Green space and health
Research on the healthy and restorative effects of green space has been rapidly expanding
over the past decades, and the field is fast moving toward maturity (van den Berg
and van den Berg, 2014; Capaldi et al., 2015). Findings converge in showing that regular
contact with green space can enhance well-being and alleviate stress, and may even
mitigate income-related health inequalities regarding chronic diseases and life expectancy
(Mitchell and Popham, 2008). In response to these insights, there has been a surge
of initiatives to (re)connect people with nature, especially those without the ability
or opportunity to engage with green space as part of their usual lifestyle. Many of
these initiatives have focused on “bringing nature to people” by greening of places
in people's nearby environment, such as schoolyards, urban public spaces, hospitals,
classrooms, and offices (Wolch et al., 2014; van den Berg et al., 2016). Other initiatives
have aimed at “bringing people to nature” by encouraging and facilitating adults and
children to actively participate in nature-based activities (Bragg and Atkins, 2016).
These activities range from health promotion programs and projects for the general
population, like green gyms or community gardening, to more therapeutic interventions
for individuals with a defined need, like care farms, walk-and-talk coaching, or horticultural
therapy.
Nature-based activities are increasingly gaining momentum as a cost-effective, easy-to-do,
low-risk, and enjoyable preventive and therapeutic intervention that is particularly
beneficial for people who do not have the ability or opportunity to engage with nature
as part of their usual lifestyle (Allen and Balfour, 2014). For example, a calculation
of the social return on investment of a health walks program in Glasgow revealed a
cost-benefit ratio of £8 for every £1 invested (Carrick, 2013). It therefore seems
important that this type of intervention becomes more routinely integrated into everyday
health care practice. In particular, general practitioners and other primary health
professionals have been targeted as key actors who can support such integration by
writing more “green prescriptions” (Hine et al., 2008; Patel et al., 2011; Nisbet
and Lem, 2015). However, although objective data are scarce, the overall impression
in most countries is that referral rates tend to be very low (Maas and Verheij, 2007;
Allen and Balfour, 2014). Therefore, a key question is: what is needed to make health
professionals write more green prescriptions?
Challenges for the health care sector
The low rates of green prescriptions may at least be partly due to the complexity
of translating innovations into health care practice, which often requires changes
on the side of both the providers of the innovative services and the health professionals
who refer patients to these services (Fleuren et al., 2004). Although, each country
has its unique context, some common challenges for providers of nature-based interventions
can be identified that transcend national boundaries (Bragg and Atkins, 2016). These
challenges include (a) the development of a consistent terminology and common language
to describe the field, (b) better collaboration between providers and streamlining
of communication to health professionals, (c) professionalization of the services
by developing quality standards and tools for monitoring and evaluation of quality
and effectiveness, and (d) improvement of access to services to health professionals,
for example, by registration in service directories and inclusion in clinical guidelines.
Regarding the development of a common language, there is an increasingly felt need
for a concise and recognizable umbrella term to promote the full range of preventive
and therapeutic health interventions and programs that make use of elements of nature.
“Green exercise” has recently been propagated as a general concept that “implies a
synergistic health benefit of being active in the presence of nature” (Barton et al.,
2016). However, the term is not suitable as an overarching term because it reduces
green space to a supportive environment for exercise, thereby ignoring the direct
beneficial effects of being immersed in green. “Green care” has been broadly defined
as “utilizing plants, animals, and landscapes to create interventions to promote health
and well-being” (Sempik and Bragg, 2013), and thus seems to offer promise as a sufficiently
broad term of choice. Contrary to this notion, however, the UK Green Care Coalition
has recently proposed to restrict the use of “green care” to interventions aimed at
people with a defined or diagnosed need (Bragg and Atkins, 2016). This restriction
seems artificial and confusing, given that many nature-based interventions (like health
walks or gardening programs) are applied in a similar manner to specific as well as
general populations. Instead of narrowing the definition, “green care” could be viewed
more broadly to include all organized nature-based health interventions and programs
for defined and general populations. More fine-grained distinctions can be expressed
by variations like “green therapeutic care” or “green community care.”
While providers and health professionals are the primary actors when it comes to the
integration of “green care” (defined in a broad manner) in medical practice, environmental
psychologists and other researchers within the green space-health domain also have
a role to play. The legitimacy of green care strongly depends on the scientific evidence
for a relationship between green space and health. The scientific research agenda
therefore has important and far-reaching implications for the acceptance of green
care and the implementation of green prescriptions.
The importance of clinical evidence
Research on beneficial effects of contact with green space has been dominated by epidemiological
studies linking green space to public health, and experimental studies on the stress-relieving
and mood-enhancing effects of interacting with green space (see for an overview, van
den Berg and van den Berg, 2014). The latter, so-called “restorative environments
studies,” show many similarities with randomized controlled trials (RCTs), such as
random assignment of participants to experimental and control conditions and the use
of validated outcome measures (Friedman et al., 2010). However, restorative environment
studies also differ from RCTs in important ways. Most studies focus on brief exposure
to (simulated) natural environments among healthy individuals; only few randomized
controlled studies have examined more long-term intervention programs and therapies
among patient samples (see for reviews, Annerstedt and Währborg, 2011; Clatworthy
et al., 2013; Kamioka et al., 2014). Furthermore, restorative environment studies
typically do not follow the strict guidelines for good clinical practice, like pre-trial
registration and the use of standardized protocols. Thus, although there is a strong
experimental tradition in green space—health research, most of the experiments do
not comply with strict criteria for RCTs, and do not qualify as clinical evidence.
