How Might Contact with Nature Promote Human Health? Promising Mechanisms and a Possible Central Pathway

Abstract

How might contact with nature promote human health? Myriad studies have linked the two; at this time the task of identifying the mechanisms underlying this link is paramount. This article offers: (1) a compilation of plausible pathways between nature and health; (2) criteria for identifying a possible central pathway; and (3) one promising candidate for a central pathway. The 21 pathways identified here include environmental factors, physiological and psychological states, and behaviors or conditions, each of which has been empirically tied to nature and has implications for specific physical and mental health outcomes. While each is likely to contribute to nature's impacts on health to some degree and under some circumstances, this paper explores the possibility of a central pathway by proposing criteria for identifying such a pathway and illustrating their use. A particular pathway is more likely to be central if it can account for the size of nature's impacts on health, account for nature's specific health outcomes, and subsume other pathways. By these criteria, enhanced immune functioning emerges as one promising candidate for a central pathway between nature and health. There may be others.

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MINI REVIEW
published: 25 August 2015
doi: 10.3389/fpsyg.2015.01093
Edited by:
Marc Glenn Berman,
The University of Chicago, USA
Reviewed by:
Sumitava Mukherjee,
Indian Institute of Management,
Ahmedabad, India
Teresa Helen Horton,
Northwestern University, USA
*Correspondence:
Ming Kuo,
Landscape and Human Health
Laboratory, Department of Natural
Resources and Environmental
Sciences, University of Illinois at
Urbana-Champaign, 1101 West
Peabody Drive, MC-637 Urbana, IL
61801, USA
fekuo@illinois.edu
Specialty section:
This article was submitted to
Cognitive Science,
a section of the journal
Frontiers in Psychology
Received: 04 March 2015
Accepted: 16 July 2015
Published: 25 August 2015
Citation:
Kuo M (2015) How might contact with
nature promote human health?
Promising mechanisms
and a possible central pathway.
Front. Psychol. 6:1093.
doi: 10.3389/fpsyg.2015.01093
How might contact with nature
promote human health? Promising
mechanisms and a possible central
pathway
Ming Kuo
*
Landscape and Human Health Laboratory, Department of Natural Resources and Environmental Sciences, University of
Illinois at Urbana-Champaign, Urbana, IL, USA
How might contact with nature promote human health? Myriad studies have linked the
two; at this time the task of identifying the mechanisms underlying this link is paramount.
This article offers: (1) a compilation of plausible pathways between nature and health;
(2) criteria for identifying a possible central pathway; and (3) one promising candidate
for a central pathway. The 21 pathways identified here include environmental factors,
physiological and psychological states, and behaviors or conditions, each of which has
been empirically tied to nature and has implications for specific physical and mental
health outcomes. While each is likely to contribute to nature’s impacts on health to some
degree and under some circumstances, this paper explores the possibility of a central
pathway by proposing criteria for identifying such a pathway and illustrating their use.
A particular pathway is more likely to be central if it can account for the size of natur e’s
impacts on health, account for nature’s specific health outcomes, and subsume other
pathways. By these criteria, enhanced immune functioning emerges as one promising
candidate for a central pathway between nature and health. There may be others.
Keywords: natural environment, greenspace, immune, mechanism, mental health, literature review
Introduction
Contact with nature has been tied to health in a plenitude of studies. Time spent in and
around tree-lined streets, gardens, parks, and forested and agricultural lands is consistently
linked to objective, long-term health outcomes. The less green a person’s surroundings, the
higher their risk of morbidity and mortality – even when controlling for socioeconomic
status and other possible confounding variables. The range of specific health outcomes tied
to nature is startling, including depression and anxiety disorder, diabetes mellitus, attention
deficit/hyperactivity disorder (ADHD), various infectious diseases, cancer, healing from surgery,
obesity, birth outcomes, cardiovascular disease, musculoskeletal complaints, migraines, respiratory
disease, and others, reviewed below. Finally, neighborhood greenness has been consistently tied to
life expectancy and all-cause mortality (see Table 3 in the Supplementary Materials).
