Nature Exposure and Its Effects on Immune System Functioning: A Systematic Review

Abstract

Given the drastic changes in our lifestyles and ecosystems worldwide, the potential health effects of natural environments have grown into a highly pervasive topic. Recent scientific findings suggest beneficial effects from nature exposure on human immune responses. This review aims at providing a comprehensive overview of literature published on immunomodulatory effects of nature exposure by inhalation of natural substances. A systematic database search was performed in SCOPUS and PubMed. The quality and potential bias of included studies (n = 33) were assessed by applying the EPHPP (Effective Public Health Practice Project) tool for human studies and the ARRIVE (Animal Research: Reporting of In Vivo Experiments) and SYRCLE (Systematic Review Centre for Laboratory Animal Experimentation) tools for animal studies. The synthesis of reviewed studies points to positive effects of nature exposure on immunological health parameters; such as anti-inflammatory, anti-allergic, anti-asthmatic effects or increased NK (natural killer) cell activity. Decreased expression of pro-inflammatory molecules, infiltration of leukocytes and release of cytotoxic mediators are outcomes that may serve as a baseline for further studies. However, partially weak study designs evoked uncertainties about outcome reproducibility and key questions remain open concerning effect sizes, duration of exposure and contributions of specific vegetation or ecosystem types.

Full text

International Journal of
Environmental Research
and Public Health
Systematic Review
Nature Exposure and Its Effects on Immune System
Functioning: A Systematic Review
Liisa Andersen * , Sus Sola Corazon and Ulrika Karlsson Stigsdotter


Citation: Andersen, L.; Corazon, S.S.;
Stigsdotter, U.K. Nature Exposure
and Its Effects on Immune System
Functioning: A Systematic Review.
Int. J. Environ. Res. Public Health 2021,
18, 1416. https://doi.org/10.3390/
ijerph18041416
Academic Editors:
Francesco Meneguzzo and
Federica Zabini
Received: 9 January 2021
Accepted: 29 January 2021
Published: 3 February 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Department of Geosciences and Natural Resource Management, University of Copenhagen, Rolighedsvej 23,
1958 Frederiksberg C, Denmark; suoe@ign.ku.dk (S.S.C.); uks@ign.ku.dk (U.K.S.)
*Correspondence: liisa.andersen@posteo.de
Abstract:
Given the drastic changes in our lifestyles and ecosystems worldwide, the potential health
effects of natural environments have grown into a highly pervasive topic. Recent scientific findings
suggest beneficial effects from nature exposure on human immune responses. This review aims
at providing a comprehensive overview of literature published on immunomodulatory effects of
nature exposure by inhalation of natural substances. A systematic database search was performed
in SCOPUS and PubMed. The quality and potential bias of included studies (n = 33) were assessed
by applying the EPHPP (Effective Public Health Practice Project) tool for human studies and the
ARRIVE (Animal Research: Reporting of In Vivo Experiments) and SYRCLE (Systematic Review
Centre for Laboratory Animal Experimentation) tools for animal studies. The synthesis of reviewed
studies points to positive effects of nature exposure on immunological health parameters; such as
anti-inflammatory, anti-allergic, anti-asthmatic effects or increased NK (natural killer) cell activ-
ity. Decreased expression of pro-inflammatory molecules, infiltration of leukocytes and release
of cytotoxic mediators are outcomes that may serve as a baseline for further studies. However,
partially weak study designs evoked uncertainties about outcome reproducibility and key questions
remain open concerning effect sizes, duration of exposure and contributions of specific vegetation or
ecosystem types.
Keywords:
BVOCs; forest bathing; green-blue space; human health; immune system; inflammation;
inhalation; natural environments; NK cells; terpenes
1. Introduction
During the last century, environmental degradation and urbanisation have caused
drastic changes in our lifestyles and living environments [
1
,
2
]. Today, more than half of the
world’s population live in urban areas [
3
], and advancements of the digital era have led to
a substantial rise in screen time and time spent indoors along with a decline in outdoor
activities, especially in the developed world [
4
]. This has caused a loss of interaction
between humans and nature and a progressing feeling of disconnection from the natural
world, which can be defined as everything that exists independently of human conduct [
5
].
The estrangement from nature and other modern lifestyle changes have considerable
consequences for human health [
6
,
7
]. However, next to being a health resource, the natu-
ral environment today also poses substantial risks to human health, not least due to air
pollution and contamination of land and water caused by human activity [
8
]. Toxic pollu-
tion ranges among the most prominent environmental health hazards and is responsible
for one out of six deaths worldwide [
9
]; other environmental burdens of disease include
exposure to extreme heat, noise, hazardous chemicals, electromagnetic fields and natural
disasters [
8
]. In recent years, negative health effects related to climate change have also
been observed [
10
], especially in urban areas that are particularly at risk of developing
urban heat islands (UHI) due to the lack of natural environments. The impacts of the
above-mentioned environmental stressors have led to a significant rise of preventable
Int. J. Environ. Res. Public Health 2021,18, 1416. https://doi.org/10.3390/ijerph18041416 https://www.mdpi.com/journal/ijerph

Int. J. Environ. Res. Public Health 2021,18, 1416 2 of 48
diseases, such as non-communicable diseases (NCDs), which are today the most frequent
cause of death worldwide [
11
]. Globally, more than 20% of all mortalities could be avoided
through healthier environments and almost two-thirds of these are related to NCDs [
12
].
Thus, the relationships between humans, the environment and health are complex and
intertwined, and exposure to intact natural environments is connected to better human
health on many levels.
A growing body of evidence suggests that various forms of being exposed to na-
ture, such as living close to, frequenting or even looking at environments dominated by
living material, are able to provide salutogenic effects on human health [
4
]. They range
from beneficial psychological to physiological outcomes such as attention restoration,
improved mood, lowered anxiety and decrease in depressive symptoms, improved cardio-
vascular, metabolic, oncogenic, respiratory and endocrine function as well as faster healing
after surgery and longer life-expectancy [
4
,
13
–
20
]. Often, these benefits are attributed
to indirect effects of nature exposure, such as increased physical activity, social inter-
actions, positive mental effects and exposure to sunlight, but recent findings have also
highlighted direct physiological mechanisms that are triggered by exposure to natural
environments [
13
,
16
,
17
]. This review focuses on direct mechanisms by which nature can
affect human health, more specifically on air-borne compounds emitted by natural envi-
ronments that have the potential to modulate immunological responses when inhaled,
such as biogenic volatile organic compounds (BVOCs), terpenes, essential oils, charged
ions, pollen, fungi and bacteria.
1.1. Nature Exposure and Immune System Functioning
A limited set of studies have pointed to potential immunological benefits from ex-
posure to natural environments [
16
,
21
,
22
]. By boosting immunological defence mech-
anisms, natural environments might be able to positively influence immunoregulatory
pathways [
16
]. Immunological defence mechanisms are complex, highly specified and
tightly regulated processes that fight foreign pathogens by inducing phagocytosis or
apoptosis, producing cytokines or antibodies and releasing inflammatory or cytotoxic
mediators [
21
,
23
]. During a lifespan, successful immune functioning is shaped by mi-
croorganisms we encounter in our environments, from other humans and animals, and is
then continuously modified by our diets or medicinal use. By being exposed to a broad
variety of organisms, the immune system learns to fine-tune the balance between attack
and tolerance mechanisms, and is able to develop the regulatory pathways needed to avoid
overshooting immune responses to self or harmless allergens [24,25].
1.2. Immunoregulation through Biodiversity
Natural environments are able to provide biologically and genetically diverse micro-
bial inputs [
26
]. Enhanced hygiene, smaller family sizes, increased antibiotic use and lower
exposure to food bacteria in today’s industrialised parts of the world increase the likelihood
of acquiring an unfavourable microbiota prone to overreact to otherwise harmless organ-
isms [
24
]. There is robust evidence that a limited gut microbial diversity leads to a higher
prevalence of chronic inflammatory conditions such as inflammatory bowel diseases or
obesity [
24
,
25
], and that reduced contact with “old friends” (bacteria and parasites common
in the natural environment) increases the risk of developing asthma, allergies or other
hypersensitivity diseases [24,27,28].
Advancing urbanisation and fragmentation of habitats along with the increase of
immunological non-communicable diseases in developed countries led to the formulation
of the biodiversity hypothesis [
29
]. It is based on the fact that nature is one of the richest
sources of microbial input, and that reduced exposure to natural environments and bio-
diversity may adversely affect our microbiota and its immunomodulatory capacity [
24
].
The biodiversity encountered in natural environments not only comprises plant, animal,
microbial and fungal varieties, but also the genetic variety of those species as well as the
variety of ecosystems that serve as their habitats [
26
,
27
,
29
]. Healthy livelihoods depend on

Int. J. Environ. Res. Public Health 2021,18, 1416 3 of 48
such bio-diverse, well-functioning environments being able to provide essential ecosystem
services, regulate infectious disease reservoirs and transmission and serve as pool for
potential medical treatments, amongst others [
26
]. Thus, biodiversity loss poses an acute
threat to human health.
1.3. Immunoregulation through Inhalation of Air-Borne, Volatile Substances
Next to a diverse microbial input, natural environments are also a rich source of air-
borne substances such as BVOCs that are emitted by above- and below-ground vegetation,
rivers and oceans, soils and other natural structures [
30
]. BVOCs are produced by terrestrial
and marine vegetation and make up approximately two thirds of total volatile organic com-
pounds (VOCs) currently emitted in the atmosphere, with forest ecosystems considered
the largest emitters of BVOCs [
31
]. Since their emission is temperature- and light-sensitive,
the amount and type of BVOC emitted varies strongly among species, diurnal and seasonal
time points and geographic and climatic regions [
30
]. Next to methane and dimethyl
sulphide (DMS) produced by oceanic plankton, the majority of emitted BVOCs belongs to
the class of terpenoids [30].
The accredited anti-inflammatory effects of terpenes include both central and pe-
ripheral mechanisms. They encompass the reduction of pro-inflammatory cytokines,
modulation of oxidative stress and inhibition of tissue infiltration by inflammatory cells,
thereby being able to reduce both acute and chronic inflammatory responses in diverse
pathological settings [
32
,
33
]. Moreover, terpenes can also exert immune-stimulatory effects
such as increasing phagocytic activity, enhancing innate immune responses, repressing the
expression of certain pro-inflammatory cytokines and increasing immunoglobulin lev-
els [
21
]. The anti-tumour effects observed are mainly associated with inducing tumour
cell apoptosis, inhibiting their proliferation and preventing metastasis [
33
]. Many of these
effects are mediated by essential immunological cellular components, such as natural killer
(NK) cells [33].
Besides terpenes, charged ions that occur in the air close to waterbodies might also
have beneficial effects on immune functioning, especially in the respiratory tract [
34
,
35
].
Water spray is also a source of microbial input [
24
], and the inhalation of charged ions,
airborne microbes and phytoncides emitted by trees is known to affect systemic immune
responses in various ways [20].
By removing airborne pollutants, forest ecosystems are also responsible for health
benefits resulting from improved air quality. Air pollution is estimated to cause 6.5 million
annual premature deaths worldwide already today [
9
]. Dry deposition of particulate
matter (PM) and absorption of gaseous pollutants by leaf stomata is able to remove up to
4 tons of airborne pollution per square mile and year [
36
]. This impacts acute and chronic
immunological mechanisms by protecting against the development of respiratory diseases
and significantly lowers mortality rates in the local population [
37
]. However, trees can
also adversely affect air pollution. BVOCs are highly reactive molecules and can form
secondary organic aerosols (SOA) with anthropogenic VOCs, thereby producing ozone [
30
].
SOAs directly affect the climate by scattering incoming solar radiation and acting as cloud
condensation nuclei, thereby significantly changing the planet’s radiative balance and
potentially leading to a net cooling effect by increasing cloud albedo [
30
,
38
]. This increased
cloud cover may locally trap pollutants and lead to adverse health effects [15].
Thus, natural environments do not exclusively have beneficial effects on the immune
system, but can sometimes even pose a threat to proper immune functioning. A wide
range of microorganisms such as pollen grains, fungal spores, mycelium, algae and bacteria
are produced by vegetation, especially grasses, and act as potential allergens and might
therefore be harmful by causing or exacerbating allergic reactions [
20
,
36
]. A growing
number of studies have tried to assess the effects of nature exposure on asthma and
allergies; however, the overall outcome of these studies is inconsistent and ranges from
positive and negative to no associations [13].

Int. J. Environ. Res. Public Health 2021,18, 1416 4 of 48
1.4. Health-Promoting Ecosystem Services and Their Effects on the Immune System
In order to understand the relationship between nature and immunological health
in detail and to provide a thorough analysis of its long- and short-term co-benefits and
potential adverse effects, it is important to consider the services that ecosystems provide
either directly or indirectly for humans to sustain their lives and enhance their wellbeing.
Many of these ecosystem services are health-supporting; they provide biodiversity and
reduce harmful exposures, e.g. to extreme heat or air and water pollution [
36
]. The multi-
tude and diversity of human health benefits observed from nature suggest a plurality of
mechanisms that either stand side by side or interact in one broad pathway of action [
16
].
The immune system is a key player in maintaining physiological homeostasis and in sus-
taining health over disease in the human body. Current literature suggests that enhanced
immune functioning can be the outcome of a vast majority of observed nature-related
health effects [
16
]. It has therefore been postulated as a promising candidate that may
incorporate many different health effects into one central pathway.
The aim of this review is to provide a comprehensive overview of literature published
on the immunomodulatory effects on human health following exposure to natural environ-
ments. What distinguishes the paper at hand is its focus on inhalation as the only way of
taking in the biogenic substances analysed. The goal was to define a baseline of reliable
data that can be used as a starting point for future in-depth immunological research, to shed
light on consistencies and potential discrepancies and to elucidate knowledge gaps in this
field.
In order to establish a holistic perspective and stimulate a broad interdisciplinary
research agenda on immunoregulation through nature exposure, different experimental
setups were included in this review. Animal experiments represent an important data
source that helps create both initial hypotheses as well as elucidate causal pathways
through which nature unfolds its various health benefits. Therefore, both human as well as
animal experimental studies were evaluated and rated for their scientific quality.
2. Methodology
The methodological approach for the present review followed the guidelines provided
by Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [39].
2.1. Search Strategy
A structured literature search was carried out in the databases Scopus and PubMed
between February and March 2020 and included all articles published up to the search
date. The search string was designed to combine different nature-based interventions with
different immune-related physiological outcomes (using the Boolean operators AND and
OR). A search for article titles including the following keywords was performed:
Nature OR “natural environment” OR forest* OR ecosystem* OR vegetation OR “green
infrastructure” OR wood* OR greenness OR greenspace OR outdoor OR biodiversity OR
shinrin yoku OR BVOC OR “biogenic volatile organic compound” OR “natural volatile
organic compound” OR phytohormon* OR phytoncide* OR “plant gas” OR “essential oil”
OR fragrance OR aromatherapy
AND
immun* OR inflamma* OR antiinflamma* OR interleukin* OR cytokin* OR allergen*
OR asthma* OR physiologic* OR “NK cell” OR “natural killer“.
When possible, the search was limited to articles and conference papers, and excluded
other document types.
The field of immunological health provisioning through nature exposure is multi-
disciplinary and entails studies with very different methodological approaches which
yet have no common narrative, let alone keywords, methodological guidelines or shared
objectives. This made it challenging to formulate a fitting keyword search that incorporated
all possible wordings and headline formulations into one comprehensive search string and
selectively targeted relevant studies in the wide-spanning field. Therefore, we included

