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date: 26 June 2017

Natural Environments, Health, and Well-Being

Summary and Keywords

Human beings are part of natural ecosystems and depend on them for their survival. In a rapidly changing environment and with increasing urbanization, this dependence is challenged. Natural environments affect human health and well-being both directly and indirectly. Urban green and blue areas provide opportunities for stress recovery and physical activity. They offer spaces for social interactions in the neighborhood and places for children’s play. Chronic stress, physical inactivity, and lack of social cohesion are three major risk factors for noncommunicable diseases, and therefore abundant urban greenery is an important asset for health promotion.

Through numerous ecosystem services natural environments play a fundamental role in protecting health. Various populations depend on nature for basic material, such as fresh water, wood, fuel, and nutritious food. Biodiverse natural areas are also necessary for regulating the environment and for mitigating and adapting to climate change. For example, tree canopy cover can reduce the urban heat island effect substantially, preventing excess morbidity during heat waves. This natural heat-reducing effect also lessens the need for air conditioning systems and as a consequence decreases energy spending. Urban trees also support storm-water management, preventing flooding and related health issues. Air pollution is a major threat to population health. Urban trees sequester pollutants and, even though the effect may be relatively small, given the severity of the problem it may still have some public-health implications.

The evidence around the effects of natural environments on health and well-being is steadily increasing. Several pathways and mechanisms are suggested, such as health services through functional ecosystems, early life exposure to biodiverse microbiota, which is important for the immune-system development, and sensory exposure, which has direct neurobiological impact supporting cognitive development and stress resilience. Support for several pathways is at hand that shows lower mortality rates and prevalence of cardiovascular and respiratory diseases, healthier pregnancy outcomes, reduced health inequalities, and improved mental health in urban areas with greater amounts of green and blue space.

Altogether, the interactions between healthy natural environments and healthy people are multiple and complex, and require interdisciplinary attention and action for full understanding and resilient development of both nature and human beings.

Keywords: green spaces, stress, physical activity, social cohesion, ecosystem services, air quality, noise, heat stress, mitigation, climate change

Human Dependence on Nature

The role of nature is fundamental to human life, health, and well-being. As human beings, we are basically dependent on healthy and functional nature and ecosystems. We are part of ecosystems ourselves, and each individual constitutes an ecosystem of its own; the human body containing 10 times more microbial cells than human cells and at least 300 times more microbial genes than human genes (O’hara & Shanahan, 2006).

In an urbanized world, the role of ecosystems for our survival is perhaps only peripherally perceived. Nevertheless, urban nature, such as urban forests, woodlands, and parks, play an increasingly important role in human well-being. Some of the health-protective effects of urban nature can be objectively defined, for example, shading from trees and the mitigation of the urban heat island effect; others may be less tangible, such as stress recovery and opportunities for social cohesion, although the latter may be as important from a broader public health perspective.

In less developed and less urbanized parts of the world, where people depend directly on natural resources like timber, soil, and game to support their livelihoods, the role of nature in survival and health is more evident.

Defining Health and Nature

Health, as defined by the World Health Organization ([WHO], 1948), is “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.” This definition has been criticized for being utopian, but it is nonetheless valuable because it recognizes the human being as part of her context and environment, including physical, mental, and social outcomes. The definition is also an alternative to the prevailing medical pathophysiological view on health and disease, a view that is often insufficient for framing the burden of disease and its determinants in the early 21st century.

Nature and natural environments can be immensely varied, and range from a neatly structured and well-planned garden to the great outdoors of untouched mountains and large forest areas. A broad perspective on nature is taken in this article, looking at urban natural environments, such as parks, water elements, and gardens. Indoor plants will not be covered, neither will the literature on the health and well-being benefits from contact with animals and pets be reviewed. Urban natural areas will be considered to a relatively large extent, in keeping with the focus on the interactions between people and environments.

