean web news from 2005 to

2019. J. KIFR 2020, 24, 63–71.

3. Lee, D.G.; Jeong, Y.M.; Lee, M.M.; Shin, W.S.; Yoon, Y.K. The effect of socio-psychological stress on mental well-being mediated by

perceived restorativeness-Focusing on visitors whose motive for visiting the forest is ‘COVID-19’. J. KIFR 2020, 24, 99–108.

4. Lee, I.; Choi, H.; Bang, K.-S.; Kim, S.; Song, M.; Lee, B. Effects of Forest Therapy on Depressive Symptoms among Adults: A

Systematic Review. Int. J. Environ. Res. Public Health 2017, 14, 321. [CrossRef]

5. Oh, B.; Lee, K.J.; Zaslawski, C.; Yeung, A.; Rosenthal, D.; Larkey, L.; Back, M. Health and well-being beneﬁts of spending time in

forests: Systematic review. Environ. Health Prev. Med. 2017, 22, 71. [CrossRef] [PubMed]

6. Chae, Y.R.; Lee, S.H. Systematic review of forest therapy program for adult patients with diseases. J. Korean Biol. Nurs. Sci. 2020,

22, 157–171. [CrossRef]

7. Woo, J.M.; Park, S.M.; Lim, S.K.; Kim, W. Synergic effect of forest environment and therapeutic program for the treatment of

depression. J. Korean Soc. For. Sci. 2012, 101, 677–685.

8. Li, Q.; Kawada, T. Effect of forest therapy on the human psycho-neuro-endocrino- immune network. Nihon Eiseigaku Zasshi 2011,

66, 645–650. [CrossRef] [PubMed]

9. Lee, B.K.; Lee, H.H. A Study on the effects of human physiology after forest phytoncide therapy. J. Naturopath. 2012, 1, 14–20.

10. Song, J.H.; Cha, J.G.; Lee, C.Y.; Choi, Y.S.; Yeon, P.S. Effects of forest healing program on stress response and spirituality in female

nursing college students and there experience. J. KIFR 2014, 18, 109–125.

11. Steptoe, A.; Hamer, M.; Chida, Y. The effects of acute psychological stress on circulating inﬂammatory factors in humans: A

review and meta-analysis. Brain Behav. Immun. 2007, 21, 901–912. [CrossRef] [PubMed]

12. Segerstrom, S.C.; Miller, G.E. Phychological stress and the human immune system: A meta-analytic study of 30 years of inquiry.

Psychol. Bull. 2004, 130, 601–630. [CrossRef]

13. Dhabhar, F.S. Effects of stress on immune function: The good, the bad, and the beautiful. Immunol. Res. 2014, 58, 193–210.

[CrossRef]

14. Walburn, J.; Vedhara, K.; Hankins, M.; Rixon, L.; Weinman, J. Psychological stress and wound healing in humans: A systematic

review and meta-analysis. J. Psychosom. Res. 2009, 67, 253–271. [CrossRef]

15. Webster Marketon, J.I.; Glaser, R. Stress hormones and immune function. Cell. Immunol. 2008, 252, 16–26. [CrossRef]

16. Roederer, M.; Quaye, L.; Mangino, M.; Beddall, M.H.; Mahnke, Y.; Chattopadhyay, P.; Tosi, I.; Napolitano, L.; Terranova, B.M.;

Menni, C.; et al. The genetic architecture of the human immune system: A bioresource for autoimmunity and disease pathogenesis.

Cell 2015, 161, 387–403. [CrossRef]

17. Brodin, P.; Jojic, V.; Gao, T.; Bhattacharya, S.; Angel, C.J.; Furman, D.; Shen-Orr, S.; Dekker, C.L.; Swan, G.E.; Butte, A.J.; et al.