Int. J. Environ. Res. Public Health 2021,18, 1416 5 of 48
snowballing as additional search strategy by screening related reviews as well as references
of the selected studies, which considerably expanded the results found (see limitations).
2.2. Study Selection
Articles retrieved from the database search were roughly screened according to title
and abstract for meeting the eligibility criteria, which were defined as follows:
Analysed species were limited to mammals, and ranged from humans (no age or
health status restrictions) to animal studies.
In vitro
studies on cell lines or primary cell
material were excluded. A wide range of different nature exposures was considered
in the inclusion criteria, such as all kinds of outdoor nature (urban nature, wilderness,
green and blue spaces
. . .
), particles and gases released or produced by nature (BVOCs,
pollen, fungi, moulds
. . .
) or man-made nature products (essential oils, fragrances, aromas,
wood panels
. . .
). Excluded were foods, roots, traditional medicine, drugs and venoms.
Only studies with no or light activities were included, since physical exercise is known
to have immunological effects in itself [
15
,
16
]. Concerning the route of administration,
only inhalation or olfactory stimulations were included. This likewise entailed being
intentionally exposed to volatile substances in an experimental setting as well as normal
breathing of ambient air while being exposed to natural environments, such as forest
bathing or “Shinrin Yoku” (Japanese term for “taking in the forest atmosphere” [
33
,
40
]).
Furthermore included were exposures to specific housing conditions or residential and
recreational stays in nature. All other kinds of exposure (like simply viewing nature from
inside) or administration routes (such as oral, topic or parenteral) were beyond the scope
of this review and therefore not considered. Furthermore excluded were exposures specific
to a certain situation or professional context, such as occupational wood dust exposure of
forest workers or wood smoke from forest fires or stoves. Studies involving viral diseases
and parasite infestations (e.g., Lyme disease, boreliose infections) were also not part of
this review.
To be eligible, studies had to examine physiologic parameters attributable to immune
system responses like cellular properties, cytokine and antibody levels, or disease-related
outcomes serving as indicators for an immune response, e.g. respiratory symptoms,
autoimmune conditions or allergic sensitisations. Due to limited resources, the language of
included articles was limited to English and the study type to peer-reviewed intervention
studies. None of the investigators were contacted, and no unpublished data was retrieved.
2.3. Quality Assessment
The included set of articles was divided into human and animal studies, and three
distinct but specific quality assessment tools were applied. Each study was independently
evaluated by two researchers. In case of disagreements, the individual assessments were
discussed and a consensus was found between the researchers.
For human studies, the risk of bias was evaluated following the quality assessment
tool developed by the Effective Public Health Practice Project (EPHPP) [
41
]. Based on six
parameters (selection bias, study design, confounders, blinding, data collection methods
and withdrawals/dropouts), the studies were rated and classified into overall strong (1),
moderate (2) or weak (3). Scoring criteria followed the publicly available EPHPP dictionary.
Two or more weak parameters classified a study as overall weakly designed. An overall
moderate study may only be rated weak in one parameter, while an overall strong study
required no weak parameters.
The quality assessment of animal studies was carried out according to the “Animals
in Research: Reporting In Vivo Experiments” (ARRIVE) guidelines for good reporting
practice in animal research [
42
]. These guidelines provide 20 questions concerning the
quality of design and reporting in animal research and give a useful indication of the
adoption of good scientific practise in animal studies. We rated each question with points
from 1 (good/reported) to 3 (weak/not reported).

Int. J. Environ. Res. Public Health 2021,18, 1416 6 of 48
Additionally, we evaluated the risk of bias of each animal study based on the System-
atic Review Centre for Laboratory Animal Experimentation (SYRCLE) tool [
43
], which was
adapted from the Cochrane Collaboration’s Risk of Bias (RoB) tool and developed specifi-
cally for the qualitative rating and systematic comparison of experimental animal studies.
The SYRCLE tool addresses 10 different domains which are categorised into assessment
of selection bias, performance bias, detection bias, attrition bias, reporting bias and other
biases. By rating the potential biases of each individual study, we assessed the study to
have a low, high or unclear risk of bias.
3. Results
3.1. Article Selection
The initial database search returned 5167 records (3278 from Scopus and 1889 from
PubMed). After a first screening of titles and abstracts and removal of duplicates, 65 studies
remained for full-text analysis. Full-text reading resulted in a total of 13 studies that met all
inclusion and exclusion criteria. Additional articles were identified through a snowballing
search based on scanning references and reviews; 20 of those met the eligibility criteria
and were therefore included in the final selection. Summing up, a total of 33 articles were
included in this systematic review, comprising both human (n = 20) and animal (n = 13)
intervention studies. Figure 1illustrates the respective stages in the study selection process.
Figure 1. Flowchart of study selection process (following PRISMA guidelines).

Int. J. Environ. Res. Public Health 2021,18, 1416 7 of 48
3.2. Characteristics of Included Studies
An overview of included human and animal studies is given in Tables 1and 2and
shows the year of publication, country of study origin, study design, sample size, sam-
ple characteristics, age and sex of sample, type of intervention and control and duration
of intervention.
3.2.1. Characteristics of Human Studies
Among the included human studies, only one study was designed as a randomised
controlled trial (RCT) [
44
] and nine studies were controlled clinical trials (CCT)
[34,35,45–51]
.
The rest were designed as one group pre-post intervention studies [
52
–
61
], three of which
were carried out as one group crossover studies [
55
,
59
,
61
]. Sample sizes ranged between 11
and 200 subjects, most studies investigating 10–20 individuals per group. Participant char-
acteristics ranged from healthy individuals to individuals suffering from diverse chronic
conditions, and comprised children, adults and elderly people from both genders aged
seven to 79 years. The types of interventions could roughly be divided into three ex-
perimental setups: forest bathing, experimental inhalation of BVOCs or fragrances and
exposure to waterfalls. Forest bathing interventions and waterfall exposures were exam-
ined in 14 [
44
–
50
,
52
–
57
] and two [
34
,
35
] studies, respectively. Four studies analysed the
effects of BVOC [58] or fragrance [59–61] inhalation on human subjects (Table 1).
3.2.2. Characteristics of Animal Studies
The majority of included animal studies were carried out in mice [
62
–
71
] (10 out
of 13), while two studies used rats [
72
,
73
] and one study worked with guinea pigs [
74
].
All studies used a pre-treatment such as lipopolysaccharide (LPS), ovalbumin (OVA),
Der p (Dermatophagoides pteronyssinus), Der f (Dermatophagoides farina) or other to exper-
imentally induce an immune reaction which served as control condition for the actual
intervention. Sample sizes ranged between 12 and 84 animals; most studies analysed
10 animals per group. Animals were between four and 10 weeks of age and comprised
both genders. The types of interventions could be divided into the following experimen-
tal setups: inhalation of BVOCs [
62
–
64
], eucalyptol [
65
,
66
,
74
], limonene [
67
,
68
,
73
], mix of
limonene/ozone [
69
,
70
], linalool [
72
] and other fragrances (lemon, oak moss, labdanum and
tuberose) [
71
]. Three studies evaluated the effects of different housing conditions in labora-
tory animal cages equipped with different wood beddings [62,63,67] (Table 2).
3.3. Quality Assessment
3.3.1. Quality Assessment of Human Studies
For methodological quality assessment, we subjected all included human studies to
a risk of bias assessment following the EPHPP quality assessment tool [41].
According to the EPHPP tool, one study was rated overall strong [
61
], four studies as
overall moderate [
48
,
49
,
52
,
56
] and 15studies got a weak overall score
[34,35,44–47,50–55,57–60]
(Table 3). The weak ratings were mainly due to lack of information on recruitment proce-
dures, the use of self-referred or non-representative samples (selection bias) and missing
information on blinding. Blinding was neither described for study assessors nor for partici-
pating subjects, the latter being hardly applicable in multi-day forest bathing studies where
the exposure to environmental surroundings is obvious. In order to more representatively
evaluate the methodological quality of included studies, we therefore chose to also provide
an alternative overall score excluding the “blinding” parameter. This resulted in five stud-
ies being rated as overall strong, nine as moderate and six as weak (Table 3, last column).
More information on scoring criteria is given in the Supplementary Material.

Int. J. Environ. Res. Public Health 2021,18, 1416 8 of 48
Table 1. Characteristics of included human studies.
Main Author Year Country Study Design
Sample Size
(Interven-
tion/Control)
Sample
Characteristics Sample Age Sample Sex Intervention Control Duration
Forest bathing
Han et al. [45] 2016 South Korea
Pre-post
(2 groups)
CCT
61 (33/28) Adults with
chronic pain 25–49 Mixed
Forest bathing
(pine, oak maple
forest)
Normal daily
routine 2 days
Im et al. [44] 2016 South Korea
Pre-post
(2 groups
crossover)
RCT
41 Healthy
students 18–35 Mixed
Forest
environment
(pine tree forest)
Urban
environment 2 h
Jia et al. [46] 2016 China
Pre-post
(2 groups)
CCT
18 (10/8) COPD patients 61–79 Mixed Forest bathing Urban stay 3 days
Kim et al. [52] 2015 South Korea Pre-post
(1 group) 11 Adults with
breast cancer 25–60 Female Forest therapy / 14 days
Li et al. [53] 2007 Japan Pre-post
(1 group) 12 Healthy adults
(office workers) 37–55 Male Forest bathing / 3 days
(2–4 h/day)
Li et al. [55] 2008a Japan Pre-post
(1 group) 13 Healthy adults
(nurses) 25–43 Female Forest bathing / 3 days
(2–4 h/day)
Li et al. [54] 2008b Japan
Pre-post
(1 group
crossover)
12 Healthy adults 35–56 Male Forest bathing Urban stay 3 days
(2–4 h/day)
Lyu et al. [47] 2019 China
Pre-post
(2 groups)
CCT
60 (45/15) Healthy adults 19–24 Male Forest bathing
(bamboo forest) Urban stay 3 days
Mao et al. [50] 2012a China
Pre-post
(2 groups)
CCT
20 (10/10) Healthy
students 20–21 Male
Forest bathing
(broad-leaved
forest)
Urban stay 2 days
Mao et al. [51] 2012b China
Pre-post
(2 groups)
CCT
24 (12/12)
Elderly patients
with
hypertension
60–75 Mixed
Forest bathing
(broad-leaved
forest)
Urban stay 7 days

Int. J. Environ. Res. Public Health 2021,18, 1416 9 of 48
Table 1. Cont.
Main Author Year Country Study Design
Sample Size
(Interven-
tion/Control)
Sample
Characteristics Sample Age Sample Sex Intervention Control Duration
Mao et al. [49] 2017 China
Pre-post
(2 groups)
CCT
33 (23/10)
Elderly patients
with chronic
heart failure
66–79 Mixed
Forest bathing
(broad-leaved
forest)
Urban stay 4 days
Mao et al. [48] 2018 China
Pre-post
(2 groups)
CCT
20 (10/10)
Elderly patients
with chronic
heart failure
66–79 Mixed
Second forest
bathing trip
(broad-leaved
forest) after 4
weeks break
Urban stay after
previous forest
bathing trip 4
weeks ago
4 days
Seo et al. [56] 2015 South Korea Pre-post
(1 group) 21 Children with
asthma 7–12 Mixed Forest bathing
(fir tree forest) / 4 days
Seo et al. [56] 2015 South Korea Pre-post
(1 group) 27
Children with
atopic
dermatitis
7–12 Mixed Forest bathing
(fir tree forest) / 4 days
Tsao et al. [57] 2018 Taiwan
Retrospective
study
(pre-post 2
groups)
200 (90/110) Healthy adults 34–56 Mixed Forest workers Urban residents 1 year
Tsao et al. [57] 2018 Taiwan Pre-post
(1 group) 11 Healthy adults / /
Forest bathing
(coniferous
forest)
/ 5 days
BVOC inhalation
Li et al. [58] 2009 Japan Pre-post
(1 group) 12 Healthy adults 37–60 Male
Inhalation of
phytoncides
(vaporized
hinoki cypress
stem oil) in
urban
hotel room
/ 3 days

Int. J. Environ. Res. Public Health 2021,18, 1416 10 of 48
Table 1. Cont.
Main Author Year Country Study Design
Sample Size
(Interven-
tion/Control)
Sample
Characteristics Sample Age Sample Sex Intervention Control Duration
Fragrance inhalation
Kiecolt-Glaser
et al. [59]2008 Ohio, USA
Pre-post
(1 group
crossover)
56 Healthy adults 18–43 Mixed
Inhalation of
fragrances
(lavender,
lemon)
Inhalation of
water vapour 1.25 h
Komori et al.
[60]1995 Japan Pre-post
(2 groups) 20 (12/8) Adults with
depression 26–53 Male
Inhalation of
citrus fragrance
mix (limonene,
citral, other EOs)
only positive
control group
4–11
weeks
Trellakis et al.
[61]2012 Germany Pre-post
(1 group
crossover) 32 Healthy adults 20–45 Mixed
Inhalation of
stimulant
fragrances
(grapefruit,
fennel, pepper) No fragrance
exposure
3 days
(30 min/day)
Inhalation of
relaxant
fragrances
(lavender,
patchouli, rose)
Waterfall exposure
Gaisberger
et al. [35]2012 Austria
Pre-post
(2 groups)
CCT
54 (27/27) Children with
allergic asthma 8–15 Mixed
Waterfall
exposure (WF+)
in national park
No waterfall
exposure (WF-)
in national park
3 weeks
(1 h/day)
Grafetstätter
et al. [34]2017 Austria
Pre-post
(3 groups)
CCT
91 (33/32/26)
Adults with stress
(Pre-treated with
oral cholera
vaccination)
19–61 Mixed
Hiking in
national park
with waterfall
exposure (WF+)
Hiking in
national park
without
waterfall
exposure (WF-)
1 week
(1 h/day)

Int. J. Environ. Res. Public Health 2021,18, 1416 11 of 48
Table 2. Characteristics of included animal studies.
Main Author Year Country Study Design
Sample Size
(Number per
Group)
Sample
Characteristics Sample Age Sample Sex Intervention Control Duration
BVOC inhalation
Ahn et al. [62]
2018a
South
Korea Animal 35 (7)
Mice
Pre-treated with
LPS
7 weeks Male
Housing with
BVOC wood
panels (C. obtusa,
P. densiflora)
LPS
Housing without
wood panels
LPS
4 weeks
Ahn et al. [63]
2018b
South
Korea Animal 49 (7)
Mice
Pre-treated with
OVA
5 weeks /
Housing with
BVOC wood
panels (C. obtusa,
P. densiflora,
P. koraiensis,
L.kaempferi)
OVA
Housing without
wood panels
OVA
27 days
Yang et al. [
64
]
2015 South Korea Animal /
Mice Dinitrochlor-
benzene
(DNCB)-induced
atopic dermatitis
(AD)-like disease
model
7 weeks / Exposure to
BVOC (C. obtusa)
Exposure to
vehicle 8 weeks
Eucalyptol inhalation
Bastos et al.
[74]2011 Brazil Animal ca. 35
(7–10)
Guineau pigs
Pre-treated with
OVA
/ Male
Eucalyptol
(1,8-cineol)
inhalation
OVA
Saline inhalation
OVA 15 min
Kennedy-
Feitosa et al.
[65]
2019 Brazil Animal 40 (10)
Mice
Pre-exposed to
cigarette smoke
(CS)
Male
Eucalyptol
(1,8-cineol)
inhalation
CS
Vehicle
inhalation
CS
120 days
(15 min/day)