The majority of the global population lives in cities, and the rate of urbanization is increasing; urban nature thus becomes an important topic in terms of population health. Another reason for the urban focus is that most of the research done on natural environments and human health has mainly been conducted in the Western world, in particular in cities. Many studies have explored urban green spaces, defined and outlined in various ways from people’s own perception of what nature is to land-use classifications in geographic information systems (GIS) to exclusively satellite-based imaging (e.g., normalized differentiation vegetation index [NDVI]). Urban natural environments as defined in this article are public green spaces (e.g., typically green areas with trees, grass, and other vegetation) and blue areas (e.g., ponds, lakes, and rivers).

The Complexity of People Environment Interactions

The examination of nature’s impact on health and well-being constitutes a broad and complex area of study. It is challenging to categorize it within a specific disciplinary context, be it medicine, public health, or environmental sciences. Its complexity somehow mirrors the global issues of the 21st century. No single discipline can solve the problems of environmental degradation and climate change, the escalating prevalence of noncommunicable diseases (NCDs), or conflict and poverty. Interdisciplinary and transdisciplinary research and systems actions are urgently required. Pioneering work and projects have been initiated, including from a policy perspective, such as in the Sustainable Development Goals ([SDGs]; United Nations, 2015), where health is expressed explicitly in Goal 3, “Ensure healthy lives and promote well-being for all at all ages.” Human health is also dependent on the achievement of the other 16 goals (McMichael, 2015).

However, further efforts are needed to enforce and strengthen collaborations across disciplinary boundaries and to create new systems-science models and methods. Environmental problems can no longer be considered a topic only for environmental scientists; they are equally a major health concern and, in a long-term perspective, economic and social issues. Similarly, human behavior, traditionally covered in behavioral medicine, neurobiology, psychology, and related disciplines, is a topic to be considered by environmentalists as human impact on the environment has wide implications for ecosystem functioning. The multiple and diverse interlinkages between natural environments and human health and well-being are increasingly important to recognize and understand and will require a substantially new way of thinking (McMichael, 2015).

The Global Disease Scenario

To understand why natural environments may play such a large role in improving public health, it is important to have an overview of the global burden of disease in the 21st century.

Noncommunicable Diseases

So-called noncommunicable diseases (NCDs) contribute to the lion’s share of global morbidity and mortality. Low- and middle-income countries are affected the most (WHO, 2014c). NCDs are chronic conditions and are not passed from person to person but are often related to social, behavioral, and environmental contexts. The main types of NCDs are cardiovascular diseases, chronic obstructive pulmonary diseases, mental disorders, cancer, diabetes, and obesity. According to a report from WHO, every year 16 million people die prematurely—before the age of 70—due to NCDs (WHO, 2014c). The etiology of NCDs is multifactorial, and intersectoral collaborations across multiple disciplines and policies are essential to efficiently tackle the issue. The main action target seems to be various prevention measures, such as encouraging physical activity or providing access to healthy environments, throughout the life course (Balbus, Barouki, Birnbaum, Etzel, Gluckman, Grandjean et al., 2013). While it is not a single, “silver bullet” solution, investing in natural environments may prove to be one efficient action. Natural environments offer settings that have the potential to reduce many of the risk factors associated with NCDs (Hartig, Mitchell, de Vries, & Frumkin, 2014).

Climate Change and Environmental Pollution

Another major health threat is climate change, generally considered to be the major threat to positive health development globally (Watts, Adger, Agnolucci, Blackstock, Byass, & Cai, 2015a; Wang & Horton, 2015). Between 2030 and 2050, climate change is expected to cause approximately 250,000 additional deaths per year, from malnutrition, malaria, diarrhea, and heat stress (WHO, 2015a). This is most likely an underestimation, as it does not account for the secondary and tertiary effects of climate change and extreme events. Natural environments, especially in urban areas, have several features that can contribute to both mitigation of and adaptation to the health consequences of climate change (Elmqvist, Maltby, Barker, Mortimer, Perrings, Aronson et al., 2012).