These findings raise the possibility that such contact is a major health determinant, and that
greening may constitute a powerful, inexpensive public health intervention. It is also possible,
however, that the consistent correlations between greener surroundings and better health reflect
self-selection – healthy people moving to or staying in greener surroundings. Examining the
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Kuo Nature-health mechanisms
potential pathways by which nature might promote health seems
paramount — both to assess the credibility of a cause-and-effect
link and to suggest possible nature-based health interventions.
Toward that end, this article offers: (1) a compilation of plausible
pathways between nature and health; (2) criteria for identifying a
possible central pathway; and (3) one promising candidate for a
central pathway.
How Nature Might Promote Health:
Plausible Pathways
How might contact with nature promote health? To date,
reviews and studies addressing multiple possible mechanisms
(Groenewegen et al., 2006, 2012; Sugiyama et al., 2008;
de Vries et al., 2013; Hartig et al., 2014)havefocused
on four – air quality, physical activity, stress, and social
integration. But the burgeoning literature on nature benefits
has revealed an abundance of possible mechanisms: as Figure 1
shows, this review identifies 21 plausible causal pathways
from nature to health. Each has been empirically tied to
contact with nature while accounting for other factors, and
is empirically or theoretically tied to specific health outcomes
(for details on the scope of this review, see Table 1 in the
Supplementary Materials). The 21 pathways identified here
include environmental factors, physiological and psychological
states, and behaviors or conditions, and are summarized below
(for more details on each of these pathways, see Table 2 in the
Supplementary Materials).
Envir onmental Conditions
Some of the plausible pathways from contact with nature
to improved health stem from specific environmental
conditions. Natural environments contain chemical and
biological agents with known health implications. Many
plants give off phytoncides
— antimicrobial volatile organic
compounds — which reduce blood pressure, alter autonomic
activity, and boost immune functioning, among other effects
(Komori et al., 1995; Dayawansa et al., 2003; Li et al., 2006,
2009). The air in forested and mountainous areas, and near
moving water, contains high concentrations of negative air ions
(Li et al., 2010), which reduce depression (Terman et al.,
1998; Goel et al., 2005), among other effects (Table 2 in the
Supplementary Materials). These environments also contain
mycobacterium vaccae
, a microorganism that appears to boost
immune functioning (see Lowry et al., 2007 for review).
Similarly, environmental biodiversity
has been proposed to
play a key role in immune function via its effects on the
microorganisms living on skin and in the gut, although the
evidence for this is mixed (Table 2 in the Supplementary
Materials).
The sights
and sounds of nature also have important
physiological impacts. Window views and images of
nature reduce sympathetic nervous activity and increase
parasympathetic activity (e.g., Gladwell et al., 2012; Brown
et al., 2013), restore attention (e.g., Berto, 2005), and promote
healing from surgery (Ulrich, 1984). Sounds of nature played
over headphones increase parasympathetic activation (Alvarsson
et al., 2010). These sympathetic and parasympathetic effects
drive the immune system’s behavior (for review, see Kenney and
Ganta, 2014), with long-term health consequences.
In built environments, trees and landscaping may promote
health not only by contributing positive factors like phytoncides
but also by reducing negative factors. Air pollution
is associated
with myocardial inflammation and respiratory conditions
(Villarreal-Calderon et al., 2012). High temperatures
can
cause heat exhaustion, heat-related aggression and violence,
and respiratory distress due to heat-related smog formation
(Anderson, 2001; Akbari, 2002; Tawatsupa et al., 2012). And
violence
affects physical and mental health (e.g., Groves et al.,
1993). Vegetation filters pollutants from the air (although see
Table 2 in the Supplementary Materials for details), dampens the
urban heat island (e.g., Souch and Souch, 1993), and appears
to reduce violence (Table 2 in the Supplementary Materials for
review).
Physiological and Psychological States
Some of the plausible pathways between contact with nature and
health involve short-term physiological and psychological effects,
which, if experienced regularly, could plausibly account for long-
term health effects.