Int. J. Environ. Res. Public Health 2021,18, 1416 12 of 48
Table 2. Cont.
Main Author Year Country Study Design
Sample Size
(Number per
Group)
Sample
Characteristics Sample Age Sample Sex Intervention Control Duration
Lee et al. [66] 2016 South
Korea Animal /
Mice
Pre-sensitised to
Der p (house dust
mite allergen;
HDM)
6 weeks Female
Eucalyptol
(1,8-cineol)
inhalation
Der p
Vehicle
inhalation
Der p
/
Limonene inhalation
Bibi et al. [67] 2015 Israel Animal 30 (10)
Mice
Pre-treated with
OVA
8 weeks Female
Housing with
Limonene-treated
wood bedding
OVA
Housing with
untreated wood
bedding
OVA
30 days
Hirota et al.
[68]2012 Japan Animal 30 (10)
Mice
Pre-sensitised to
Der f (house dust
mite allergen;
HDM)
6 weeks Male
Limonene
inhalation
Der f
No inhalation
Der f 31 days
Keinan et al.
[73]2005 Israel Animal 40 (10)
Rats
Pre-treated with
OVA
4 weeks /
Limonene
inhalation (ozone
scavenger)
Eucalyptol
inhalation (inert
to ozone)
OVA
No inhalation
OVA 1 week
Limonene/ozone inhalation
Hansen et al.
[69]2013 Denmark Animal ca. 40 (9–10)
Mice
Pre-treated with
OVA
5-6 weeks Female
Limonene
inhalation
Limonene + ozone
inhalation
OVA
No inhalation
Ozone inhalation
OVA
14 weeks

Int. J. Environ. Res. Public Health 2021,18, 1416 13 of 48
Table 2. Cont.
Main Author Year Country Study Design
Sample Size
(Number per
Group)
Sample
Characteristics Sample Age Sample Sex Intervention Control Duration
Hansen et al.
[70]2016 Denmark Animal 40 (10)
Mice
Pre-treated with
OVA
6 weeks Female Limonene
inhalation Air inhalation 3 days
(60 min/day)
Linalool inhalation
Nakamura
et al. [72]2009 Japan Animal 12 (4)
Rats
Stressed by
restraining in tube
7–8 weeks Male
Linalool
inhalation
Stress
No inhalation
Stress 2 h
Fragrance inhalation
Fujiwara et al.
[71]1998 Japan Animal 84 (12)
Mice
Stressed with high
pressure
8–10 weeks Male
Fragrance
exposure (lemon,
oak moss,
labdanum,
tuberose)
Stress
No fragrance
exposure
Stress
24 h

Int. J. Environ. Res. Public Health 2021,18, 1416 14 of 48
Table 3. Quality assessment of human intervention studies following the EPHPP tool.
Overall Score
Selection
Bias
Study
Design
Con-
founders Blinding Data
Collection Dropouts Without
Blinding *
Gaisberger
et al. 2012 2 1 1 3 1 3 3 2
Grafetstätter
et al. 2017 3 1 2 3 1 3 3 3
Han et al. 2016 2 1 1 3 1 3 3 2
Im et al. 2016 2 1 1 3 1 3 3 2
Jia et al. 2016 3 1 1 3 1 3 3 3
Kiecolt-Glaser
et al. 2008 2 2 3 1 3 1 3 3
Kim et al. 2015 2 2 NA 3 1 1 2 1
Komori et al. 1995 3 2 2 3 1 3 3 3
Li et al. 2007 3 2 NA 3 1 2 3 2
Li et al. 2008a 3 2 NA 3 1 3 3 3
Li et al. 2008b 3 2 NA 3 1 2 3 2
Li et al. 2009 3 2 NA 3 1 2 3 2
Lyu et al. 2019 3 1 2 3 1 2 3 2
Mao et al. 2012a 3 1 1 3 1 2 3 2
Mao et al. 2012b 3 1 1 3 1 2 3 2
Mao et al. 2017 2 1 1 3 1 1 2 1
Mao et al. 2018 2 1 1 3 1 1 2 1
Seo et al. 2015 2 2 NA 3 1 2 2 1
Trellakis et al. 2012 2 2 NA 2 1 1 1 1
Tsao et al. 2018 3 2 1 3 1 3 3 3
1 = strong (green), 2 = moderate (yellow), 3 = weak (red), NA = not applicable (one-group studies). An alternative overall score excludes
the “blinding” category (*).
3.3.2. Quality Assessment of Animal Studies
All animal studies were evaluated following two standardised quality assessment
guidelines: ARRIVE assessment tool [
42
] and SYRCLE’s risk of bias tool for animal stud-
ies [
43
]. The results from the ARRIVE assessment, entailing 20 categories, are summarised
in Table 4. Specific information on assessment criteria for individual ratings are provided
in the Supplementary Material.
Most studies provided a good to moderate title, abstract and introduction section.
In the methods section, most studies met the criteria concerning the description of exper-
imental procedures, animal details and housing and husbandry conditions (categories
7–9), while no study appropriately provided a calculation of sample sizes (category 10)
or described the method of allocation to experimental groups (category 11). Almost all
studies provided sufficient information regarding the experimental outcomes (category 12)
and included a description of the statistical analysis of the results (category 13).
In the results section, we detected more reporting shortcomings than in the other
categories. A failure in reporting group-specific baseline data of experimental animal
characteristics (category 14) as well as reporting adverse events (category 17) was asserted
in almost all studies. The majority of studies failed to monitor and report specific baseline
characteristics such as body weight. Moreover, many studies did not report any information
on the numbers of animals included in the final analysis (category 15), nor refer to drop
out rates or reasons for exclusions. All studies included a measure of precision (e.g.,
bars representing standard deviations) in their outcome report (category 16), but only one
study [
69
] also provided data on the number of individual data points within one group,
which should be considered the gold standard of outcome reporting.

Int. J. Environ. Res. Public Health 2021,18, 1416 15 of 48
Table 4. Quality assessment of animal intervention studies following the ARRIVE guidelines.
(a)
1 2 3 4 5 6 7 8 9 10
Title Abstract Introduction Methods
Background Objectives and
Hypotheses
Ethical
Statement
Study Design
(Number of
Experimental
Groups,
Blinding,
Experimental
Unit)
Experimental
Procedure
Animal
Details
(Species, Sex,
Age, Source,
Weight)
Housing and
Husbandry
Conditions
Sample size
Calculation
(Number per
Group,
Number of
Independent
Replicates)
Ahn et al. 2018a 2 1 1 1 1 3 1 1 1 2
Ahn et al. 2018b 2 1 1 1 2 2 1 2 1 2
Bastos et al. 2011 1 1 1 1 1 2 1 2 3 3
Bibi et al. 2015 1 2 1 1 2 2 1 2 1 2
Fujiwara et al. 1998 1 1 2 2 3 2 2 2 2 1
Hansen et al. 2013 2 1 1 2 1 2 1 1 1 2
Hansen et al. 2016 1 1 1 1 1 1 1 1 1 2
Hirota et al. 2012 1 2 2 3 2 1 1 1 1 2
Keinan et al. 2005 1 1 1 1 2 3 1 3 3 3
Kennedy-
Feitosa
et al.
2019 1 1 2 2 2 2 1 1 1 2
Lee et al. 2016 2 1 1 1 2 3 1 1 3 3
Nakamura
et al. 2009 2 2 2 3 3 2 1 2 1 2
Yang et al. 2015 1 1 1 1 1 3 3 2 1 3
(b)
11 12 13 14 15 16 17 18 19 20
Methods Results Discussion
Allocation to
Experimen-
tal Groups
(Randomi-
sation,
Matching)
Experi-
mental
Outcomes
Statistical
Analysis
Baseline Data
Monitoring
Number of
Animals
Analysed
Outcome
Report and
Estimation
(Individual
Values,
Standard
Deviation)
Adverse
Events
Interpreta-
tion and
Limitations
Generalis-
ability and
Translational
Relevance
Funding
Ahn et al. 2018a 3 1 1 3 3 2 3 2 2 1
Ahn et al. 2018b 3 1 1 3 3 2 3 2 2 3
Bastos et al. 2011 3 1 1 3 3 2 3 2 2 2
Bibi et al. 2015 3 1 1 3 1 2 3 2 1 1
Fujiwara et al. 1998 3 3 2 3 3 2 3 2 2 3
Hansen et al. 2013 3 1 1 1 2 1 3 1 1 1
Hansen et al. 2016 2 1 1 1 1 2 3 1 1 1
Hirota et al. 2012 2 1 1 3 1 2 3 2 2 2
Keinan et al. 2005 3 2 2 3 3 2 3 3 2 3
Kennedy-
Feitosa
et al.
2019 3 1 1 3 1 2 3 1 1 1
Lee et al. 2016 3 1 1 3 3 2 3 2 2 1
Nakamura
et al. 2009 3 1 1 3 1 2 3 2 2 3
Yang et al. 2015 3 1 1 3 3 2 3 2 2 1
1 = reported/strong (green), 2 = moderate (yellow), 3 = not reported/weak (red).
Regarding the discussion section, most studies interpreted the implications of their
findings within the current scientific literature; however many failed to elaborate on
study limitations (category 18) and generalisability of study outcomes (category 19), i.e.,
translation of outcomes into the human system.
A risk of bias assessment following the SYRCLE’s risk of bias (RoB) tool for animal
studies [
43
] was conducted to complement the ARRIVE quality assessment. According
to the SYRCLE RoB tool, many studies did not adequately report measures taken to
reduce potential risk of biases in several categories. Randomisation and blinding of
the experimental setup were rarely reported, and no study had performed a preceding
calculation of sample sizes. All results of the SYRCLE risk of bias assessment are provided
in the Supplementary Material (Table S1).
3.4. Outcomes and Synthesis
3.4.1. Outcomes of Human Studies
The majority of studies measured either anti-inflammatory or cytotoxic effects follow-
ing nature exposure. Experimental parameters analysed were mainly expression of pro- or
anti-inflammatory cytokines (mostly IL-6 and TNF
α
) in serum, numbers and percentages
of immune cell subsets (mostly NK cells and T cells) and expression of cytotoxic mediators
(perforin, granzyme A/B and granulysin) as well as cytotoxic NK cell activity. Some studies
examined several outcomes. Overall, a positive effect was observed on most immunolog-
ical parameters measured; four studies showed no significant changes. An overview of
outcomes measured in studies with human subjects is presented in Figure 2.

Int. J. Environ. Res. Public Health 2021,18, 1416 16 of 48
Figure 2. Overview of outcomes from human studies.
Most studies with human subjects examined the effects of forest bathing trips of vary-
ing lengths on different immunological parameters. Seven (out of 14) forest bathing studies
analysed inflammatory cytokine expression and unanimously reported either a decrease in
pro-inflammatory and/or an increase in anti-inflammatory cytokine levels
[44,46,48–51,56]
,
implicating anti-allergic or anti-asthmatic outcomes from being exposed to forest environ-
ments. One of the studies also evaluated changes in clinical scores of patients with asthma
and atopic dermatitis after a forest trip and concluded a relief of clinical symptoms and
beneficial effects of forest bathing on spirometric outcomes [56].
Ten (out of 14) forest bathing studies elucidated the effects on the distribution of
immune cell subsets as well as on their distinct effector activities, with a special focus on
NK cells. The majority of these studies (8 out of 10) reported an increase either in NK
cell number or in NK cell activity [
45
,
47
,
52
–
55
,
57
], which was measured either directly
by flow cytometry or indirectly by assessing the level of cytotoxic mediators circulating
in the blood. Two forest bathing studies did not observe any significant changes in NK
cell outcomes
[46,50]
. One additional study reported an increase in NK cell number and
activity after the isolated inhalation of phytoncides under laboratory conditions [
58
], con-
cluding that volatile substances released by plants are responsible for the effects observed
in natural environments.
Three studies examined the outcomes of inhaling different fragrances (citrus mix,
lavender, lemon, grapefruit, fennel, pepper, patchouli and rose) on immunological parame-
ters such as cytokine and chemokine levels, cell ratios and strength of immune response
to infection. Two of these studies reported no significant effect of fragrance exposure on
levels of circulating cytokines and chemokines [
59
,
61
], while one of them observed a lower
hypersensitivity to candida infection compared to control [
59
]. Another study reported
beneficial effects on immune cell ratios after fragrance exposure [60].
Two studies looked at potential health effects of charged ions in ambient air in the
vicinity of waterfalls. One of them detected significantly decreased pro-inflammatory
cytokine levels combined with enhanced lung function and reduced clinical symptoms in
children with allergic asthma, which was suggested to be due to an induction of circulating
regulatory T cells [
35
]. The other study reported that exposure to waterfalls led to an
activated immune system and also improved lung function [34].
Table 5provides a comprehensive summary of all outcomes from human studies.

Int. J. Environ. Res. Public Health 2021,18, 1416 17 of 48
Table 5.
Outcomes of human studies. Significances are given with p< 0.05, p< 0.01 and p< 0.001; NS = not significant, NA = not applicable. A non-significant trend is described as
“Decrease/Increase, NS”. If significances are not given, it is described as “Decrease/Increase, NA”.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Forest bathing
Han et al.
[45]2016 The effects of forest therapy on
coping with chronic widespread
pain NK cell activity Increase p < 0.001 NS Increase NA
NK cell activity
increases after a
forest bathing trip in
adults with chronic
pain.
Significant
baseline
differences
between
control and
intervention
group in NK
cell activity.
Im et al. [44]2016
Comparison of effect of two-hour
exposure to forest and urban
environments on cytokine,
anti-oxidant and stress levels in
young adults
IL-6 level in serum NS Pro-inflammatory
cytokine level (IL-8
and TNFα, but not
IL-6) is reduced in
healthy students
after a forest bathing
trip.
No pre-
intervention
data shown.
IL-8 level in serum Decrease p < 0.001
TNFαlevel in serum Decrease p < 0.001
Glutathione
peroxidase (GPx)
level in serum Increase p < 0.05
Jia et al. [46]2016
Health effects of forest bathing
trip on elderly patients with
chronic obstructive pulmonary
disease
CD8+ T cell number
and % NS NS NS
Forest bathing
reduces
pro-inflammatory
cytokine levels, but
not proportions of
CD8+ T, NK or NKT
cells or GrB
expression in COPD
patients.
Result on
perforin
expression is
questionable,
since there is
also an effect
in control
group.
NK cell number
and % (CD3-CD56+) NS NS NS
NKT cell number
and % (CD3+CD56+) NS NS NS
Perforin expression in
CD8+ T cells
(flow cytometry) Decrease p < 0.001 Decrease NA Decrease p < 0.01
Perforin expression in
NK cells
(flow cytometry) Decrease p < 0.001 Decrease NA NS
Perforin expression in
NKT cells
(flow cytometry) Decrease p < 0.001 Decrease NA Decrease p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 18 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Granzyme B
expression in NK
cells
(flow cytometry)
NS NS NS
Granzyme B
expression in NKT
cells (flow cytometry)
NS NS NS
Granzyme B
expression in CD8+ T
cells
(flow cytometry)
NS NS NS
IL-6 level in serum Decrease p < 0.01 NS Decrease p < 0.05
IL-8 level in serum Decrease p < 0.05 NS Decrease p < 0.01
IFN-y level in serum Decrease p < 0.01 NS Decrease p < 0.05
IL-1b level in serum NS NS Decrease p < 0.05
CRP level in serum NS NS Decrease p < 0.05
TNFαlevel in serum NS NS NS
Kim et al.
[52]2015
Forest adjuvant anti-cancer therapy
to enhance natural cytotoxicity in
urban women with breast cancer: A
preliminary prospective
interventional study
NK cell number
(CD3-CD56+) Increase p < 0.01 Forest therapy
enhances natural
cytotoxicity in breast
cancer patients by
increasing NK cells
and cytotoxic
mediators.
Perforin level in
serum (ELISA) Increase p < 0.02
Granzyme B level in
serum (ELISA) Increase p < 0.02