Environmental pollution is contributing to considerable morbidity and mortality across the world; ambient air pollution alone accounts for around 600,000 deaths annually in the pan-European region (Van den Bosch, Cave, Kock, & Nieuwenhuijsen, 2016). Replacing motorized infrastructure with walkable green infrastructure can contribute to improved air quality of cities and making healthy living easier (Zupancic, 2015b). Other types of chemical pollutants in the environment are major health issues (United Nations Environment Programme [UNEP]/United Nations Economic Commission for Europe [UNECE], 2016). For example, persistent pharmaceutical pollutants are a big problem because they impose sometimes irreversible changes to flora and fauna, degrading ecosystems on which we depend. One way forward could be to replace chemical drugs with other treatment options, such as nature-based interventions (Annerstedt & Währborg, 2011).

A common feature of these health threats is that they are unequally distributed. NCDs, climate change, and environmental pollution—all disproportionately affect the developing parts of the world and vulnerable populations even in the developed nations (Di Cesare, Khang, Asaria, Blakely, Cowan, Farzadfar et al., 2013; Martuzzi, Mitis, & Forastiere, 2010; Friel, Marmot, McMichael, Kjellstrom, & Vågerö, 2008). Safeguarding ecosystem functioning in poorer parts of the world and providing high-quality green spaces in deprived urban areas may mitigate some of the health inequalities (Mitchell & Popham, 2008; McMichael, 2000).

Theories Behind Research on Nature and Health

Connections between natural environments and health and well-being have been studied from a myriad of perspectives over time. The Father of Medicine, Hippocrates (460–370 bc), was the first physician to reject theories of illness as the consequences of mystical forces or punishments from the gods; rather, he attributed his patients’ conditions to environmental factors, diet, and living habits. Among his treatment options were walks in a botanical garden, especially for mentally ill patients. Over the centuries and in modern time, research on the associations between exposure to nature and health has developed slowly, but steadily. During the early 21st century, we have witnessed an exponentially growing body of research, confirming varied health effects of interactions with nature (Hartig et al., 2014).

Environmental Psychology, Theories, and Mechanisms

Environmental psychology may be considered among the first scientific disciplines to initiate theory development and the search for empirical support regarding nature’s positive impact on human health. Various theories have been suggested; among the more established is the attention restoration theory (ART), developed by the environmental psychologists Rachel Kaplan and Stephen Kaplan (Kaplan, 1995; Kaplan & Kaplan, 1989). This theory states that an overload of directed attention, which is mentally demanding (as in many daily working tasks), eventually leads to “mental fatigue.” A nondemanding but still fascinating environment such as nature provides an opportunity for psychological recovery and reflection, thus restoring mental attention capacity. This is a common intuitive experience of many people who spend time in serene and peaceful nature (Berto, 2014). Empirically, this theory has found some support in studies showing, for example, improved cognitive capacity and attention skills after visits to natural environments, including studies of children playing in natural environments (Berman, Jonides, & Kaplan, 2008; Berman, Kross, Krpan, Askren, Burson, Deldin et al., 2012; Mårtensson, Boldemann, Söderström, Blennow, Englund, Grahn et al., 2009b).

Another theoretical framework is offered by Roger Ulrich’s stress reduction theory (SRT) or psycho-social stress recovery theory. This theory postulates that we react unconsciously to sensory input from nature, leading to immediate stress recovery, both psychologically and physiologically (Ulrich, 1983). The idea is, at least partly, framed within the context of the biophilia hypothesis, an understanding of the instinctive and innate connection of humans to nature based on our evolutionary origin in a largely natural environment (Wilson, 1984). This means that we are biologically programmed to respond to environmental and natural cues and stimuli, something that may not always be respected in the modern, anthropocentric environment many of us inhabit. Several studies have been able to confirm stress-reducing effects after shorter or longer exposure to nature (De Vries, Van Dillen, Groenewegen, & Spreeuwenberg, 2013a; Ulrich, Simons, Losito, Fiorito, Miles, & Zelson, 1991; Annerstedt, Jönsson, Wallergård, Johansson, Karlson, Grahn et al., 2013).

Another theory, which may contribute to an explanation of the stress-reducing potential of natural environments, is the fractal hypothesis. Fractal objects feature patterns that recur on finer and finer scales, and they can, for example, be found in nature (see Figure 1).