Blood tests before and after walks in different environments
reveal that levels of health-protective factors increase after
forest but not urban walks. Didehydroepiandrosterone
(DHEA)
increases after a forest walk (Li et al., 2011); DHEA has cardio
protective, anti-obesity, and anti-diabetic properties (Bjørnerem
et al., 2004). Similarly, time in nature increases adiponectin
(Li et al., 2011), which protects against atherosclerosis, among
other things (Table 2 in the Supplementary Materials), and
the immune system
’s anti-cancer (so-called “Natural Killer,” or
NK) cells and related factors (Table 2 in the Supplementary
Materials). NK cells play important protective roles in cancer,
viral infections, pregnancy, and other health outcomes (Orange
and Ballas, 2006).
Further, walks in forested, but not urban areas, reduce
the levels of health risk factors, specifically inflammatory
cytokines (Mao et al., 2012), and elevated blood glucose
(Ohtsuka
et al., 1998). Inflammatory cytokines are released by the
immune system
in response to threat, and have been implicated
in diabetes, cardiovascular disease, and depression (Table 2 in the
Supplementary Materials). Chronically elevated blood glucose
carries multiple health risks, including blindness, nerve damage,
and kidney failure (Sheetz and King, 2002). The powerful effects
of a walk in a forest on blood glucose are particularly striking
(Table 2 in the Supplementary Materials for review).
Contact with nature has a host of other physiological
effects related to relaxation
or stress reduction (Table 2 in the
Supplementary Materials). The experience of nature helps shift
individuals toward a state of deep relaxation and parasympathetic
activity, which improves sleep (El-Sheikh et al., 2013), boosts
immune function in a number of ways (Kang et al., 2011), and
counters the adverse effects of stress on energy metabolism,
insulin secretion, and inflammatory pathways (Bhasin et al.,
2013). Evidence suggests this pathway contributes substantially to
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Kuo Nature-health mechanisms
FIGURE 1 | The nature-health link: filling in the details. This Figure
summarizes the state of the scientific literature on nature and health, listing (1)
the “active ingredients” in nature that have been identified as having impacts on
health or health antecedents; (2) physiological/psychological states, behaviors,
and conditions tied to both nature and health; and (3) specific health outcomes
that have been tied to nature (controlling for socioeconomic variables). Note that
physical activity (in brackets) is only sometimes tied to nature; and that allergies,
asthma, and eczema are sometimes positively and sometimes negatively tied to
nature. DHEA: didehydroepiandrosterone; acute UTI: acute urinary tract
infection; ADHD: attention deficit hyperactivity disorder; IDIC: infectious disease
of the intestinal canal; MUPS: medically unexplained physical symptoms; URTI:
upper respiratory tract infection.
the link between nature and health (Table 2 in the Supplementary
Materials).
Three psychological effects of nature — experiences of awe
(Shiota et al., 2007), enhanced vitality (Ryan et al., 2010), and
attention restoration (Table 2 in the Supplementary Materials) —
offer additional possible pathways between nature and health.
Regular experiences of awe are tied to healthier, lower levels of
inflammatory cytokines (Stellar et al., 2015); the ties between
nature and awe, and awe and cytokines, respectively, may help
explain the effects of forest walks on cytokines above. Similarly,
feelings of vitality predict resistance to infection (Cohen et al.,
2006) and lowered risk of mortality (Penninx et al., 2000).
Attention restoration could theoretically reduce accidents caused
by mental fatigue and, by bolstering impulse control, reduce risky
health behaviors such as smoking, overeating, and drug or alcohol
abuse (Wagner and Heatherton, 2010).
Behaviors and Conditions
The remaining four possible pathways between contact with
nature and health identified here involve behaviors and
conditions: physical activity
,obesity, sleep,andsocialties.
Physical activity is a major contributor to health (Centers for
Disease Control and Prevention [CDC], 2015), and intuitively
we associate green space with physical activity — but empirically
this relationship is surprisingly inconsistent (Table 2 in the
Supplementary Materials) and may hold only under certain
conditions and for certain populations. Perhaps still more
surprising, while greener residential areas do not consistently
predict physical activity, they do consistently predict lower
rates of obesity (for review, see Table 2 in the Supplementary
Materials); this suggests the pathway between nature and obesity
may depend less on nature’s effects on physical activity and more
on its effects on adiponectin, stress, and impulse control. Both
sleep and social ties are major contributors to health (Table 2 in
the Supplementary Materials); contact with nature contributes to
both better sleep (Morita et al., 2011; Astell-Burt et al., 2013)and
stronger social ties (see Table 2 in the Supplementary Materials
for review).