Int. J. Environ. Res. Public Health 2021,18, 1416 19 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to
Baseline)
p-Value
Control
(Com-
pared
to Base-
line)
p-Value
Comparison
(Interven-
tion
Com-
pared to
Control)
p-Value Conclusion Comment
Li et al. [53]2007
Forest bathing enhances
human natural killer activity
and expression of anti-cancer
proteins
NK cell number
and % (CD16+) Increase p < 0.01
Forest bathing
enhances NK cell
activity and
numbers in
healthy male
adults.
Cytolytic NK cell
activity (Cr-release
assay)
Increase p < 0.01
% of T cells (CD3+)
Decrease NA
% of perforin-
expressing cells Increase p < 0.01
% of granzyme
A/B-
expressing cells
Increase p < 0.01
% of granulysin-
expressing
cells
Increase p < 0.01
Li et al. [55]2008a
A forest bathing trip increases
human natural killer activity
and expression of anti-cancer
proteins in female subjects
NK cell number
and % (CD16+) Increase p < 0.01
Forest bathing
enhances NK cell
activity and
numbers in
healthy female
adults.
Cytolytic NK cell
activity (Cr-release
assay)
Increase p < 0.01
% of T cells (CD3+)
Decrease p < 0.05
% of perforin-
expressing cells Increase p < 0.01
% of granzyme
A/B-
expressing cells
Increase p < 0.01
% of granulysin-
expressing cells Increase p < 0.01

Int. J. Environ. Res. Public Health 2021,18, 1416 20 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Li et al. [54]2008b
Visiting a forest, but not a city,
increases human natural killer
activity and expression of
anti-cancer proteins
NK cell number and
% (CD16+) Increase p < 0.01 NS Increase p < 0.05
Forest bathing
enhances NK cell
activity and numbers
in healthy adults.
Cytolytic NK cell
activity (Cr-release
assay) Increase p < 0.01 NS Increase p < 0.05
% of T cells (CD3+) NS NS
% of perforin-
expressing cells Increase p < 0.01 NS
% of granzyme A/B-
expressing cells Increase p < 0.01 NS
% of granulysin-
expressing cells Increase p < 0.01 NS
Lyu et al. [
47
]
2019
Benefits of a three-day bamboo
forest therapy session on the
psychophysiology and immune
system responses of male college
students
Cytolytic NK cell
activity Increase p < 0.05 NS NA
Forest bathing in a
bamboo forest
enhances NK cell
activity and
percentages in
healthy adults.
% of NK cells
(CD16+CD56+) Increase p < 0.05 NS NA
Perforin level (ELISA)
Increase p < 0.05 NS NA
Granulysin level
(ELISA) NS NS NA
Granzyme A/B level
(ELISA) Increase p < 0.05 NS NA

Int. J. Environ. Res. Public Health 2021,18, 1416 21 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Mao et al.
[50]2012a
Effects of short-term forest
bathing on human health in a
broad-leaved evergreen forest in
Zhejiang Province, China
IL-6 level in serum
(radioimmunoassay) NS Increase NA Decrease p < 0.05
Forest bathing
decreases
pro-inflammatory
cytokine levels (IL-6
and TNFα) in
healthy young adults
but has no effect on
immune cell
distribution.
Questionable
effect
(increase) in
control group
compared to
baseline for
IL-6 and
TNFαlevels.
TNFαlevel in serum
(radioimmunoassay) Decrease NA Increase NA Decrease p < 0.05
HCRP level in serum NS
% of B cells
(CD5+CD19+) Increase p < 0.05 No pre-
intervention
data shown
for leukocyte
distributions.
% of T cells (CD3+) NS
% of Th cells
(CD3+CD4+) NS
% of cytotoxic T cells
(CD3+CD8+) NS
% of NK cells
(CD3-CD16+CD56+) NS
Mao et al.
[51]2012b
Therapeutic effect of forest
bathing on human hypertension
in the elderly
IL-6 level in serum
(radioimmunoassay) Decrease p < 0.05 NS NS Forest bathing
decreases
pro-inflammatory
cytokine level (IL-6,
but not TNFα) in
elderly patients with
hypertension.
TNFαlevel in serum
(radioimmunoassay) NS NS NS
Mao et al.
[49]2017 The salutary influence of forest
bathing on elderly patients with
chronic heart failure
IL-6 level in serum
(ELISA) NS NS Decrease p < 0.05 Forest bathing
decreases
pro-inflammatory
cytokine level (IL-6,
but not TNFα) in
elderly patients with
chronic heart failure.
No effect in
intervention
group
compared to
baseline.
TNFαlevel in serum
(ELISA) NS NS NS
HCRP level in serum NS NS NS

Int. J. Environ. Res. Public Health 2021,18, 1416 22 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Mao et al.
[48]2018
Additive benefits of twice forest
bathing trips in elderly patients
with chronic heart failure
IL-6 level in serum
(ELISA) NS NS NS A second forest
bathing trip further
decreases
pro-inflammatory
cytokine level (TNFα,
but not IL-6) in elderly
patients with chronic
heart failure.
TNFαlevel in serum
(ELISA) Decrease p < 0.05 NS Decrease p < 0.05
Seo et al. [56]2015
Clinical and immunological
effects of a forest trip in children
with asthma and atopic dermatitis
Asthma:
Forest environment
improves clinical
symptoms in
asthmatic children.
Forced vital capacity
(spirometry, FCV) Increase p < 0.05
Fractional exhaled
nitric oxide (FeNO) Decrease NA
Seo et al. [56]2015
Clinical and immunological
effects of a forest trip in children
with asthma and atopic dermatitis
Atopic dermatitis:
Forest environment
improves clinical
symptoms and has
immunological
effects in chronic
allergic skin disease.
Atopic dermatitis
index (SCORAD) Decrease NA
Thymus and
activation-regulated
chemokine/
CCL17 level
NS
Macrophage-derived
chemokine/
CCL22 level Decrease p < 0.01
Tsao et al.
(forest
workers) [57]2018
Health effects of a forest
environment on natural killer cells
in humans: an observational pilot
study
% of NK cells in
blood
(CD3-CD56+) Increase p < 0.05 Living in a forest
environment
increases NK cell
percentage, but not
the amount of
activated NK cells.
% of activated NK
cells in blood
(CD3-CD56+CD69+) NS

Int. J. Environ. Res. Public Health 2021,18, 1416 23 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Tsao et al.
(forest
bathing) [57]2018
Health effects of a forest
environment on natural killer cells
in humans: an observational pilot
study
% of NK cells in
blood on d5 NS
Short-term forest trip
enhances fraction of
activated NK cells in
healthy adults, and
effect lasts for at least
4 days.
% of activated NK
cells in blood on d5 Increase p < 0.01
% of NK cells in
blood on d9 (4 days
after intervention) NS
% of activated NK
cells in blood on d9 (4
days after
intervention)
Increase p < 0.01
BVOC inhalation
Li et al. [58]2009
Effect of phytoncide from trees on
human natural killer cell function
Cytolytic NK cell
activity (Cr-release
assay) Increase p < 0.05
Phytoncide exposure
enhances NK cell
activity and % in
healthy adults.
% of NK cells
(CD16+) Increase p < 0.01
% of T cells (CD3+) Decrease p < 0.01
% of perforin-
expressing cells Increase p < 0.05
% of granzyme A/B-
expressing cells Increase p < 0.01
p < 0.05
% of granulysin-
expressing cells Increase p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 24 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Kiecolt-
Glaser et al.
[59]2008
Olfactory influences on mood and
autonomic, endocrine and
immune function
Delayed
hypersensitivity to
candida (DTH) Increase NA Increase NA Decrease
p= 0.02
(lavender)
p= 0.06
(lemon)
Greater DTH
response after water
inhalation indicates
better immune
response than in
fragrance groups,
but no difference in
cytokine levels
detectable.
* Differing
effects in
blinded and
informed
groups for
blastogenesis
responses.
PBL proliferation
(blastogenesis) NA*
IL-6 level in PBLs
(ELISA) NS
IL-10 level in PBLs
(ELISA) NS
Fragrance inhalation
Komori et al.
[60]1995 Effects of citrus fragrance on
immune function and depressive
states
Deviation from
normal CD4/CD8
ratio Decrease NA Citrus fragrance has
a beneficial effect on
immune cell
distribution in
depressive patients
and can reduce the
dose antidepressants
needed.
Deviation from
normal NK cell
activity
(Cr-release assay)
Decrease NA
Trellakis et al.
[61]2012 Subconscious olfactory influences
of stimulant and relaxant odors on
immune function
IL-8 level in serum
(ELISA) NS
No significant effect
of any stimulatory or
relaxing fragrance
exposure on immune
parameters in
healthy adults.
IL-6 level in serum
(ELISA) NS
TNFαlevel in serum
(ELISA) NS
CCL3 (MIP-1a) level
in serum (ELISA) NS
CCL4 (MIP-1b) level
in serum (ELISA) NS
CCL5 (RANTES)
level in serum
(ELISA) NS
CXCL8 (IL-8) release
by neutrophils NS

Int. J. Environ. Res. Public Health 2021,18, 1416 25 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Waterfall exposure
Gaisberger
et al. [35]2012
Effects of ionized waterfall aerosol
on pediatric allergic asthma
IL-5 level in serum
(ELISpot) Decrease p < 0.05 NS Decrease NS
Exposure to
waterfalls reduces
pro-inflammatory
cytokines and
allergic asthma
symptoms, enhances
lung function and
induces Treg cells.
IL-10 level in serum
(ELISpot) Increase p < 0.05 NS Increase NS
IL-13 level in serum
(ELISpot) Decrease p < 0.01 Decrease p < 0.01 NS
IL-10 expression
(PCR) Increase NA Increase NA NS
IL-13 expression
(PCR) Decrease NA NS NA Decrease p < 0.05
IFNg expression
(PCR) Increase NA Increase NA NS
Treg cells (%) Increase p < 0.01 Increase p < 0.05 NS
Eosinophilic cationic
protein (ECP) levels
in serum Decrease p < 0.05 NS NS
Fractional exhaled
nitric oxide (FeNO) at
d20 Decrease p < 0.001 Decrease p < 0.001 NA
Fractional exhaled
nitric oxide (FeNO) at
d80 Decrease p < 0.01 NS NA
Peak expiratory flow
rate (PEF) Increase p < 0.01 Increase p < 0.01 NA
Other spirometric
parameters (FEV,
FEV%FVC, FEF25,
FEF50, MMEF2575)
Increase p < 0.05
p < 0.01 NS NA

Int. J. Environ. Res. Public Health 2021,18, 1416 26 of 48
Table 5. Cont.
Main
Author Year Title Measure
Intervention
(Compared
to Baseline)
p-Value
Control
(Com-
pared to
Base-
line)
p-Value
Comparison
(Interven-
tion
Compared
to Control)
p-Value Conclusion Comment
Grafetstätter
et al. [34]2017
Does waterfall aerosol influence
mucosal immunity and chronic
stress? A randomized controlled
clinical trial
IgA level in saliva
(d6) Increase NA Increase NA Increase p= 0.001 Exposure to
waterfalls activates
the immune system
and improves lung
function.
IgA level in saliva
(d66) Increase NA NS Increase p < 0.05
Peak expiratory flow
rate (PEF) (d6) Increase p= 0.023 Increase NA NS

Int. J. Environ. Res. Public Health 2021,18, 1416 27 of 48
3.4.2. Synthesis of Human Studies
The synthesis of human studies analysing the immunological effects of forest bathing
points to largely positive evidence of anti-inflammatory and anti-asthmatic effects along
with a promising evidence of enhanced cytotoxicity stemming from increased NK cell levels
or activities. However, the synthesis of anti-inflammatory effects was obscured by differing
results derived from analysing cytokine levels, specifically IL-6 and TNF
α
. Some studies
only observed an alteration in one of the cytokines and no change in the other, while other
studies reported a change in the respective other cytokine value. Since the results reveal
overall anti-inflammatory effects, the evidence base is still regarded as largely positive,
but not entirely conclusive. Several forest bathing studies also measured immune cell
subset distributions. While no significant changes in T cell numbers and/or percentages
were observed, most studies showed an increase of NK cell levels along with elevated
percentages of cells with cytotoxic content (perforin, granzyme A/B and granulysin).
Studies without control groups and/or without sufficient baseline values to ade-
quately control for confounders were heavily represented among forest bathing studies. Al-
most half of the studies were carried out as one group pre-post intervention designs
[52–57]
;
therefore, their findings have less strength but are backed up by similar results from the
included CCT studies. Furthermore, a considerable number of forest bathing studies did
not provide a thorough calculation of significances for certain outcome measures that were
nevertheless interpreted by the study authors as showing an effect. Moreover, most studies
comprised a low number of study participants (<30 subjects), which weakened the evidence
base for the measured effects significantly. Only four forest bathing studies analysed larger
group sizes [
44
,
45
,
47
,
57
]. One of them showed retrospective results from a big cohort
of subjects living in a forest environment, but lacked baseline values of respective mea-
sures [
57
]. Three other studies with larger sample sizes either provided no pre-intervention
data [
44
], no appropriate significance calculations [
47
] or ignored considerable baseline
differences [
45
], which weakened their results. The findings on NK cell distributions were
underlined by positive results from one laboratory experiment simulating BVOC exposure
in natural environments [
58
]; however, this study provided no control group and must
therefore be interpreted cautiously. Overall, the evidence base for the effects of forest
bathing on immune functioning can be regarded as promising, despite substantial study
design shortcomings.
The three studies analysing fragrance inhalation showed highly heterogeneous results
that ranged from beneficial immune responses to no measurable effects at all. The different
study designs and measured parameters resulted in differing outcomes, which barely
suffice to formulate overall tendencies. It is noteworthy that the two studies describing
certain positive effects were relatively old [
59
,
60
] and one of them used out-dated methods
(for details, see Supplementary Material) [
59
]; accordingly, these results must be questioned
in terms of reliability and validity.
Lastly, the two studies analysing the effects of waterfall exposure on immunological
health provided good study designs with appropriate controls and group sizes as well
as methodologically transparent approaches [
34
,
35
]. Thus, their results showing anti-
inflammatory and anti-allergic outcomes as well as beneficial clinical outcomes provided a
reliable evidence base which remains to be confirmed by a greater number of studies.
In conclusion, human studies provide a promising evidence base for immunomodula-
tory effects following exposure to natural environments, though general shortcomings in
the study designs weaken their soundness.
3.5. Outcomes and Synthesis of Animal Studies
3.5.1. Outcomes of Animal Studies
Most animal studies examined the effects of phytoncide or aroma inhalation on a
pre-induced immune response. Experimental parameters analysed were levels of various
cytokines and/or antibodies in sera and/or bronchoalveolar lavage fluids (BALF) of the
respiratory tract, leukocyte numbers (mostly neutrophils, eosinophils, macrophages and

Int. J. Environ. Res. Public Health 2021,18, 1416 28 of 48
lymphocytes) as well as histological or clinical symptoms such as cellular or structural
changes, inflammatory cell infiltrations into lung tissue or respiratory markers. Some stud-
ies assessed several outcome measures. Overall, a positive effect was observed on most im-
munological parameters and only one study showed no significant changes. An overview
of outcomes from animal studies is presented in Figure 3.
Figure 3. Overview of outcomes from animal studies.
By measuring a decrease in pro-inflammatory and/or an increase in anti-inflammatory
(IL-10) cytokines as well as a reduced number of leukocytes, 13 animal studies reported
anti-inflammatory, anti-allergic and/or anti-asthmatic effects following exposure to natural
substances. All studies analysing BVOC [
62
,
64
,
75
] and eucalyptol [
65
,
66
,
74
] inhalation
attested to protective and therapeutic anti-inflammatory effects of these substances in
asthma- or allergy-challenged animals. The results for limonene were more heterogeneous,
with four out of five studies observing potentially beneficial, anti-inflammatory effects in
challenged animals [
67
,
68
,
70
,
73
], while one study showed no significant effects of limonene
inhalation [
69
]. The mixture of limonene and ozone inhalation was able to protect from
the adverse effects elicited by inhalation of only ozone in two studies [
69
,
70
]. Furthermore,
inhalation of linalool was able to change the immune cell distribution of previously stressed
rats, as one study reported [
72
]. Another study observed that exposure to certain fragrances
was able to induce a general immune activation, which they diagnosed by measuring the
number of plaque-forming cells in spleen and the thymic weight [71].
The findings of all animal studies are summarised in Table 6.