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Figure 1. Ferns in nature displaying a midrange fractal pattern.

Photograph by Lis West_CC BY 2.0.

In general, people seem to find natural fractals aesthetically pleasing (Hägerhäll, Purcell, & Taylor, 2004), and a several studies have suggested that this may induce activity in brain regions associated with a state of tranquility (Hägerhäll, Laike, Taylor, Küller, Küller, & Martin, 2008; Taylor, Spehar, Van Donkelaar, & Hagerhall, 2011).

It is not only visual input from nature that induces psychophysiological responses; other sensory stimuli, such as natural sounds and smells, give rise to certain reactions. Some studies indicate that such responses are mirrored by brain activity associated with self-reflection and feelings of tranquility (Hunter, Eickhoff, Pheasant, Douglas, Watts, Farrow et al., 2010). Other studies have demonstrated improved physiological stress recovery after exposure to sounds of nature, such as twittering birds or a stream of water (Annerstedt et al., 2013; Alvarsson, Wiens, & Nilsson, 2010). Some research has explored olfactory stimuli and found positive effects on cognitive performance, stress recovery, and mood (Diego, Jones, Field, Hernandez-Reif, Schanberg, Kuhn et al., 1998; Chen, Kumar, Chen, Tsao, Chien, Chang et al., 2015). However, much of this research is still in its infancy and more controlled studies are required before the evidence can be confirmed.

A specific field of research regarding sensory stimuli of natural environments is contained within the context of Shinrin-yoku, “forest-bathing,” a concept proposed in Japan as a health-promoting activity (Tsunetsugu, Park, & Miyazaki, 2010). During a forest bathing trip, one breathes in the forest atmosphere, and studies have shown various effects on the neuroendocrine and immune system when compared with walks in a built environment. A few studies have demonstrated a reduction in urinary dopamine and adrenaline, and enhanced natural killer-cell activity has been found in peripheral blood (Li, 2010; Li, Morimoto, Kobayashi, Inagaki, Katsumata, Hirata et al., 2008; Li, Otsuka, Kobayashi, Wakayama, Inagaki, Katsumata et al., 2011). This would point toward stress recovery effects in the autonomic nervous system, as well as an improvement in the immune function. However, much of this research has been done on a regional level and the generalizability is unclear.

Mediating Factors of Nature Health Associations

Following the pioneering theory development, the field has expanded to include a range of areas, spanning interventions such as garden therapy (Annerstedt & Währborg, 2011) to healthy urban planning by including green spaces in cities for improved public health (Annerstedt van den Bosch, Mudu, Uscila, Barrdahl, Kulinkina, Staatsen et al., 2016). Much research is focused on mediating factors that may explain why people may get healthier by living close to nature or by being exposed to various forms of nature. Although mediating factors or pathways, such as stress recovery or physical activity, are positive outcomes, they do not themselves represent illnesses or diagnoses; but it is likely that they contribute to disease prevention. Most of the mediating factors depend on various other factors, such as the size of and the distance to green space or the population in question. Thus, exposure to nature may indicate neighborhood availability, views of nature, passive or active interactions, or specific activities involving natural elements.

Stress Recovery

Stress can be defined as an imbalance between the demands placed on us and our ability to manage them. Although stress is a functional physiological reaction to an emerging threat, prolonged stress can give rise to a range of health problems, such as chronic fatigue, sleep problems, and diffuse muscle pains. In the modern society, stress has become a major primary or secondary risk factor for many common disorders, such as mental and cardiovascular illnesses and other NCDs (McEwen & Stellar, 1993; Crestani, 2016).

Stress recovery is considered a mediating factor between health and nature. This means that if views of, or interactions with natural environments contribute to reduced stress, chronic diseases may be prevented through healthy urban planning, providing access to public green and blue spaces (see Figure 2).