Variation in the human immune system is largely driven by non-heritable inﬂuences. Cell 2015, 160, 37–47. [CrossRef] [PubMed]

18. Li, Q.; Morimoto, K.; Nakadai, A.; Inagaki, H.; Katsumata, M.; Shimizu, T.; Hirata, Y.; Hirata, K.; Suzuki, H.; Miyazaki, Y.; et al.

Forest bathing enhances human natural killer activity and expression of anti-cancer proteins. Int. J. Immunopathol. Pharmacol.

2007, 20, 3–8. [CrossRef]

19. Li, Q.; Morimoto, K.; Kobayashi, M.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Shimizu, T.; Li, Y.J.; Wakayama, Y.; et al. A

forest bathing trip increases human natural killer activity and expression of anti-cancer proteins in female subjects. J. Biol. Regul.

Homeost. Agents. 2008, 22, 45–55. [PubMed]

20. Li, Q.; Morimoto, K.; Kobayashi, M.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Suzuki, H.; Li, Y.J.; Wakayama, Y.; et al.

Visiting forest, but not a city, increases human natural killer activity and expression of anti-cancer proteins. Int. J. Immunopathol.

Pharmacol. 2008, 21, 117–127. [CrossRef]

21. Mao, G.X.; Lan, X.G.; Cao, Y.B.; Chen, Z.M.; He, Z.H.; Lv, Y.D.; Wang, Y.Z.; Hu, X.L.; Wang, G.F.; Yan, J. Effects of short-term forest

bathing on human health in a broad-leaved evergreen forest in Zhejiang province, China. Biomed. Environ. Sci. 2012, 25, 317–324.

[PubMed]

22. Nakau, M.; Imanishi, J.; Imanishi, J.; Watanabe, S.; Imanishi, A.; Baba, T.; Hirai, K.; Ito, T.; Chiba, W.; Morimoto, Y. Spiritual care

of cancer patients by integrated medicine in urban green space: A pilot study. Explore 2013, 9, 87–90. [CrossRef] [PubMed]

23. Ideno, Y.; Hayashi, K.; Abe, Y.; Ueda, K.; Iso, H.; Noda, M.; Lee, J.; Suzuki, S. Blood pressure-lowering effect of Shinrin-yoku

(Forest bathing): A systematic review and meta-analysis. BMC Complement. Altern. Med. 2017, 17, 409. [CrossRef]

24. Moher, D.; Liberati, A.; Tetzlaff, J.; Altman, D.G.; PRISMA Group. Preferred reporting items for systematic reviews and

meta-analyses: The PRISMA statement. Int. J. Surg. 2010, 8, 336–341. [CrossRef]

Int. J. Environ. Res. Public Health 2021, 18, 8440

16 of 17

25. Sterne, J.A.C.; Savovic´, J.; Page, M.J.; Elbers, R.G.; Blencowe, N.S.; Boutron, I.; Cates, C.J.; Cheng, H.-Y.; Corbett, M.S.; Eldridge,

S.M.; et al. RoB 2: A revised tool for assessing risk of bias in randomized trials. Br. Med. J. 2019, 366, l4898. [CrossRef] [PubMed]

26. Kim, S.Y.; Park, J.E.; Seo, Y.J.; Jang, B.H.; Son, H.J.; Suh, H.S.; Shin, C.M. NECA’s Guidance for Undertaking Systematic Reviews and

Meta-Analyses for Intervention; National Evidence-based Healthcare Collaborating Agency: Seoul, Korea, 2011; pp. 65–78.

27. Jung, W.H.; Woo, J.M.; Ryu, J.S. Effect of a forest therapy program and the forest environment on female workers’ stress. Urban

For. Urban Green. 2015, 14, 274–281. [CrossRef]

28. Kim, B.J.; Jeong, H.; Park, S.; Lee, S. Forest adjuvant anti-cancer therapy to enhance natural cytotoxicity in urban women with

breast cancer: A preliminary prospective interventional study. Eur. J. Integr. Med. 2015, 7, 474–478. [CrossRef]

29. Jia, B.B.; Yang, Z.X.; Mao, G.X.; Lyu, Y.D.; Wen, X.L.; Xu, W.H.; Lyu, X.L.; Cao, Y.B.; Wang, G.F. Health effect of forest bathing trip

on elderly patients with chronic obstructive pulmonary disease. Biomed. Environ. Sci. 2015, 29, 212–218.