Exploring the Possibility of a Central
Pathway
Each of the mechanisms above is likely to contribute to nature’s
impacts on health to some degree and under some circumstances.
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Kuo Nature-health mechanisms
Most likely, some pathways will play a larger role than others.
This paper explores the possibility that one or a few pathways
may explain the lion’s share of the link between nature and
health by proposing criteria for identifying central pathways and
illustrating the application of these criteria.
First, a pathway is more likely to be central if it can account
for the size of nature’s impacts on health. A study of over 345,000
people living in greener and less green residential surroundings
revealed large differences in the prevalence of disease; even
after controlling for socioeconomic status, prevalence for 11
major categories of disease was at least 20% higher among the
individuals living in less green surroundings (Maas et al., 2009).
For a single pathway to plausibly account for the bulk of the tie
between nature and health, the mechanism involved would need
to have substantial effects on health, and be substantially affected
by contact with nature.
Second, a pathway is more likely to be central if it can
account for specific health outcomes tied to nature. Although
health is often treated as a unitary construct in the nature-health
literature, poor health takes a multiplicity of separable, largely
independent forms. A pathway that leads to one health outcome
may not lead to others; for example, reduced air pollution may
lessen respiratory symptoms, but is not likely to affect ADHD
symptoms. A central pathway between nature and health should
accountformany,ifnotmost,ofthespecifichealthoutcomestied
to nature.
Third, a pathway is more likely to be central if it subsumes
other pathways. To the extent that multiple nature-health
pathways feed into a particular pathway between nature and
health, that pathway is more central to the relationship between
nature and health.
These three criteria can be applied to any given pathway to
determine its centrality. In this paper, they are applied to one
particular pathway: enhanced immune functioning.
Criterion #1: Accounting for the Size of the
Nature-Health Link
Determining whether a particular pathway can account for the
size of the nature–health link requires examining the effect sizes
for the nature–mechanism and mechanism–health relationships.
For the immune system, the existing literature reveals both these
effect sizes to be large.
Time spent in nature has substantial beneficial effects on
the immune system, raising positive indicators, and lowering
negative ones. Two 2-h forest walks on consecutive days
increased the number and activity of anti-cancer NK cells by 50
and 56%, respectively, and activity remained significantly boosted
even a month after returning to urban life — 23% higher than
before the walks (Li, 2010). Moreover, extended time in a forest
decreased inflammatory cytokines implicated in chronic disease
by roughly one-half (Mao et al., 2012). Urban walks have no such
effect.
The immune system, in turn, has powerful effects on health.
The cytotoxic activity of NK cells is important in preventing
cancer – in an 11-year study, the incidence of cancer among the
individuals in the middle and top third of cytotoxic activity was
roughly 40% less than that among individuals with low levels
of NK activity (Imai et al., 2000). NK cells also play important
health-promoting roles in fighting viral and other infections, in
autoimmune disorders, and in pregnancy (see Orange and Ballas,
2006 for review). Moreover, inflammatory cytokines are thought
to play an important role in a host of chronic diseases, including
diabetes, cardiovascular disease, and depression (Cesari et al.,
2003; Wellen and Hotamisligil, 2005; Dowlati et al., 2010).
It appears that enhanced immune function fulfills the first
criterion for a central pathway: it can account for the size of
nature’s apparent impacts on health.
Criterion #2: Accounting for the Specific
Health Outcomes Tied to Nature
Each specific health outcome tied to nature constitutes a testable
hypothesis for any proposed central pathway; the more specific
health outcomes a pathway can account for, the more central its
role.
Contact with nature has been linked to a plethora of
specific health outcomes; in general, the more contact with
nature, the better the health outcome, even after controlling
for socioeconomic status and other factors. For each of the
following, available evidence points to a favorable impact: acute
urinary tract infections, anxiety disorder, ADHD, birth outcomes,
cancer, cardiovascular disease, depression, diabetes mellitus,
healing from surgery, infectious disease of the intestinal canal,
musculoskeletal complaints, medically unexplained physical
symptoms (MUPS), migraines, upper respiratory tract infections,
respiratory disease, and vertigo (for details, see Table 3 in the
Supplemental Materials). For allergies, asthma, and eczema, the
apparent impact of nature varies; depending on the specific
measures used and the place, the relationships are positive,
negative, or null (Table 3 in the Supplemental Materials).