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Figure 2. Stress increases the prevalence of mental disorders and other noncommunicable diseases (NCDs). These conditions constitute a large part of the global burden of disease and impose a heavy load on society’s public health concerns and costs. Through the mediating role of stress recovery, natural environments may contribute to preventing many stress-related health issues and thus have a positive impact on the general burden of health and related costs.

This area has gained particular attention in urban settings because stress is considered to be higher in cities, and there is a higher prevalence of mental disorders in urban compared to rural settings (Peen, Schoevers, Beekman, & Dekker, 2010; Sundquist, Frank, & Sundquist, 2004). With neuroimaging techniques, structural and functional discrepancies in the brains of urban and rural populations have been found, suggesting a biological basis for increased stress vulnerability as a consequence of urban living (Lederbogen, Kirsch, Haddad, Streit, Tost, Schuch et al., 2011).

Using various methods, research designs, and definitions of natural spaces, several studies have demonstrated improved or generally better mental health in greener areas, which may be associated with lower stress levels. For example, a longitudinal study from UK showed that moving to greener areas was associated with sustained mental health improvement (Alcock, White, Wheeler, Fleming, & Depledge, 2014). Other large-scale cohort studies have demonstrated generally better mental health and well-being in populations living in green urban areas compared to built “grey” areas (Dadvand, Bartoll, Basagaña, Dalmau-Bueno, Martinez, Ambros et al., 2016; Reklaitiene, Grazuleviciene, Dedele, Virviciute, Vensloviene, Tamosiunas et al., 2014; Triguero-Mas, Dadvand, Cirach, Martínez, Medina, Mompart et al., 2015). Studies that have included biomarker monitoring have suggested health protective effects of living in green neighborhoods, indicated by reduced levels and healthier circadian release of the stress hormone cortisol, an effect that seems to be particularly pronounced in deprived communities (Ward Thompson, Aspinall, Roe, Robertson, & Miller, 2016; Ward Thompson, Roe, Aspinall, Mitchell, Clow, & Miller, 2012; Gidlow Jones, Hurst, Masterson, Clark-Carter, Tarvainen et al., 2016). Merely having a view of nature or green space may be perceived as restorative—a feeling of replenishment of energy previously drained by stress—and associated with autonomic stress reduction (Gladwell, Brown, Barton, Tarvainen, Kuoppa, Pretty et al., 2012), as may small so-called urban pocket parks (Nordh, Hartig, Hägerhäll, & Fry, 2009).

Whether a natural environment provides stress recovery or not may be determined by various factors, such as the quality of the green spaces, crowding, fear, scenery, presence of water features, noise, frequency, and length of stay, and so on (Grahn & Stigsdotter, 2010; Scopelliti, Carrus, Adinolfi, Suarez, Colangelo, Lafortezza et al., 2016; Arnberger, 2012; Annerstedt, Norman, Boman, Mattsson, Grahn, & Währborg, 2010; White, Pahl, Ashbullby, Herbert, & Depledge, 2013; Völker & Kistemann, 2013; Sreetheran & Konijnendijk van den Bosch, 2014). Many of these factors are dynamic and difficult to untangle and objectively relate to measurements of stress recovery. Some research includes dose-response analyses, suggesting that longer duration of visits (> 30 minutes) is related to enhanced stress reduction (e.g., using reduced blood pressure as a proxy for stress reduction resulted in almost 10% improvement; Shanahan, Bush, Gaston, Lin, Dean, Barber et al., 2016). Some studies suggest a dose-response relation between self- reported stress and nature as measured by tree-cover density (Jiang, Li, Larsen, & Sullivan, 2014), as well as between reduced blood pressure and the length of the nature exposure (Hartig, Evans, Jamner, Davis, & Gärling, 2003). However, many studies are more general in terms of the quantification of stress effects, natural features or exposure type, and health outcomes. Also individual factors, such as age, gender, or socioeconomic group, influence which environmental qualities provide the potential for stress recovery (Astell-Burt, Mitchell, & Hartig, 2014; Baran, Smith, Moore, Floyd, Bocarro, Cosco et al., 2014), although some studies show more generalizable population effects (Wheeler, Lovell, Higgins, White, Alcock et al., 2015). In addition, for example cultural belonging and in which environment a person has grown up, are likely to influence the perceived effects on stress (Kloek, Buijs, Boersema, & Schouten, 2013).