Int. J. Environ. Res. Public Health 2021,18, 1416 29 of 48
Table 6.
Outcomes of animal studies. Significances are given with p< 0.05, p< 0.01 and p< 0.001; NS = not significant, NA=not applicable. A non-significant trend is described as
“Decrease/Increase, NS”. If significances are not given, it is described as “Decrease/Increase, NA”.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
BVOC inhalation
Ahn et al. [62]2018a
Alleviation effects
of natural volatile
organic
compounds from
Pinus densiflora
and
Chamaecyparis
obtuda on
systemic and
pulmonary
inflammation
IgE level in serum
(ELISA) Increase p < 0.05 Decrease p < 0.05
BVOCs (VOCCo,
VOCPd) excert
anti-inflammatory
effects in mice.
Prostaglandin E2 (PGE2)
level in serum (ELISA) Increase p < 0.05 Decrease p < 0.05
COX-2 mRNA
expression in PBMCs Increase p < 0.05 Decrease p < 0.05
TNF
α
mRNA expression
in PBMCs Increase p < 0.05 Decrease p < 0.05
IL-1b mRNA expression
in PBMCs Increase p < 0.05 Decrease p < 0.05
IL-13 mRNA expression
in PBMCs Increase p < 0.05 Decrease p < 0.05
COX-2 mRNA
expression in lung tissue
Increase p < 0.05 Decrease p < 0.05
NF-kB mRNA
expression in lung tissue
Increase p < 0.05 Decrease p < 0.05
TNF
α
mRNA expression
in lung tissue Increase p < 0.05 NS
COX-2, NF-kB, IL-1b,
TNFα, IL-13 mRNA in
BALF cells Increase p < 0.05 Decrease p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 30 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Ahn et al. [63]2018b
Anti-asthmatic
effects of
volatile organic
compounds
from
Chamaecyparis
obtusa, Pinus
densiflora,
Pinus koraiensis,
and Larix
kaempferi wood
panels
Thickening of
bronchiolar wall
(hypertrophy)
Increase NA Decrease NA
BVOCs (VOCCo,
VOCPd, VOCPk,
VOCLk) excert
anti-allergic effects
in asthmatic mice.
IL-4 level in serum
(ELISA) Increase p < 0.05 Decrease p < 0.05
TNFαlevel in serum
(ELISA) Increase p < 0.05 Decrease
(C. obtusa) p < 0.05
IL-4 mRNA
expression in
bronchioles
Increase p < 0.05 Decrease p < 0.05
IL-5 mRNA
expression in
bronchioles
Increase p < 0.05 NS
IL-9 mRNA
expression in
bronchioles
Increase p < 0.05 Decrease
(C. obtusa) p < 0.05
IL-13 mRNA
expression in
bronchioles
Increase p < 0.05 Decrease p < 0.05
Yang et al. [64]2015
Estimation of the
environmental
effect of natural
volatile organic
compounds from
Chamaecyparis
obtusa and their
effect on atopic
dermatitis-like
skin lesions in
mice
IgE level in serum Increase p < 0.05 Decrease p < 0.05
Exposure to
BVOCs
(C. obtusa)
ameliorates
inflammatory skin
reactions in mice
with atopic
dermatitis.
Mast cell infiltration
into skin lesions Increase p < 0.05 Decrease p < 0.05
IL-1b mRNA
expression in skin
lesions
Increase p < 0.05 Decrease p < 0.05
IL-6 mRNA
expression in skin
lesions
Increase p < 0.05 Decrease p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 31 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Eucalyptol inhalation
Bastos et al. [74]2011
Inhaled
1,8-Cineole
reduces
inflammatory
parameters in
airways of
ovalbumin-
challenged
guinea pigs
TNFαlevel in BALF
(ELISA) Increase p < 0.05 Decrease NS
Eucalyptol
(1,8-cineol) inhibits
antigen-induced
airway
inflammation in
guinea pigs.
IL-1b level in BALF
(ELISA) Increase p < 0.05 Decrease NS
IL-10 level in BALF
(ELISA) Decrease p < 0.05 Increase NS
Leukocyte number
in BALF
(eosinophils and
neutrophils)
Increase p < 0.05 Decrease p < 0.05
MPO activity Increase p < 0.05 Decrease p < 0.05
Kennedy-
Feitosa et al.
[65]2019
Eucalyptol
promotes lung
repair in mice
following
cigarette
smoke-induced
emphysema
TNFαlevel in lung
tissue Increase p < 0.01 Decrease p < 0.01
Eucalyptol reduces
pro-inflammatory
cytokines and
neutrophil
activation marker
(MPO) after lung
damage by
cigarette smoke.
IL-1b level in lung
tissue Increase p < 0.01 Decrease p < 0.05
IL-6 level in lung
tissue Increase p < 0.01 Decrease p < 0.01
TGFß-1 level in lung
tissue Increase p < 0.05 Decrease p < 0.05
MPO activity in lung
tissue Increase p < 0.01 Decrease p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 32 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Lee et al. [66]2016
Effect of
1,8-cineol in Der-
matophagiodes
pteronyssinus-
stimulated
bronchial
epithelial cells
and mouse
model of asthma
IL-4 level in BALF
(ELISA) Increase p < 0.01 Decrease p < 0.05
Eucalyptol reduces
pro-inflammatory
cytokine
expression (IL-4,
IL-13, IL-17A) in
house dust mite-
allergic/asthmatic
mice.
IL-13 level
in BALF (ELISA) Increase p < 0.05 Decrease p < 0.05
IL-17A
level
in BALF (ELISA) Increase p < 0.05 Decrease p < 0.05
Neutrophil number
in BALF Increase p < 0.05 Decrease p < 0.05
Eosinophil number
in BALF Increase p < 0.05 Decrease p < 0.05
Lymphocyte number
in BALF Increase p < 0.05 Decrease p < 0.05
Der p-specific IgG1 in
serum (ELISA) Increase p < 0.01 Decrease p < 0.05
Airway restriction
(Penh) Increase p < 0.01 Decrease p < 0.05
Limonene inhalation
Bibi et al. [67]2015
Treatment of
asthma by an
ozone scavenger
in a mouse
model
Aldehyde (ozone
oxydation product)
levels in BALF
Increase NA Decrease NA
Prophylactic
limonene
inhalation protects
against allergic
asthma in mice.
Aldehyde (ozone
oxydation product)
levels in lung tissue
Increase NA Decrease NA
Aldehyde (ozone
oxydation product)
levels in spleen
Increase NA Decrease NA
Neutrophil number
in BALF Increase NA Decrease p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 33 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Eosinophil number
in BALF Increase NA NS
Infiltration of
inflammatory cells
into lung tissue
Increase NA Decrease NA
Hirota et al. [68]2012
Limonene
inhalation
reduces allergic
airway
inflammation in
Der-
matophagoides
farinae-treated
mice
Der f-specific IgG in
serum (ELISA) Increase p < 0.001 Decrease p < 0.01
Limonene reduces
pro-inflammatory
cytokines and cell
numbers in mice
pre-sensitized to
house dust mite
allergen.
Total IgE in serum
(ELISA) Increase NS Decrease NS
Eosinophil number
in BALF Increase p < 0.001 Decrease p < 0.001
Lymphocyte number
in BALF Increase p < 0.001 Decrease p < 0.05
Neutrophil number
in BALF Increase p < 0.001 Decrease p < 0.05
Macrophage number
in BALF Increase p < 0.001 Decrease p < 0.05
IL-5 level in BALF Increase p < 0.001 Decrease p < 0.001
IL-13 level in BALF Increase p < 0.001 Decrease p < 0.001
Eotaxin level in BALF Increase p < 0.001 Decrease p < 0.001
MCP-1 level in BALF Increase p < 0.001 Decrease p < 0.001
TGF-b level in BALF Increase p < 0.001 Decrease p < 0.05
IFNy level in BALF Decrease p < 0.01 Increase p < 0.05
Bronchorestriction
with Ach Increase p < 0.01 Decrease p < 0.01

Int. J. Environ. Res. Public Health 2021,18, 1416 34 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Keinan et al.
[73]2005
Natural ozone
scavenger
prevents asthma
in sensitized rats
Limonene
inhalation: Limonene reduces
inflammatory cell
infiltrates into lung
tissue and improves
airway restriction in
lungs of rats with
allergic asthma. No significances
provided in graphical
outcome report
Inflammatory cell
infiltrates Increase NA Decrease NA
Airway restriction
(Penh) Increase NA Decrease NA
Eucalyptol
inhalation:
Eucalyptol reduces
inflammatory cell
infiltrates into lung
tissue, but to a lesser
extent than
limonene, but does
not improve airway
restriction.
Inflammatory cell
infiltrates Increase NA Decrease NA
Airway restriction
(Penh) Increase NA NS
Limonene/ozone inhalation
Hansen et al.
[69]2013
Adjuvant and
inflammatory
effects in mice
after subchronic
inhalation of
allergen and
ozone-initiated
limonene
reaction
products
Limonene
inhalation:
Limonene
inhalation has no
significant effect on
inflammatory
response in
pre-sensitized mice.
No naive (baseline
before
OVA-sensitisation)
data shown.
OVA-specific IgE in
serum (ELISA) Increase NA NS
Eosinophil number in
BALF NS NS
Lymphocyte number
in BALF Increase NA NS
Neutrophil number in
BALF Increase NA NS
Macrophage number
in BALF Increase NA NS

Int. J. Environ. Res. Public Health 2021,18, 1416 35 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Ozone
inhalation:
Limonene +
ozone inhalation:
Limonene/ozone
mixture reduces
allergen-specific
reactions in
pre-sensitized mice.
No naive (baseline
before
OVA-sensitisation)
data shown.
OVA-specific IgE in
serum (ELISA) Increase NA Increase p < 0.05
Eosinophil number in
BALF Increase NA Decrease p < 0.05
Lymphocyte number
in BALF Increase NA Decrease NS
Neutrophil number in
BALF Increase NA Decrease p < 0.05
Macrophage number
in BALF Increase NA NS
Hansen et al.
[70]2016
Limonene and
its
ozone-initiated
reaction
products
attenuate
allergic lung
inflammation in
mice
Air
inhalation:
Limonene
inhalation:
Limonene
potentially reduces
airway
inflammation in
allergic mice,
however no
significances given.
Unclear graphical
and verbal
description of
outcomes and
signifi-cances.
No OVA-only
control.
No comparisons
between relevant
groups.
OVA-specific IgE in
serum (ELISA) Increase NA Decrease NA
OVA-specific IgG1 in
serum (ELISA) Increase NA NS
Eosinophil number in
BALF Increase NA NS
Lymphocyte number
in BALF Increase NA Increase NA
Neutrophil number in
BALF Increase NA Decrease NA
Macrophage number
in BALF Increase NA Decrease NA
IL-5 expression in
BALF Increase NA Decrease NA

Int. J. Environ. Res. Public Health 2021,18, 1416 36 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Ozone
inhalation:
Limonene +
ozone inhalation:
Limonene/ozone
mixture potentially
attenuates allergic
inflammation and
ozone-induced
pulmonary
irritation in allergic
mice.
Unclear graphical
and verbal
description of
outcomes and
signifi-cances.
No OVA-only
control.
No comparisons
between relevant
groups.
OVA-specific IgE in
serum (ELISA) Increase NA Decrease NA
OVA-specific IgG1 in
serum (ELISA) Increase NA Increase NA
Eosinophil number in
BALF Increase NA Decrease p < 0.05
Lymphocyte number
in BALF Increase NA Decrease NA
Neutrophil number in
BALF NS NA
Macrophage number
in BALF Increase NA Decrease NA
IL-5 expression in
BALF Increase NA Decrease NA
Linalool inhalation
Nakamura et al.
[72]2009
Stress repression
in restrained rats
by R-(-)-linalool
inhalation and
gene expression
profiling of their
whole blood cells
% of neutrophils Increase p < 0.05 Decrease NA Linalool inhalation
reverts
stress-induced
changes in
neutrophil and
lymphocyte
fractions.
% of lymphocytes Decrease p < 0.05 Increase NA

Int. J. Environ. Res. Public Health 2021,18, 1416 37 of 48
Table 6. Cont.
Main Author Year Title Measure Pre-treatment p-Value
Intervention
(Compared to
Pre-treatment)
p-Value Conclusion Comment
Fragrance inhalation
Fujiwara et al.
[71]1998
Effects of a
long-term
inhalation of
fragrances on
the
stress-induced
immunosup-
pression in
mice
Number of plaque forming cells (PFC)
in spleen:
Exposure to
natural fragrances
reverses
stress-induced
thymic involution
and activates the
immune system.
Lemon inhalation Decrease p < 0.05 Increase p < 0.05
Oakmoss inhalation Decrease p < 0.05 Increase p < 0.05
Labdanum inhalation Decrease p < 0.05 Increase p < 0.05
Tuberose inhalation Decrease p < 0.05 Increase p < 0.05
Thymic weight:
Lemon inhalation Decrease p < 0.05 Increase p < 0.05
Oakmoss inhalation Decrease p < 0.05 Increase p < 0.05
Labdanum inhalation Decrease p < 0.05 Increase p < 0.05
Tuberose inhalation Decrease p < 0.05 Increase p < 0.05

Int. J. Environ. Res. Public Health 2021,18, 1416 38 of 48
3.5.2. Synthesis of Animal Studies
Overall, animal studies point to largely coherent and unambiguous evidence for anti-
inflammatory effects of phytoncide inhalation in immune-challenged animals. Most exper-
imental setups could easily be compared to each other, since they were relatively homoge-
neous concerning their methodological approaches, sample sizes and outcome measures.
Many studies examined a broad range of inflammatory cytokines and specific antibodies,
some measuring both mRNA expression and protein levels. All studies reported widely
concordant outcomes, as almost all interventions were able to reverse the inflammation
induced by the distinct pre-treatments. Differences were found in the statistical analysis
and evaluation of results, with some studies providing rigorous statistical significance
calculations and others none. Especially the studies examining BVOC and eucalyptol
inhalation comprised a good study design and statistical evaluation, rendering strong
evidence for the beneficial effects of these treatments.
Out of the three studies examining limonene inhalation alone, two lacked significance
calculations [
67
,
73
] while one provided good statistical evaluations [
68
]. This made it
difficult to reliably conclude on the effects observed from limonene inhalation, but points
to a positive immune response derived from these experiments.
In contrast, two studies analysing the combined effects of limonene and ozone inhala-
tion in pre-sensitised animals failed to provide essential baseline values before sensitisation,
did not show any sensitisation-only control values and lacked significances and statisti-
cal comparisons between relevant groups [
69
,
70
]. These extensive shortcomings made it
impossible to draw any conclusions based on these experiments.
Lastly, single experiments measured the effects of linalool [
72
] and fragrance [
71
]
inhalation, respectively. One of them used out-dated methods (see Supplementary Material
for further details), which made the results less robust [71].
In conclusion, animal studies included in this review provide a solid evidence base
for anti-inflammatory, anti-asthmatic and anti-allergic effects upon inhalation of nature-
derived substances, especially BVOCs and eucalyptol.
4. Limitations of the Review
Limitations are found in the study selection process, which might be biased due to
the interdisciplinarity of the topic and the difficulties generated in performing a fitting
keyword search. Studies in the field of immunological health and nature have yet no
common narrative, making it difficult to formulate a search string that selectively targeted
studies from different disciplines. Also the variety of interventions and methodological
approaches made it challenging to incorporate all possible wordings into one comprehen-
sive search string. Especially within the field of animal studies, many studies focused
on the analysis of a single substance and only included this specific term in their titles
and keywords. This problem was handled by including snowballing as additional search
strategy. Nevertheless, it cannot be ruled out that individual studies which would also meet
this review’s inclusion criteria are missing from the present review. Furthermore, due to
the large number of articles resulting from the initial keyword search, it was not possible to
include more than two databases in the search. However, included databases gave entirely
overlapping outputs, with Scopus being the database having most relevant hits.
Association studies were excluded from this review, since they require different
quality assessment approaches and a separate synthesis, which was beyond the scope of
the review. Nevertheless, the field provides a high amount of large and rigorously designed
population-based association studies that render compelling insights into and provide
supplementary evidence for nature’s effect on the immune system. It therefore remains an
open task to systematically assess association studies published on this topic and evaluate
their outcomes, which could subsequently be merged with the final synthesis of the review
at hand.