Health professionals widely believe that only RCTs can produce trustworthy results
(Concato et al., 2000). Moreover, RCTs can provide information on optimal dosing,
treatment duration, and effectiveness of different types of interventions for different
groups. This would seem to suggest a straightforward recommendation for more RCTs.
However, a practical issue in applying RCTs to nature-based interventions to is that
patients and administrators cannot be easily blinded to the intervention: it is usually
very noticeable that the treatment takes place in natural surroundings, instead of
the standard care environment. If blinding is not possible, negative or positive views
of the intervention by patients or administrators may be a major source of bias, and
the outcomes may not be taken as seriously as the outcomes of (double) blinded trials.
Thus, although the availability of RCTs is a key requirement for a greater acceptance
by health professionals, RCTs do not lend themselves well to nature-based interventions,
and therefore are unlikely to be sufficient in convincing health professionals to
write more green prescriptions.
The need for a convincing explanatory framework
Besides a strong clinical evidence base, an important prerequisite for the acceptance
of green care is a convincing explanatory framework that specifies the main pathways
and causal mechanisms of these interventions. Since the natural environment is a central
component, an explanatory framework for green care should specifically elucidate how
interacting with nature and green space may promote health. The available evidence
suggests three main pathways: (a) regulation of immunological and physiological (stress)
responses, (b) enhancement of psychological states like mood, self-esteem, vitality,
and attention, and (c) facilitation of health-promoting behaviors such as exercise
and social contacts. In a recent review, Kuo (2015) proposed that most known pathways
between green space and health can be subsumed under the central biological pathway
of enhanced immune functioning. This is an important new insight that, if substantiated,
would provide a plausible medical explanation that may increase adoption of green
care by health professionals.
Researchers have also worked toward unraveling the causal mechanisms involved in the
direct physiological and psychological effects of green space. Traditionally, two
theories have been invoked to explain these effects. First, stress-reduction theory
holds that natural settings evoke an “automatic positive affective response,” which
blocks negative thoughts and feelings and reduces physiological activation (Ulrich
et al., 1991). Second, attention restoration theory proposes that natural settings
evoke “soft fascination,” capturing attention in a pleasant, effortless bottom-up
manner, without taxing executive processes (Kaplan, 1995). What remains unclear in
both theories, however, is precisely which environmental cues trigger the “automatic
positive affective response” and “soft fascination” (Valtchanov and Ellard, 2015).
In other words, if green space is a medicine, then what are its active ingredients?
Potential active ingredients of green space as medicine
Using advanced image decomposition techniques, recent research has shown that positive
responses to viewing nature scenes images are causally related to low level spatial
and color features of the scenes (Kardan et al., 2015; Valtchanov and Ellard, 2015).
Of these low level features, particularly fractals have gained much interest (Joye
and van den Berg, 2011). Fractals are self-similar patterns that can be found throughout
the natural world, but are uncommon in human-made structures (Mandelbrot, 1983). Among
other things, it has been demonstrated that viewing computer-generated natural fractals
can increase EEG-recorded alpha waves, an indicator of a wakefully relaxed state (Hägerhäll
et al., 2008, 2015).
People's interactions with nature typically involve a full, multisensory experience,
instead of mere visual exposure. A convincing explanatory framework would therefore
include not only visual, but also other types of sensory cues. In this respect, olfactory
cues, such as phytoncides, have recently come into focus (Craig et al., 2016). Phytoncides
are chemical substances with antibacterial and fungal qualities that are secreted
by many plants and trees to protect themselves. Preliminary findings indicate that
inhaling phytoncides can evoke physiological and psychological responses in humans
similar to the responses evoked by a visit to nature and green space (Li et al., 2006).
The recent advances in basic research on fractals and other low level features of
natural environments mark an important turning-point in the understanding of health
benefits of green space and green care, because they speak against the idea that these
benefits are merely a “placebo effect,” produced by people's culturally and personally
shaped expectations of what nature and green space can do for them. It is therefore
important that researchers continue to explore this line of work. However, to avoid
the pitfalls of reductionism, other more holistic and experiential perspectives should
also be taken into consideration. In particular, the concept of “connectedness to
nature” has recently come to the fore as a mechanism that strikes a chord with many
who have personally experienced the benefits of interacting with green space (Capaldi
et al., 2015).
Conclusion
In the past decades, research on the health benefits of green space has made great
strides, which has led many countries to actively work toward implementing nature-based
interventions that make use of the preventive and therapeutic potential of green space.
However, health care professionals are still reluctant in prescribing these interventions
to their patients. To sway these professionals, providers need to agree on a common
language for describing their services, and researchers need to give more attention
to clinical trials as well as to basic research on the biological pathways and explanatory
mechanisms. With the help of these developments, a transition can be made from green
space to green prescriptions.
Author contributions
The author confirms being the sole contributor of this work and approved it for publication.
Conflict of interest statement
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.