Many people claim get a certain restorative experience in nature which is perceived as untouched by human beings and wilderness areas (Cole & Hall, 2010; Kaplan & Talbot, 1983; Johnsen, 2013; Mutz & Müller, 2016), and this has also been used therapeutically for people with stress-related disorders (Sonntag-Öström, Nordin, Lundell, Dolling, Wiklund, Karlsson et al., 2014). However, this is also likely to depend on a familiarity with “wild nature.”

Encouraging Physical Activity

Another proposed mediator is physical activity. Sedentary behaviors and the lack of daily physical activity is a major risk factor for morbidity and premature mortality (WHO, 2010a). Physical inactivity accounts for over five million deaths annually through its effects on multiple NCDs. Therefore, various interventions are needed to enhance the level of physical activity in populations. A multitude of cultural, social, and environmental factors influence individual and population levels of physical activity. In addition, environmental exposure during childhood and epigenetic mechanisms play a role in the subsequent physical activity levels in adulthood (Gluckman & Hanson, 2008). From this perspective, it is imperative to replace so-called obesogenic environments, especially in cities, where opportunities for active transport are restricted, creating walkable areas and public open spaces for engaging in vigorous activities from childhood and onward throughout life.

Many studies show an association between the access to urban green spaces and increased physical activity in a community (Sallis, Cerin, Conway, Adams, Frank, Pratt et al., 2016; Sugiyama, Giles-Corti, Summers, du Toit, Leslie, & Owen, 2013). Having access to a green area in the neighborhood seems to encourage the use of and being active in an environment that is perceived as healthy and having good air quality. Vegetated areas are also cooler, and thus more thermally comfortable to exercise in (Bowler, Buyung-Ali, Knight, & Pullin, 2010). Some research even shows further improved mental health outcomes from being physically active in a green area compared to, for example, in an indoor gym, though various exercise environments may have different effects on various outcomes (Mitchell, 2013; Thompson Coon, Boddy, Stein, Whear, Barton, & Depledge, 2011).

Various factors determine the association between physical activity and natural environments. The walking distance from the residence is often mentioned (Toftager, Ekholm, Schipperijn, Stigsdotter, Bentsen, Gronbaek et al., 2011); and having a green space nearby is suggested to automatically encourage a more active behavior, for example, among children (Almanza, Jerrett, Dunton, Seto, & Pentz, 2012). Coastal proximity has also been associated with physical activity (White, Wheeler, Herbert, Alcock, & Depledge, 2014). The size of the area and the facilities in terms of trails and the lighting of paths are other modifying factors (Schipperijn, Bentsen, Troelsen, Toftager, & Stigsdotter, 2013; Toftager et al., 2011). While dose-response relations are difficult to determine, it is suggested that the duration of and frequency of visits are associated with the level of physical activity (Shanahan et al., 2016).

Promoting Social Cohesion

Social cohesion may mediate some of the health effects of natural environments. Social isolation is a significant health risk factor, at the same level as tobacco smoking (Pantell, Rehkopf, Jutte, Syme, Balmes, & Adler, 2013). Social cohesion contributes positively to mental and physical health, and is influenced by both neighborhood characteristics (such as socioeconomic deprivation) and individual characteristics (such as age or mobility constraints; Pearson, Ivory, Breetzke, & Lovasi, 2014). Since neighborhood green spaces are often perceived as attractive places to visit, they provide ample opportunities for residents to meet other people and interact in informal ways. The integrative potential of these spaces is therefore acknowledged, because they may facilitate meetings and acquaintances with persons of different social and ethnic backgrounds (Peters, 2010; Peters, Elands, & Buijs, 2010). Shopping malls, restaurants, pubs, cafes, and other public spaces can also promote meetings, but those venues may not encourage interactions across cultures to the same extent. They tend to be culturally or ethnically defined and also require consumption, which can limit equal accessibility.