To determine whether enhanced immune functioning could
account for these specific health outcomes, the literature on
immune functioning and each of the 18 specific outcomes
was collected and reviewed. Available evidence indicates that
enhanced immune functioning may be able to account, wholly
or partially, for all 18 (see Figure 2).
Oneofthechieffunctionsoftheimmunesystemisto
ward off infectious disease, protecting the body from bacterial,
parasitic, fungal, and viral infections (National Institutes of
Health [NIH], 2012). Thus enhanced immune function could
clearlyexplainwhycontactwithnature is tied to lower rates of
acute urinary tract infections, infectious disease of the intestinal
canal, and upper respiratory tract infections. Further, for health
outcomes with multiple possible origins, enhanced immune
function can account for cases with infectious origins — for
example, infectious forms of respiratory disease but perhaps
not forms of respiratory disease with non-infectious origins.
Similarly, enhanced immune function can account for MUPS and
vertigo with infectious origins (Bovo et al., 2006; Deary et al.,
2007, respectively).
Two other key roles of the immune system are to assist
in wound healing, and to seek out and destroy tumor cells.
Thus enhanced immune function can account for the effect of a
hospital view of nature on recovery from surgery (Ulrich, 1984),
as well as the relationship between residential greenness and
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Kuo Nature-health mechanisms
FIGURE 2 | Enhanced immune function as one possible central
pathway. All items in white text are currently known or have been
proposed to be causally tied to immune function. As the Figure
shows, enhanced immune function can account, at least partially, for
each of the specific health outcomes currently tied to nature and
may subsume, at least partially, all but two of the other pathways.
Other pathways may also contribute the nature-health link, and other
central pathways may exist.
lower rates of cancer (Li et al., 2010). Further, the immune system
governs inflammation, which is involved in allergies (National
Institutes of Health [NIH], 2015), anxiety disorder (Salim et al.,
2012), asthma (Murdoch and Lloyd, 2010), cardiovascular disease
(Mari et al., 2002; Ho et al., 2010; Schiffrin, 2013), depression
(Calabrese et al., 2014), diabetes mellitus (Pedicino et al.,
2013), eczema (National Institutes of Health [NIH], 2014), and
musculoskeletal complaints (Ji et al., 2002; Wang et al., 2011).
Finally, immune functioning is important in healthy birth weight
(Moffett et al., 2014), and is suspected to play a role in ADHD
(Segman et al., 2002; Budziszewska et al., 2010) and migraines
(Bruno et al., 2007).
Available evidence indicates that enhanced immune function
fulfills the second criterion for a central pathway.
Criterion #3: Subsuming Other Pathways
between Nature and Health
A nature–health pathway is more central if it subsumes other
pathways; the more other pathways it subsumes, the more central
its role. As Figure 2 shows, the current literature suggests
enhanced immune function can subsume as many as 18 out of
the 20 other possible pathways between nature and health.
Enhanced immune function is known to wholly or partially
subsume 11 other pathways. Each of the following is known
to enhance immune function — adiponectin (Fantuzzi, 2013),
reduced air pollution (e.g., Nadeau et al., 2010), awe (Stellar
et al., 2015), normalized levels of blood glucose (as compared
to elevated levels, Geerlings and Hoepelman, 1999), reduced
obesity (de Heredia et al., 2012), physical activity (Shepherd
et al., 1991), phytoncides (Li et al., 2009), better sleep
(Besedovsky et al., 2012), social ties (Kiecolt-Glaser et al.,
2002; Robles and Kiecolt-Glaser, 2003), relaxation and stress
reduction (Abboud et al., 2012; Bhasin et al., 2013), and
reduced immediate and long-term traumatic stress due to
violence (e.g., Baum et al., 1993). Note that these pathways
may be partially or wholly subsumed by the enhanced immune
functioning pathway between nature and health; if a pathway
contributes to health via both the immune system and other
effects, it is partially subsumed by the immune function
pathway.