Int. J. Environ. Res. Public Health 2021,18, 1416 39 of 48
5. Discussion
Until recently, scientific studies on nature and human health have largely been sep-
arated into traditional research fields such as environmental science, ecology, biology,
geography, landscape architecture, medicine, psychology, epidemiology and public health.
This has generated an impressive amount of data that is now starting to be fruitfully
brought together in a holistic perspective to stimulate a broad, interdisciplinary research
agenda on environmental health. A considerable number of reviews have emerged that
illuminate connections between nature and many human health challenges [
11
,
76
–
78
]; how-
ever, studies focusing on immunological health benefits have so far been underrepresented.
This review examined both human and animal studies and found a promising evidence
base for immunomodulatory effects following exposure to natural volatile substances or
environments, comprising anti-inflammatory, anti-asthmatic, anti-allergic and cytotoxic
responses from inhalation of diverse nature-derived compounds.
Whether or not natural environments have the potential to alleviate or even prevent
immunological health problems remains an open question that needs more investigation
from a multi-dimensional perspective. Animal studies are a strong tool to formulate a
robust initial starting point and can be used to back up findings from studies with human
subjects. In this review they represent an important data source concerning changes in
expression levels of immunological key molecules resulting from the inhalation of bio-
genic substances. They are used to dissect mRNA and protein expression of inflammatory
molecules in various tissues (lung, bronchioles, skin) and not only in blood, as it is com-
monly done in human experiments. The advantage of animal experiments is that they can
easily screen a set of potential hypotheses with relatively low effort and be carried out in
standardised, controllable conditions. Unequivocally, translating results into the human
setting is challenging, since the standardised experimental setups do not correspond to
the multi-faceted aspects of human life. Further, the ethical aspects of using experimental
animals need to be diligently balanced with the scientific gains and alternatives considered
whenever possible. At present, laboratory experiments are fundamental in disentangling
mechanistic pathways and establishing a data base that can eventually be tested in the
human system. The positive evidence base derived from animal experiments included in
the present review supports the notion that immune functioning might represent a direct,
central pathway of how nature and health are connected.
5.1. A Baseline for Future Research
Next to providing a comprehensive literature overview, the goal of this review was to
define a baseline of existing data for future research. However, since most included studies
diverged in their methodological approaches and only performed a superficial screening
of varying parameters, this baseline cannot be explicitly defined. Animal and human
experiments analysed largely distinct parameters. While animal studies screened a wide
range of different cytokines along with measuring detailed leukocyte subset distributions,
most human studies did not provide such a thorough analysis of inflammatory parameters
and focused more on specific cytokines and cytotoxic mediators. Thus, the human study
outcomes are relatively fragmented and lack a comprehensive insight into the distribu-
tion of other immune cell types and properties. Therefore, data derived from included
human and animal studies in this review can hardly be linked. Nevertheless, a clear anti-
inflammatory and cytotoxic tendency can be observed in the majority of studies. Decreased
expression levels of many pro-inflammatory molecules in various tissue and blood samples
along with an infiltration of leukocyte subsets and an increase of NK cell activity and
release of cytotoxic granules are results that may serve as a baseline for further studies.
5.2. Study Shortcomings and Recommendations for Future Study Designs
Overall, the synthesis of study findings carried out in this review presents some
promising evidence for the positive influence of nature exposure on various aspects of
immune functioning. However, considerable shortcomings in the design and conduct

Int. J. Environ. Res. Public Health 2021,18, 1416 40 of 48
especially of included human studies weaken their solidity. Most studies examined only
one independent replicate (trials were carried out once), leaving the study inherently
prone to random and systematic errors that can only be ruled out by trial repetitions
(at least three independent replicates). Moreover, the high number of human studies
completely lacking controls gives rise to major concerns regarding the reproducibility
and reliability of study outcomes. It has been shown that “within-subject” studies are
susceptible to bias since an individual’s initial physiological outcome value can influence
the extent and direction of post-intervention responses [
79
]. Furthermore, most studies
neither monitored environmental conditions nor adjusted for potential changes in airborne
parameters such as temperature, humidity or BVOC concentrations, which makes it hard
to account for individual parameters that might influence study outcomes. The majority of
animal studies did not show comparable shortcomings; however, selected studies lacked
important baseline values as well as statistical comparisons between relevant groups.
In general, included animal trials were also not repeated to generate three independent
replicates, which again represents a main drawback concerning the solidity of outcomes.
Ideally, future studies should encompass relatable animal and human experiments
including sufficient and adequate controls (placebo controls, controls of external conditions
as well as positive and negative control groups). Moreover, they should calculate effect
sizes and provide dose-response relationships. In order to guarantee outcome reproducibil-
ity, results should be consolidated by a minimum of three trial repetitions. Along with
improving the rigor of study designs, the study field should be expanded to bigger group
sizes and diverse environments and use more in-depth, state-of-the art analytical methods
such as next generation sequencing and big data technologies.
5.3. What Is the Optimal Type, Length, Season and Dosage of Nature Exposure?
Studies included in this review originate from different research fields and approach
the main question from distinct scientific angles. Although the included studies cover
different target groups, exposure types and durations, some essential questions concerning
the maximal efficacy of nature exposure still remain open. Which type of nature is best
suited for governing positive immunoregulatory effects? How long should nature exposure
last and which dosage do we need to ensure a prolonged, but safe, effect? When is the
optimal timeframe for nature contact in order to gain the maximum effect?
A factor that might influence the efficacy and magnitude of immune response to
natural environments is the type and variability of vegetation that humans are exposed
to, also termed eco- or geodiversity [
20
]. This encompasses both diverse landscape types
such as forests, meadows, mountains, coastal areas or oceans as well as the specific species
that can be found in these habitats. Human studies in this review mainly analyse forest
environments, but encompass a range of different forest types such as broad-leaved, conif-
erous or bamboo forests. Animal studies also highlight immunomodulatory properties of
different BVOCs and natural fragrances, but fail to relate them to specific vegetation types
in the natural world. Therefore, it cannot be concluded from the studies included here
which type of nature is best at conferring immunological benefits. However, catalogues
of different species’ BVOC emission rates from needles and leaves are available [
80
,
81
].
BVOC emission potentials depend on environmental factors such as temperature and
light, and species-specific factors such as plant age, developmental stage, intercellular
CO
2
concentration, stomatal conductance, leaf structure and gas storage potential [
31
,
82
].
Seasonal and diurnal variability have also been observed, and summer seemed to be the
best time for using forest environments for medical purposes due to highest temperatures
and best light conditions [
31
,
82
]. Preliminary evidence points to a characteristic daily
emission pattern which might at least apply on clear and calm days, while cloudy and
windy conditions appear to be least beneficial for the uptake of BVOCs from forest envi-
ronments [
31
,
82
], but further research is necessary to link these findings with policies that
promote public health gains.

Int. J. Environ. Res. Public Health 2021,18, 1416 41 of 48
Concerning the duration of exposure, this review included studies with exposures
ranging from a few hours to several days and weeks. Animal studies were especially
useful to elucidate longer exposures of up to four months. However, no conclusion on
minimal durations or dosage of nature exposure can be drawn due to the various exper-
imental setups and substances tested. Along with the missing calculation of effect sizes,
exposure characteristics need to be analysed in much more detail in order to correctly
interpret the study results. Very limited research has so far been carried out to address the
question of explicit exposure-response relationships between nature and human health,
and no studies included in this review provided relevant answers on this topic. However,
some large-scale, population-based research has tried to establish an association between
the duration of nature exposure and observed health effects. One study showed that
spending a minimum of two hours a week in nature improved overall health and well-
being, with positive associations peaking at approximately four hours of exposure [
83
].
Another study examining the associations between frequency, duration and intensity of
nature exposure observed a reduction in the prevalence of depression and high blood
pressure following nature visits longer than 30 min a week [
84
]. However, these studies
can only provide fractional answers and do not address immunological responses at all.
Another interesting question relates to the optimal exposure time point in life. The re-
view at hand includes studies with adults, elderly and children, but none of the studies
compared different exposure timeframes in different target groups; thus, this question
cannot be answered here. However, association studies provide evidence for optimal time
windows of nature exposure concerning immunological effects. Some studies show that
very early exposures during pregnancy and childhood exhibit a greater immunomodula-
tory effect than exposures later in life [
85
,
86
]. Other studies report that exposures during
late childhood up to the age of 10–15 years are associated with a lower risk of developing
multiple sclerosis [
87
,
88
]. However, it seems that later exposures during adult life are
also beneficial for certain immune parameters such as the immunoregulatory capacity of
helminth infections [
89
]. To gain profound knowledge on the optimal exposure time point
in life, longitudinal research monitoring immunological effects over longer timespans in
different target groups is needed.
5.4. Understanding Biomedical Mechanisms
A wide range of studies exists that describes immunomodulatory effects of natural sub-
stances
in vitro
or
in vivo
by other administrative pathways than mere inhalation. Recently,
the main molecular targets of terpenes in inflammatory diseases have been summarised
and categorised into six groups [
22
]: inflammatory mediators (interleukins, TNF
α
, NO and
COX2), transcription factors (NF-
κ
B, Nrf2), signal transduction molecules (MAPK sig-
nalling molecules such as ERK, p38, JNK, but also STAT3, TRPVs, CB(2)R), oxidative stress
(ROS, H
2
O
2
) and autophagy (by targeting apoptotic genes). Disentangling key molecular
components and action pathways provides important information for understanding the
biomedical mechanisms of how nature affects the immune system, which is essential for its
effective and targeted application. However, most studies used parenteral, oral or topical
administration, which possibly increases effect sizes compared to studies set in natural
environments. Therefore, it remains an open task to verify this evidence in more natural
interventions that mimic real life exposures. The review at hand focuses on studies that
only use inhalation of volatile substances as administrative pathway. Advantages of this
approach are a low implementation threshold and a relatively easy transferability into ther-
apy and policy measures. However, smaller and less coherent outcomes stemming from
experiments performed in natural environments should not be confused with evidence
obtained from invasive administrations. The challenge of the former will be to define effect
sizes and molecular action pathways as clearly and in as detailed a manner as in invasive
drug studies.

Int. J. Environ. Res. Public Health 2021,18, 1416 42 of 48
5.5. Nature-Based Clinical Applications
Considering the wide range of potential health benefits that nature provides, possibili-
ties of how to effectively harvest these natural goods for future therapeutic applications
can be envisioned. Owing to the rapid rise of diseases related to misled immunoregulation
and the frequent severe side effects of currently used immunomodulatory drugs [
90
,
91
],
there is a growing need for discovering new therapeutic options that are better tolerated.
Nature-based interventions might represent such an option; however, potential adverse
effects need to be considered when using nature in therapeutic and preventive clinical ap-
plications or implementing it in policy recommendations. Many terpenes are not harmful
themselves, but can easily oxidise upon air exposure and create allergenic or inflamma-
tory secondary molecules. One study reported irritations of the respiratory tract in mice
after exposure to oxidation products from
α
-pinene and d-limonene [
92
]. Another study
analysing the association between indoor VOCs and lung function reported
α
-pinene as
one of 10 substances negatively influencing human lung function [
93
]. Auto-oxidation of
various terpenes such as
α
- and
β
-pinene, limonene, camphor and
β
-phellandrene has
also been observed to have negative allergenic effects on atopic dermatitis [
94
–
96
]. Thus,
the observed adverse effects are concentration- and exposure-dependent, and call for a
detailed evaluation of the safe concentration range of terpenes. Furthermore, the use of
terpenes through “milder” exposure routes such as forest bathing or nature trips may
constitute a potentially safer therapeutic strategy than their direct intake or skin applica-
tion. Considering the small amount of data that exists on the potential adverse effects of
volatile biogenic substances, it is yet too early to draw any conclusion from these studies.
Nevertheless, the synthesis of the studies included in this review supports the notion that
breathing in nature-derived compounds is overall beneficial for reducing inflammation
and promoting immune homeostasis.
6. Outlook
Besides the possible clinical use of nature, the findings of the studies in this review
also point to potential benefits from promoting nature in official policy frameworks. In-
haling substances emitted from trees in close living proximity or being exposed to wood
in daily housing environments display promising examples of co-benefits derived from
sustainable developments tailored to support public health gains [
13
,
36
]. Possible pol-
icy measures could be to promote the conservation of natural environments, enforce the
construction of houses from natural materials such as wood and enhance green infrastruc-
ture in urban regions. Green infrastructure could be a very effective and multifunctional
measure to mitigate negative health impacts e.g., from urban heat islands in cities [
36
].
It may comprise vegetation planted in urban areas but also engineered structures that
fulfil specific functions such as sustainable urban drainage systems (SUDS) [97], and may
range from street trees, green roofs and walls, parks, allotment gardens to rain gardens and
water basins. Pioneering policy recommendations have defined nature as a cost-effective
planning tool for healthy cities [
98
]. Exposure to nature may therefore be an option to
address a range of health challenges and be most effective if designed to harvest both
direct as well as indirect benefits from nature (such as physical activity, social cohesion,
improved mood, etc.).
Key environmental pressures, such as disruption of ecosystems, loss of biodiversity,
habitat destruction, pollution and climate change are driven by human behaviour and
threaten the resilience of natural systems [
99
]. Many studies have shown that these changes
have a direct effect on living conditions on earth and significant implications for the public
health agenda worldwide [
2
,
11
]. Human health is profoundly dependent on undamaged
natural environments and functioning ecosystems that provide life supporting services
and resources [
1
]. It is well known that climate change caused by human interventions
in biogeochemical cycles negatively impacts ecological balances and concomitant ecosys-
tem services [
100
]. The Lancet Countdown on Health and Climate Change has recently
outlined pathways by which climate change will affect human health worldwide, span-

Int. J. Environ. Res. Public Health 2021,18, 1416 43 of 48
ning from exacerbation of existing health problems to introduction of new threats [
10
].
Anthropogenic climate change also has the potential to affect ecosystem services in terms of
immunological health provisioning and regulation, especially by impacting the occurrence
and spread of infectious diseases [
10
,
101
]. Examples are a change in the geographic range
and resulting rise of vector-borne diseases [
102
], ancient viruses emerging from thawing
permafrost [
103
,
104
] or the increase in allergenic pollen [
105
]. Mitigating climate change
might therefore yield considerable co-benefits for human health, and joined-up policy mak-
ing could be seen as a great opportunity for enforcing global health priorities in the 21st
century. This study may contribute to a novel, uncommon argumentation in future nature
protection and climate change mitigation debates and encourage green policy measures
that benefit human and planetary health alike. At large, this review supports the notion
that the conservation and creation of healthy human habitats along with the promotion
of a nature-connected lifestyle create a new opportunity to support immunological health
provisioning.
7. Conclusions
This systematic review gathers promising evidence that nature exposure influences
measurable immunological parameters in healthy individuals as well as in people suffering
from acute or chronic inflammatory conditions, and that inhaling certain volatile natural
compounds can have a beneficial effect on the elicited immune response. According to the
synthesis of the studies included in this review, nature exposure supports immunological
homeostasis and might offer promising strategies for therapeutic and preventive clinical
use. However, there is a lack of studies that rigorously address questions of selectivity,
effectivity or adverse effects deriving from nature exposure, let alone providing mecha-
nistic pathway analyses or a solid calculation of effect sizes. This is highly necessary to
guarantee outcome reproducibility and safety of nature-based therapies for future broader
applications. There is a need for expanding the study field to a larger scale and bigger
study cohorts, including different study populations and environments, a standardised
control for confounders and environmental conditions as well as the use of more in-depth,
state-of-the-art analytical methods and tools. This is essential to draw adequate conclusions
and envision a future potential for nature exposure in immunological disease prevention
or treatment.
Supplementary Materials:
The following are available online at https://www.mdpi.com/1660-4
601/18/4/1416/s1, Table S1: Risk of bias assessment of animal intervention studies following the
SYRCLE guidelines.
Author Contributions:
Each author made substantial contributions to the study. Conceptualisation:
L.A., S.S.C. and U.K.S.; methodology: L.A. and S.S.C.; data retrieval and curation: L.A.; assess-
ment: L.A. and S.S.C.; formal analysis: L.A. writing: L.A., S.S.C. and U.K.S.; project administration
and funding acquisition: U.K.S. All authors have read and agreed to the published version of
the manuscript.
Funding:
This work was supported by the 15th of June Foundation, Denmark (grant number 2020-
0374). The foundation had no involvement in the review.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: We have no archived datasets, since we did not produce any primary
data in this review.
Conflicts of Interest: The authors declare no conflict of interest.