30. Han, J.W.; Choi, H.; Jeon, Y.H.; Yoon, C.H.; Woo, J.M.; Kim, W. The effect of forest therapy on coping with chronic widespread

pain: Physiological and psychological differences between participants in a forest therapy program and a control group. Int. J.

Environ. Res. Public Health. 2016, 13, 255. [CrossRef]

31. Mao, G.; Cao, Y.; Wang, B.; Wang, S.; Chen, Z.; Wang, J.; Xing, W.; Ren, X.; Lv, X.; Dong, J.; et al. The salutary inﬂuence of forest

bathing on elderly patients with chronic heart failure. Int. J. Environ. Res. Public Health. 2017, 14, 368. [CrossRef]

32. Lee, M.O.; Shin, C.S.; Yeon, P.S.; Shin, M.J.; Lee, J.S.; Choi, J.H. The effects of forest-walking exercise on NK cells and blood

melatonin levels of women in their 50s. J. KIFR. 2017, 21, 39–52.

33. Tsao, T.M.; Tsai, M.J.; Hwang, J.S.; Cheng, W.F.; Wu, C.F.; Chou, C.K.; Su, T.C. Health effects of a forest environment on natural

killer cells in humans: An observational pilot study. Oncotarget 2018, 9, 16501–16511. [CrossRef]

34. Lyu, B.; Zeng, C.; Xie, S.; Li, D.; Lin, W.; Li, N.; Jiang, M.; Liu, S.; Chen, Q. Beneﬁts of a three-day bamboo forest therapy session

on the psychophysiology and immune system responses of male college students. Int. J. Environ. Res. Public Health. 2019, 16, 4991.

[CrossRef]

35. Lee, I.; Bang, K.S.; Kim, S.; Song, M.; Kang, K. Status of health promotion programs utilizing forest—Based on 2015–2016 regional

healthcare plans in korea. J. Naturopath. 2016, 20, 39–52.

36. Chae, Y.R.; Kim, J.H.; Kang, H.J. Literature review of forest healing therapy on Korean adult. J. Korean Biol. Nurs. Sci. 2018, 20,

122–131. [CrossRef]

37. Vivier, E.; Tomasello, E.; Baratin, M.; Walzer, T.; Ugolini, S. Functions of natural killer cells. Nat. Immunol. 2008, 9, 503–510.

[CrossRef]

38. Park, B.J.; Tsunetsugu, Y.; Kasetani, T.; Kagawa, T.; Miyazaki, Y. The physiological effects of Shinrin-yoku (taking in the forest

atmosphere or forest bathing): Evidence from ﬁeld experiments in 24 forests across Japan. Environ. Health Prev. Med. 2010, 15,

18–26. [CrossRef] [PubMed]

39. Trinchieri, G. Biology of natural killer cells. Adv. Immunol. 1989, 47, 187–376.

40. Na, J.C. Exercise Immunology; DaeKyung Book Publishing: Seoul, Korea, 2002.

41. Li, Q.; Nakadai, A.; Ishizaki, M.; Morimoto, K.; Ueda, A.; Krensky, A.M.; Kawada, T. Dimethyl 2,2-dichlorovinyl phosphate

(DDVP) markedly decreases the expression of perforin, granzyme A and granulysin in human NK-92CI cell line. Toxicology 2005,

213, 107–116. [CrossRef]

42. Smyth, M.J.; Kelly, J.M.; Sutton, V.R.; Davis, J.E.; Browne, K.A.; Sayers, T.J.; Trapani, J.A. Unlocking the secrets of cytotoxic granule

proteins. J. Leukoc. Biol. 2001, 70, 18–29. [CrossRef]

43. Okada, S.; Li, Q.; Whitin, J.C.; Clayberger, C.; Krensky, A.M. Intracellular mediators of granulysin-induced cell death. J. Immunol.