For seven additional pathways, while there is no direct
evidence tying them to human immune function, there is
indirect evidence suggesting such a tie. DHEA (Hazeldine
et al., 2010), mycobacterium vaccae (Lowry et al., 2007), and
negative air ions (Yamada et al., 2006)areallknownto
improve immune function in mice. Vitality enhances resistance
to upper respiratory tract infections (see vigor findings in Cohen
et al., 2006), an effect mostly likely mediated via enhanced
immune functioning. Both visual (e.g., Brown et al., 2013)and
auditory (e.g., Alvarsson et al., 2010) nature stimuli are likely to
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Kuo Nature-health mechanisms
boost immune function by way of their demonstrated effects
on parasympathetic activity, and the subsequent effects of
parasympathetic activity on immune function (Kenney and
Ganta, 2014). And environmental biodiversity has been proposed
to help train and regulate the immune system, although the
findings here are correlational and mixed (e.g., Ruokolainen et al.,
2014).
Enhanced immune function fulfills the third criterion for a
central pathway.
Conclusion
This review reveals a multiplicity of mechanisms by which
contact with nature might promote health, as well as a promising
candidate for a central pathway. There may be other mechanisms,
such as other physiological effects, reduced accidents, and
healthier behaviors. There may also be other central contributors
to the nature-health link – of those reviewed here, deep
relaxation, attention restoration and impulse control, sleep, and
social ties seem particularly worthy of attention. No doubt some
of the plausible pathways identified here will prove either not
to contribute substantially to nature’s impact on health, or to
contribute only under certain limited circumstances; here, the
roles of improved air quality, environmental biodiversity and
microbiota, and physical activity merit closer study.
These limitations notwithstanding, this review makes a
number of contributions to our understanding of nature and
health, to future investigation in this area, and to the creation
of healthy human habitats. The multiplicity of nature–health
pathways identified here lends credibility to the hypothesis that
nature actually promotes health, aswellasapotentialexplanation
for the startling size and scope of nature’s apparent impact.
With so many contributing pathways operating in concert, the
cumulative effect could be quite large even if many of the
individual pathways contribute only a small effect – that is, the
effect of exposure to phytoncides plus exposure to mycobacteria
vaccae plus increased adiponectin plus stronger social ties plus
better sleep, etc., could indeed be quite large, and if some of the
pathways, such as enhanced immune function, contribute a large
effect, the combined effect would be larger still.
For future work in this area, the criteria here give researchers
interested in central mechanisms a means of using existing
literature to assess the centrality of a particular mechanism. In
addition the detailed reviews provided in the Supplementary
Materials may provide a useful starting point for researchers
interested in specific pathways or specific health outcomes of
nature.
Finally, the findings here can help guide the creation of
healthy human habitats. The existing literature speaks to the
value not only of “wild” nature but also “everyday” nature –
the views and green spaces where we live. That physical activity
is not consistently related to greener environments suggests
that our conceptualization of health-promoting greenspaces
should center at least as much on oases as on ball fields,
and on greenspaces for walking and quiet contemplation as
much as on recreation areas. The findings here suggest that
such oases should incorporate plants — especially trees, soil,
and water (preferably moving) — and should be designed to
induce feelings of deep relaxation, awe, and vitality. Providing
these green oases, especially in areas where health risks are
high and landscaping is sparse, might be an inexpensive,
powerful public health intervention and address persisting health
inequalities.
Acknowledgments
I am grateful to the TKF Foundation for encouragement and
financial assistance in gathering some of the necessary literature
for this work through its Nature Sacred National Awards
program. This review is more clear, better structured, more
interesting, and more complete than it would have been without
the thoughtful intervention of Kirstin Wilcox, Rafter S. Ferguson,
Dexter Locke, Meghan Frankel, Natalia Maurer, and reviewers
Sumitava Mukherjee and Teresa Helen Horton: thank you!
Supplementary Material
The Supplementary Material for this article can be found
online at: http://journal.frontiersin.org/article/10.3389/fpsyg.
2015.01093
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