Int. J. Environ. Res. Public Health 2021,18, 1416 44 of 48
References
1.
Whitmee, S.; Haines, A.; Beyrer, C.; Boltz, F.; Capon, A.G.; De Souza Dias, B.F.D.S.; Ezeh, A.; Frumkin, H.; Gong, P.; Head, P.; et al.
Safeguarding human health in the Anthropocene epoch: Report of The Rockefeller Foundation–Lancet Commission on planetary
health. Lancet 2015,386, 1973–2028. [CrossRef]
2. Myers, S.S. Planetary health: Protecting human health on a rapidly changing planet. Lancet 2017,390, 2860–2868. [CrossRef]
3.
United Nations Departement of Economic and Social Affairs. Population Division World Urbanization Prospects: The 2018 Revision;
UN DESA: New York, NY, USA, 2018.
4.
Frumkin, H.; Bratman, G.N.; Breslow, S.J.; Cochran, B., Jr.; Khan, P.H.; Lawler, J.J.; Levin, P.S.; Tandon, P.S.; Varanasi, U.; Wolf, K.L.;
et al. Nature Contact and Human Health: A Research Agenda. Environ. Health Perspect.
2017
,125, 075001. [CrossRef] [PubMed]
5.
Lauesen, L.M. Natural Environment BT. In Encyclopedia of Corporate Social Responsibility; Idowu, S.O., Capaldi, N., Zu, L., Das
Gupta, A., Eds.; Springer: Berlin/Heidelberg, Germany, 2013; pp. 1734–1742, ISBN 978-3-642-28036-8.
6.
Mitten, D.; Brymer, E. Outdoor and Environmental Education: Nature and Wellbeing. In Encyclopedia of Teacher Education;
Springer: Singapore, 2019; pp. 1–7. [CrossRef]
7. WHO. WHO Guidelines on Physical Activity and Sedentary Behaviour; WHO: Geneva, Switzerland, 2020; ISBN 9789241550536.
8.
European Environment Agency. Healthy Environment, Healthy Lives: How the Environment Influences Health and Well-being in Europe;
EEA: Copenhagen, Denmark, 2020; ISBN 9789294802125.
9.
Landrigan, P.J.; Fuller, R.; Acosta, N.J.R.; Adeyi, O.; Arnold, R.; Basu, N.; Baldé, A.B.; Bertollini, R.; Bose-O’Reilly, S.; Boufford, J.I.;
et al. The Lancet Commission on pollution and health. Lancet 2018,391, 462–512. [CrossRef]
10.
Watts, N.; Amann, M.; Ayeb-Karlsson, S.; Belesova, K.; Bouley, T.; Boykoff, M.; Byass, P.; Cai, W.; Campbell-Lendrum, D.;
Chambers, J.; et al. The Lancet Countdown on health and climate change: From 25 years of inaction to a global transformation for
public health. Lancet 2018,391, 581–630. [CrossRef]
11.
Frumkin, H.; Haines, A. Global Environmental Change and Noncommunicable Disease Risks. Annu. Rev. Public Health
2019
,40,
261–282. [CrossRef]
12. World Health Organization. Preventing noncommunicable diseases (NCDs) by reducing environmental risk factors. 2017.
13.
Fong, K.C.; Hart, J.E.; James, P. A Review of Epidemiologic Studies on Greenness and Health: Updated Literature Through 2017.
Curr. Environ. Health Rep. 2018,5, 77–87. [CrossRef]
14.
Corazon, S.S.; Sidenius, U.; Poulsen, D.V.; Gramkow, M.C.; Stigsdotter, U.K. Psycho-Physiological Stress Recovery in Outdoor
Nature-Based Interventions: A Systematic Review of the Past Eight Years of Research. Int. J. Environ. Res. Public Health
2019
,16,
1711. [CrossRef]
15. Hartig, T.; Mitchell, R.; De Vries, S.; Frumkin, H. Nature and Health. Annu. Rev. Public Health 2014,35, 207–228. [CrossRef]
16.
Kuo, M. How might contact with nature promote human health? Promising mechanisms and a possible central pathway. Front.
Psychol. 2015,6, 1093. [CrossRef]
17.
Haluza, D.; Schönbauer, R.; Cervinka, R. Green Perspectives for Public Health: A Narrative Review on the Physiological Effects
of Experiencing Outdoor Nature. Int. J. Environ. Res. Public Health 2014,11, 5445–5461. [CrossRef] [PubMed]
18.
McMahan, E.A.; Estes, D. The effect of contact with natural environments on positive and negative affect: A meta-analysis. J. Posit.
Psychol. 2015,10, 507–519. [CrossRef]
19.
Twohig-Bennett, C.; Jones, A. The health benefits of the great outdoors: A systematic review and meta-analysis of greenspace
exposure and health outcomes. Environ. Res. 2018,166, 628–637. [CrossRef] [PubMed]
20.
Craig, J.; Logan, A.C.; Prescott, S.L. Natural environments, nature relatedness and the ecological theater: Connecting satellites
and sequencing to shinrin-yoku. J. Physiol. Anthr. 2016,35, 1. [CrossRef]
21.
Kim, T.; Song, B.; Cho, K.S.; Lee, I. Therapeutic Potential of Volatile Terpenes and Terpenoids from Forests for Inflammatory
Diseases. Int. J. Mol. Sci. 2020,21, 2187. [CrossRef]
22.
Peterfalvi, A.; Miko, E.; Nagy, T.; Reger, B.; Simon, D.; Miseta, A.; Czéh, B.; Szereday, L. Much More Than a Pleasant Scent: A
Review on Essential Oils Supporting the Immune System. Molecules 2019,24, 4530. [CrossRef]
23.
Janeway, C.J.; Travers, P.; Walport, M. Immunobiology: The Immune System in Health and Disease, 5th ed.; Garland Science: New
York, NY, USA, 2001; ISBN 0-8153-3642-X.
24.
Rook, G. Regulation of the immune system by biodiversity from the natural environment: An ecosystem service essential to
health. Proc. Natl. Acad. Sci. USA 2013,110, 18360–18367. [CrossRef]
25.
Aerts, R.; Honnay, O.; Van Nieuwenhuyse, A. Biodiversity and human health: Mechanisms and evidence of the positive health
effects of diversity in nature and green spaces. Br. Med Bull. 2018,127, 5–22. [CrossRef]
26.
World Health Organisation. Connecting Global Priorities: Biodiversity and Human Health. A State of Knowledge Review; WHO: Geneva,
Switzerland, 2015.
27.
Hanski, I.; Von Hertzen, L.; Fyhrquist, N.; Koskinen, K.; Torppa, K.; Laatikainen, T.; Karisola, P.; Auvinen, P.; Paulin, L.; Mäkelä,
M.J.; et al. Environmental biodiversity, human microbiota, and allergy are interrelated. Proc. Natl. Acad. Sci. USA
2012
,109,
8334–8339. [CrossRef]
28.
Ruokolainen, L.; Von Hertzen, L.C.; Fyhrquist, N.; Laatikainen, T.J.; Lehtomäki, J.; Auvinen, P.; Karvonen, A.M.; Hyvärinen, A.M.;
Tillmann, V.; Niemelä, O.; et al. Green areas around homes reduce atopic sensitization in children. Allergy
2015
,70, 195–202.
[CrossRef]

Int. J. Environ. Res. Public Health 2021,18, 1416 45 of 48
29.
Von Hertzen, L.; Hanski, I.; Haahtela, T. Natural immunity. Biodiversity loss and inflammatory diseases are two global megatrends
that might be related. EMBO Rep. 2011,12, 1089–1093. [CrossRef] [PubMed]
30.
Laothawornkitkul, J.; Taylor, J.E.; Paul, N.D.; Hewitt, N. Biogenic volatile organic compounds in the Earth system: Tansley review.
New Phytol. 2009,183, 27–51. [CrossRef] [PubMed]
31. Chen, J.; Tang, J.; Yu, X. Environmental and physiological controls on diurnal and seasonal patterns of biogenic volatile organic
compound emissions from five dominant woody species under field conditions. Environ. Pollut.
2020
,259, 113955. [CrossRef]
[PubMed]
32.
Quintans, J.S.; Shanmugan, S.; Heimfarth, L.; Araújo, A.A.S.; Almeida, J.R.G.d.S.; Picot, L.; Quintans-Júnior, L.J. Monoterpenes
modulating cytokines—A review. Food Chem. Toxicol. 2019,123, 233–257. [CrossRef] [PubMed]
33.
Cho, K.S.; Lim, Y.-R.; Lee, K.; Lee, J.; Lee, J.H.; Lee, I. Terpenes from Forests and Human Health. Toxicol. Res.
2017
,33, 97–106.
[CrossRef]
34.
Grafetstätter, C.; Gaisberger, M.; Freidl, J.; Ritter, M.; Kolarž, P.; Pichler, C.; Thalhamer, J.; Hartl, A. Does waterfall aerosol influence
mucosal immunity and chronic stress? A randomized controlled clinical trial. J. Physiol. Anthr. 2017,36, 10. [CrossRef]
35.
Gaisberger, M.; Šanovi´c, R.; Dobias, H.; Kolarž, P.; Moder, A.; Thalhamer, J.; Selimovi´c, A.; Huttegger, I.; Ritter, M.; Hartl, A.
Effects of Ionized Waterfall Aerosol on Pediatric Allergic Asthma. J. Asthma 2012,49, 830–838. [CrossRef]
36.
Coutts, C.; Hahn, M. Green Infrastructure, Ecosystem Services, and Human Health. Int. J. Environ. Res. Public Health
2015
,12,
9768–9798. [CrossRef]
37.
World Health Organisation. Evolution of WHO Air Quality Guidelines: Past, Present and Future; WHO Regional Office for Europe:
Copenhagen, Denmark, 2017.
38.
Wu, K.; Yang, X.; Chen, D.; Gu, S.; Lu, Y.; Jiang, Q.; Wang, K.; Ou, Y.; Qian, Y.; Shao, P.; et al. Estimation of biogenic VOC
emissions and their corresponding impact on ozone and secondary organic aerosol formation in China. Atm. Res.
2020
,231,
104656. [CrossRef]
39.
Moher, D.; Shamseer, L.; Clarke, M.; Ghersi, D.; Liberati, A.; Petticrew, M.; Shekelle, P.; Stewart, L.A.; PRISMA-P Group. Preferred
reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015 statement. Syst. Rev. 2015,4, 1. [CrossRef]
40.
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, 1–11. [CrossRef] [PubMed]
41.
Effective Public Health Practice Project. Quality Assessment Tool for Quantitative Studies; Effective Public Health Practice Project:
Hamilton, ON, USA, 1998.
42.
Kilkenny, C.; Browne, W.J.; Cuthill, I.C.; Emerson, M.; Altman, D.G. Improving Bioscience Research Reporting: The ARRIVE
Guidelines for Reporting Animal Research. PLoS Biol. 2010,8, e1000412. [CrossRef] [PubMed]
43.
Hooijmans, C.R.; Rovers, M.M.; De Vries, R.B.M.; Leenaars, M.; Ritskes-Hoitinga, M.; Langendam, M.W. SYRCLE’s risk of bias
tool for animal studies. BMC Med. Res. Methodol. 2014,14, 43. [CrossRef]
44.
Im, S.G.; Choi, H.; Jeon, Y.-H.; Song, M.-K.; Kim, W.; Woo, J.-M. Comparison of Effect of Two-Hour Exposure to Forest and Urban
Environments on Cytokine, Anti-Oxidant, and Stress Levels in Young Adults. Int. J. Environ. Res. Public Health
2016
,13, 625.
[CrossRef]
45.
Han, J.-W.; Choi, H.; Jeon, Y.-H.; Yoon, C.-H.; Woo, J.-M.; Kim, W. The Effects 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] [PubMed]
46.
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. 2016,29, 212–218. [CrossRef] [PubMed]
47.
Lvu, 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]
48.
Mao, G.X.; Cao, Y.B.; Yang, Y.; Chen, Z.M.; Dong, J.H.; Chen, S.S.; Wu, Q.; Lyu, X.L.; Jia, B.B.; Yan, J.; et al. Additive Benefits of
Twice Forest Bathing Trips in Elderly Patients with Chronic Heart Failure. Biomed. Environ. Sci 2018,31, 159–162. [CrossRef]
49.
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]
50.
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.
[CrossRef]
51.
Mao, G.-X.; Cao, Y.-B.; Lan, X.-G.; He, Z.-H.; Chen, Z.-M.; Wang, Y.-Z.; Hu, X.-L.; Lv, Y.-D.; Wang, G.-F.; Yan, J. Therapeutic effect
of forest bathing on human hypertension in the elderly. J. Cardiol. 2012,60, 495–502. [CrossRef] [PubMed]
52.
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]
53.
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] [PubMed]