2003, 171, 2556–2562. [CrossRef] [PubMed]

44. Krensky, A.M.; Clayberger, C. Granulysin: A novel host defense molecule. Am. J. Transplant. 2005, 5, 1789–1792. [CrossRef]

45. Smyth, M.J.; Cretney, E.; Kelly, J.M.; Westwood, J.A.; Street, S.E.; Yagita, H.; Takeda, K.; van Dommelen, S.L.; Degli-Esposti, M.A.;

Hayakawa, Y. Activation of NK cell cytotoxicity. Mol. Immunol. 2005, 42, 501–510. [CrossRef] [PubMed]

46. Kägi, D.; Ledermann, B.; Bürki, K.; Seiler, P.; Odermatt, B.; Olsen, K.J.; Podack, E.R.; Zinkernagel, R.M.; Hengartner, H. Cytotoxicity

mediated by T cells and natural killer cells is greatly impaired in perforin-deﬁcient mice. Nature 1994, 369, 31–37. [CrossRef]

[PubMed]

47. Heusel, J.W.; Wesselschmidt, R.L.; Shresta, S.; Russell, J.H.; Ley, T.J. Cytotoxic lymphocytes require granzyme B for the rapid

induction of DNA fragmentation and apoptosis in allogeneic target cells. Cell 1994, 76, 977–987. [CrossRef]

48. Tsunetsugu, Y.; Tsunetsugu, Y.; Park, B.J.; Miyazaki, Y. Trends in research related to “Shinrin-yoku” (taking in the forest

atmosphere or forest bathing) in Japan. Environ. Health Prev. Med. 2010, 15, 27–37. [CrossRef] [PubMed]

49. Li, Q.; Kobayashi, M.; Wakayama, Y.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Shimizu, T.; Kawada, T.; Park, B.-J.; et al.

Effect of phytoncide from trees on human natural killer cell function. Int. J. Immunopathol. Pharmacol. 2009, 22, 951–959. [CrossRef]

[PubMed]

50. Li, Q.; Nakadai, A.; Matsushima, H.; Miyazaki, Y.; Krensky, A.M.; Kawada, T.; Morimoto, K. Phytoncides (wood essential oils)

induce human natural killer cell activity. Immunopharmacol. Immunotoxicol. 2006, 28, 319–333. [CrossRef] [PubMed]

51. Hochman, P.S.; Cudkowicz, G.; Dausset, J. Decline of natural killer cell activity in sublethally irradiated mice. J. Natl. Cancer Inst.

1978, 61, 265–268. [CrossRef] [PubMed]

52. Shibata, H.; Fujiwara, R.; Iwamoto, M.; Matsuoka, H.; Yokoyama, M.M. Immunological and behavioral effects of fragrance in

mice. Int. J. Neurosci. 1991, 57, 151–159. [CrossRef] [PubMed]

Int. J. Environ. Res. Public Health 2021, 18, 8440

17 of 17

53. Komori, T.; Fujiwara, R.; Tanida, M.; Nomura, J.; Yokoyama, M.M. Effects of citrus fragrance on immune function and depressive

states. Neuroimmunomodulation 1995, 2, 174–180. [CrossRef] [PubMed]

54. Park, S.N. A Study on Anti-Stress Effect of Phytoncides. Ph.D. Thesis, Daejeon University, Daejeon, Korea, 2016; Unpublished.

55. Kim, H.M.; Cho, J.K.; Ahn, K.M.; Kim, T.Y.; Lee, J.H.; Jang, D.M.; Min, C.K. Antitumor effects of phytoncides: Vitalization of

natural killer cells. J. Sci. Educ. Gift. 2013, 5, 96–104.