Int. J. Environ. Res. Public Health 2021,18, 1416 46 of 48
54.
Li, Q.; Morimoto, K.; Kobayashi, M.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Suzuki, H.; Li, Y.; Wakayama, Y.; et al.
Visiting a 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]
55.
Li, Q.; Morimoto, K.; Kobayashi, M.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Shimizu, T.; Li, Y.; 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]
56.
Seo, S.C.; Park, S.J.; Park, C.-W.; Yoon, W.S.; Choung, J.T.; Yoo, Y. Clinical and immunological effects of a forest trip in children
with asthma and atopic dermatitis. Iran. J. Allergy Asthma Immunol. 2015,14, 28–36. [PubMed]
57.
Tsao, T.-M.; Tsai, M.-J.; Hwang, J.-S.; Cheng, W.-F.; Wu, C.-F.; Chou, C.-C.; 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]
58.
Li, Q.; Kobayashi, M.; Wakayama, Y.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Shimizu, T.; Kawada, T.; Park, B.; et al.
Effect of Phytoncide from Trees on Human Natural Killer Cell Function. Int. J. Immunopathol. Pharmacol.
2009
,22, 951–959.
[CrossRef]
59.
Kiecolt-Glaser, J.K.; Graham, J.E.; Malarkey, W.B.; Porter, K.; Lemeshow, S.; Glaser, R. Olfactory influences on mood and
autonomic, endocrine, and immune function. Psychoneuroendocrinology 2008,33, 328–339. [CrossRef]
60.
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]
61.
Trellakis, S.; Fischer, C.; Rydleuskaya, A.; Tagay, S.; Bruderek, K.; Greve, J.; Lang, S.; Brandau, S. Subconscious olfactory influences
of stimulant and relaxant odors on immune function. Eur. Arch. Oto-Rhino-Laryngol.
2011
,269, 1909–1916. [CrossRef] [PubMed]
62. Ahn, C.; Lee, J.-H.; Kim, J.-W.; Park, M.-J.; Lee, S.-S.; Jeung, E.-B. Alleviation effects of natural volatile organic compounds from
Pinus densiflora and Chamaecyparis obtusa on systemic and pulmonary inflammation. Biomed. Rep.
2018
,9, 405–414. [CrossRef]
[PubMed]
63.
Ahn, C.; Jang, Y.-J.; Kim, J.-W.; Park, M.-J.; Yoo, Y.-M.; Jeung, E.-B. Anti-asthmatic effects of volatile organic compounds from
Chamaecyparis obtusa, Pinus densiflora, Pinus koraiensis, or Larix kaempferi wood panels. J. Physiol. Pharmacol. Off. J. Pol.
Physiol. Soc. 2019,69, 933–941. [CrossRef]
64.
Yang, H.; Ahn, C.; Choi, I.-G.; Choi, W.-S.; Park, M.-J.; Lee, S.-S.; Choi, D.; Jeung, E.-B. Estimation of the environmental effect of
natural volatile organic compounds from Chamaecyparis obtusa and their effect on atopic dermatitis-like skin lesions in mice.
Mol. Med. Rep. 2012,12, 345–350. [CrossRef] [PubMed]
65.
Kennedy-Feitosa, E.; Cattani-Cavalieri, I.; Barroso, M.V.; Romana-Souza, B.; Brito-Gitirana, L.; Valença, S.S. Eucalyptol promotes
lung repair in mice following cigarette smoke-induced emphysema. Phytomedicine 2019,55, 70–79. [CrossRef] [PubMed]
66.
Lee, H.-S.; Park, D.-E.; Song, W.-J.; Park, H.; Kang, H.; Cho, S.-H.; Sohn, S.-W. Effect of 1.8-Cineole in Dermatophagoides
pteronyssinus-Stimulated Bronchial Epithelial Cells and Mouse Model of Asthma. Biol. Pharm. Bull.
2016
,39, 946–952. [CrossRef]
[PubMed]
67.
Bibi, H.; Reany, O.; Waisman, D.; Keinan, E. Prophylactic treatment of asthma by an ozone scavenger in a mouse model. Bioorganic
Med. Chem. Lett. 2015,25, 342–346. [CrossRef] [PubMed]
68.
Hirota, R.; Nakamura, H.; Bhatti, S.A.; Ngatu, N.R.; Muzembo, B.A.; Dumavibhat, N.; Eitoku, M.; Sawamura, M.; Suganuma, N.
Limonene inhalation reduces allergic airway inflammation inDermatophagoides farinae-treated mice. Inhal. Toxicol.
2012
,24,
373–381. [CrossRef]
69.
Hansen, J.S.; Nielsen, G.D.; Sørli, J.B.; Clausen, P.A.; Wolkoff, P.; Larsen, S.T. Adjuvant and Inflammatory Effects in Mice After
Subchronic Inhalation of Allergen and Ozone-Initiated Limonene Reaction Products. J. Toxicol. Environ. Health Part A
2013
,76,
1085–1095. [CrossRef]
70.
Hansen, J.S.; Nørgaard, A.W.; Koponen, I.K.; Sørli, J.B.; Paidi, M.D.; Hansen, S.W.K.; Clausen, P.A.; Nielsen, G.D.; Wolkoff, P.;
Larsen, S.T. Limonene and its ozone-initiated reaction products attenuate allergic lung inflammation in mice. J. Immunotoxicol.
2016,13, 793–803. [CrossRef]
71.
Fujiwara, R.; Komori, T.; Noda, Y.; Kuraoka, T.; Shibata, H.; Shizuya, K.; Miyahara, S.; Ohmori, M.; Nomura, J.; Yokoyama, M.M.
Effects of a long-term inhalation of fragrances on the stress-induced immunosuppression in mice. Neuroimmunomodulation
1998
,
5, 318–322. [CrossRef] [PubMed]
72.
Nakamura, A.; Fujiwara, S.; Matsumoto, I.; Abe, K. Stress Repression in Restrained Rats by (R)-(
−
)-Linalool Inhalation and Gene
Expression Profiling of Their Whole Blood Cells. J. Agric. Food Chem. 2009,57, 5480–5485. [CrossRef] [PubMed]
73.
Keinan, E.; Alt, A.; Amir, G.; Bar-Yoseph, R.; Bibi, H.; Shoseyov, D. Natural ozone scavenger prevents asthma in sensitized rats.
Bioorg. Med. Chem. 2005,13, 557–562. [CrossRef] [PubMed]
74.
Bastos, V.P.; Gomes, A.S.; Lima, F.J.; Brito, T.S.; Soares, P.M.; Pinho, J.P.; Silva, C.S.; Santos, A.A.; Souza, M.H.; Magalhães, P.J.
Inhaled 1,8-Cineole Reduces Inflammatory Parameters in Airways of Ovalbumin-Challenged Guinea Pigs. Basic Clin. Pharmacol.
Toxicol. 2010,108, 34–39. [CrossRef] [PubMed]
75.
Lee, J.-E.; Lee, E.-H.; Park, H.-J.; Kim, Y.-J.; Jung, H.-Y.; Ahn, D.-H.; Cho, Y.-J. Inhibition of inflammatory responses in
lipopolysaccharide-induced RAW 264.7 cells byPinus densifloraroot extract. J. Appl. Biol. Chem. 2018,61, 275–281. [CrossRef]
76.
Wheeler, B.; Lovell, R.; Higgins, S.L.; White, M.P.; Alcock, I.; Osborne, N.J.; Husk, K.; Sabel, C.; Depledge, M. Beyond greenspace:
An ecological study of population general health and indicators of natural environment type and quality. Int. J. Health Geogr.
2015,14, 1–17. [CrossRef]

Int. J. Environ. Res. Public Health 2021,18, 1416 47 of 48
77.
White, M.P.; Elliott, L.R.; Gascon, M.; Roberts, B.; Fleming, L.E. Blue space, health and well-being: A narrative overview and
synthesis of potential benefits. Environ. Res. 2020,191, 110169. [CrossRef]
78.
Gascon, M.; Zijlema, W.; Vert, C.; White, M.P.; Nieuwenhuijsen, M.J. Outdoor blue spaces, human health and well-being:
A systematic review of quantitative studies. Int. J. Hyg. Environ. Health 2017,220, 1207–1221. [CrossRef]
79.
Song, C.; Ikei, H.; Miyazaki, Y. Elucidation of a Physiological Adjustment Effect in a Forest Environment: A Pilot Study. Int. J.
Environ. Res. Public Health 2015,12, 4247–4255. [CrossRef]
80.
Acosta Navarro, J.C.; Smolander, S.; Struthers, H.; Zorita, E.; Ekman, A.M.L.; Kaplan, J.O.; Guenther, A.; Arneth, A.; Riipinen,
I. Global emissions of terpenoid VOCs from terrestrial vegetation in the last millennium. J. Geophys. Res. Atmos.
2014
,119,
6867–6885. [CrossRef]
81.
Guenther, A.B.; Jiang, X.; Heald, C.L.; Sakulyanontvittaya, T.; Duhl, T.; Emmons, L.K.; Wang, X. The Model of Emissions of Gases
and Aerosols from Nature version 2.1 (MEGAN2.1): An extended and updated framework for modeling biogenic emissions.
Geosci. Model Dev. 2012,5, 1471–1492. [CrossRef]
82.
Meneguzzo, F.; Albanese, L.; Bartolini, G.; Zabini, F. Temporal and Spatial Variability of Volatile Organic Compounds in the
Forest Atmosphere. Int. J. Environ. Res. Public Health 2019,16, 4915. [CrossRef] [PubMed]
83.
White, M.P.; Alcock, I.; Grellier, J.; Wheeler, B.W.; Hartig, T.; Warber, S.L.; Bone, A.; Depledge, M.H.; Fleming, L.E. Spending at
least 120 minutes a week in nature is associated with good health and wellbeing. Sci. Rep. 2019,9, 1–11. [CrossRef] [PubMed]
84.
Shanahan, D.F.; Bush, R.; Gaston, K.J.; Lin, B.B.; Dean, J.; Barber, E.; Fuller, R.A. Health Benefits from Nature Experiences Depend
on Dose. Sci. Rep. 2016,6, 28551. [CrossRef]
85.
Riedler, J.; Braun-Fahrländer, C.; Eder, W.; Schreuer, M.; Waser, M.; Maisch, S.; Carr, D.; Schierl, R.; Nowak, D.; Von Mutius, E.
Exposure to farming in early life and development of asthma and allergy: A cross-sectional survey. Lancet
2001
,358, 1129–1133.
[CrossRef]
86.
Ege, M.J.; Herzum, I.; Büchele, G.; Krauss-Etschmann, S.; Lauener, R.; Roponen, M.; Hyvärinen, A.; Vuitton, D.A.; Riedler, J.;
Brunekreef, B.; et al. Prenatal exposure to a farm environment modifies atopic sensitization at birth. J. Allergy Clin. Immunol.
2008
,
122, 407–412. [CrossRef]
87. Gale, C.R.; Martyn, C.N. Migrant studies in multiple sclerosis. Prog. Neurobiol. 1995,47, 425–448. [CrossRef]
88.
Ahlgren, C.; Odén, A.; Lycke, J. A nationwide survey of the prevalence of multiple sclerosis in immigrant populations of Sweden.
Mult. Scler. J. 2012,18, 1099–1107. [CrossRef]
89.
Correale, J.; Farez, M.F. The impact of parasite infections on the course of multiple sclerosis. J. Neuroimmunol.
2011
,233, 6–11.
[CrossRef]
90.
Oray, M.; Abu Samra, K.; Ebrahimiadib, N.; Meese, H.; Foster, C.S. Long-term side effects of glucocorticoids. Expert Opin. Drug
Saf. 2016,15, 457–465. [CrossRef]
91.
Michot, J.; Bigenwald, C.; Champiat, S.; Collins, M.; Carbonnel, F.; Postel-Vinay, S.; Berdelou, A.; Varga, A.; Bahleda, R.;
Hollebecque, A.; et al. Immune-related adverse events with immune checkpoint blockade: A comprehensive review. Eur. J.
Cancer 2016,54, 139–148. [CrossRef] [PubMed]
92.
Rohr, A.C.; Wilkins, C.K.; Clausen, P.A.; Hammer, M.; Nielsen, G.D.; Wolkoff, P.; Spengler, J.D. Upper airway and pulmonary
effects of oxidation products of (+)-
α
-pinene, d -limonene and isoprene in balb/c mice. Inhal. Toxicol.
2002
,14, 663–684.
[CrossRef] [PubMed]
93.
Cakmak, S.; Dales, R.E.; Liu, L.; Kauri, L.M.; Lemieux, C.L.; Hebbern, C.; Zhu, J. Residential exposure to volatile organic
compounds and lung function: Results from a population-based cross-sectional survey. Environ. Pollut.
2014
,194, 145–151.
[CrossRef] [PubMed]
94.
Wei, Q.; Wei, C.-N.; Harada, K.; Minamoto, K.; Okamoto, Y.; Otsuka, M.; Ueda, A. Evaluation of Allergenicity of Constituents of
Myoga Using the Murine Local Lymph Node Assay. Int. J. Immunopathol. Pharmacol. 2010,23, 463–470. [CrossRef]
95.
Pesonen, M.; Suomela, S.; Kuuliala, O.; Henriks-Eckerman, M.-L.; Aalto-Korte, K. Occupational contact dermatitis caused
byD-limonene. Contact Dermat. 2014,71, 273–279. [CrossRef] [PubMed]
96.
Dharmagunawardena, B.; Takwale, A.; Sanders, K.J.; Cannan, S.; Rodger, A.; Ilchyshyn, A. Gas chromatography: An investigative
tool in multiple allergies to essential oils. Contact Dermat. 2002,47, 288–292. [CrossRef]
97.
Kumar, P.; Druckman, A.; Gallagher, J.; Gatersleben, B.; Allison, S.; Eisenman, T.S.; Hoang, U.; Hama, S.; Tiwari, A.;
Sharma, A.; et al. The nexus between air pollution, green infrastructure and human health. Environ. Int.
2019
,133, 105181.
[CrossRef]
98.
Shanahan, D.F.; Lin, B.B.; Bush, R.; Gaston, K.J.; Dean, J.H.; Barber, E.; Fuller, R.A. Toward Improved Public Health Outcomes
from Urban Nature. Am. J. Public Heal. 2015,105, 470–477. [CrossRef]
99.
Steffen, W.; Broadgate, W.; Deutsch, L.; Gaffney, O.; Ludwig, C. The trajectory of the Anthropocene: The Great Acceleration.
Anthr. Rev. 2014,2, 81–98. [CrossRef]
100.
Maes, J.; Teller, A.; Nessi, S.; Bulgheroni, C.; Konti, A.; Sinkko, T.; Tonini, D.; Pant, R. Mapping and Assessment of Ecosystems and
Their Services: An EU Ecosystem Assessment; European Comission: Maastricht, The Netherlands, 2020; ISBN 9789276178330.
101.
Altizer, S.; Ostfeld, R.S.; Johnson, P.T.J.; Kutz, S.; Harvell, C.D. Climate Change and Infectious Diseases: From Evidence to
a Predictive Framework. Science 2013,341, 514–519. [CrossRef]
102.
Rohr, J.R.; Dobson, A.P.; Johnson, P.T.; Kilpatrick, A.M.; Paull, S.H.; Raffel, T.R.; Ruiz-Moreno, D.; Thomas, M.B. Frontiers in
climate change–disease research. Trends Ecol. Evol. 2011,26, 270–277. [CrossRef] [PubMed]

Int. J. Environ. Res. Public Health 2021,18, 1416 48 of 48
103.
Houwenhuyse, S.; Macke, E.; Reyserhove, L.; Bulteel, L.; Decaestecker, E. Back to the future in a petri dish: Origin and impact of
resurrected microbes in natural populations. Evol. Appl. 2018,11, 29–41. [CrossRef] [PubMed]
104.
Legendre, M.; Bartoli, J.; Shmakova, L.; Jeudy, S.; Labadie, K.; Adrait, A.; Lescot, M.; Poirot, O.; Bertaux, L.; Bruley, C.; et al.
Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology. Proc. Natl. Acad.
Sci. USA 2014,111, 4274–4279. [CrossRef] [PubMed]
105.
Ziska, L.H.; Makra, L.; Harry, S.K.; Bruffaerts, N.; Hendrickx, M.; Coates, F.; Saarto, A.; Thibaudon, M.; Oliver, G.; Damialis, A.; et al.
Temperature-related changes in airborne allergenic pollen abundance and seasonality across the northern hemisphere: A retro-
spective data analysis. Lancet Planet. Health 2019,3, e124–e131. [CrossRef]