Several studies have found that objective measures of green are related to social cohesion, in terms of residents being more socially active, knowing more neighbors, and feeling that their neighbors are helpful and supportive (Kuo, Sullivan, Coley, & Brunson, 1998; Sugiyama, Leslie, Giles-Corti, & Owen, 2008; Kemperman & Timmermans, 2014). A few studies suggest that it is already existing social groups, like families or friends, which are most likely to interact socially, thus contributing less to creating new social bonds (Peters et al., 2010), while others suggest that especially children are encouraged to interact across group boundaries by playing in green spaces (Seeland, Dübendorfer, & Hansmann, 2009)

Social cohesion effects are probably most likely to occur in urban natural environments; and some groups seem to be more likely to experience the beneficial effects, such as people with mobility or financial constraints, children and youths, parents with young children, and the unemployed (Kemperman & Timmermans, 2014; Seeland et al., 2009). Other factors that may influence the effects of natural environments on social cohesion are availability, aesthetic attractiveness, maintenance, recreational facilities, organized events, and perceptions of safety in the area (De Vries, Van Dillen, Groenewegen, & Spreeuwenberg, 2013b; Francis, Giles-Corti, Wood, & Knuiman, 2012; Kaźmierczak, 2013). The size of the area seems to matter. Smaller parks and community or allotment gardens can be particularly important for encouraging social interactions (Van den Berg, Van Winsum-Westra, De Vries, & Van Dillen, 2010; Wakefield, Yeudall, Taron, Reynolds, & Skinner, 2007). Similarly, the frequency of visits and length of stay show some association with the magnitude of social-capital gain (Shanahan et al., 2016).

Buffering Health Inequalities

Socioeconomic health inequalities are a major concern in global public health (Commission on the Social Determinants of Health, 2008). There are no biological explanations for health discrepancies that depend on socioeconomic conditions, and these conditions are preventable by various societal interventions, for example providing clean and healthy living environments (Solar & Irvin, 2010). Assuming that natural environments promote health, investing in green spaces in deprived areas may be considered from a healthy urban planning perspective. Research has shown that socioeconomic health inequalities seem to be buffered by living in green areas. A large population study from UK demonstrated significantly decreased health gaps, in cardiovascular and all-cause mortality, in greener areas (as defined by the area of land designated as green space in a neighborhood; Mitchell & Popham, 2008). Another study, including 34 European countries, demonstrated a 40% reduction in socioeconomic related inequalities in mental well-being in green areas (Mitchell, Richardson, Shortt, & Pearce, 2015). Similarly, stress reduction and other positive health effects, such as pregnancy outcomes, seem to be more pronounced among poorer populations (Ward Thompson et al., 2016; Dadvand, de Nazelle, Figueras, Basagaña, Su, Amoly et al., 2012a). Most of these studies have controlled for selection bias, but the evidence is still inconclusive (Sugiyama, Villanueva, Knuiman, Francis, Foster et al., 2016). It is also not clear whether any particular features of green spaces are required for the positive effects on health inequalities.

Immune System Development

Another effort to explain the benefits of natural environments to human health includes a microbiological viewpoint (Rook, 2013). As the human immune system develops by exposure to various microorganisms beginning at the fetus stage through birth and up to infancy, the system must be exposed to a broad variety of bacteria and other microorganisms for optimal development. In urban areas, often indoors, and with higher hygiene standards our immune systems tend to be insufficiently developed (Rook, Lowry, & Raison, 2015). This is potentially one of the factors behind the increasing prevalence of autoimmune diseases, such as allergies, inflammatory bowel diseases, and multiple sclerosis (Rehman, Lepage, Nolte, Hellmig, Schreiber, & Ott, 2010; Fleming, 2013; Rook, 2012). Because natural environments with healthy ecosystems contain a broad biodiversity, it should be essential to expose people to such spaces to improve the chances for functional immune system development and less disease (Rook, Lowry, & Raison, 2014). In fact, it has been demonstrated that households containing animal feces predict a better ability to control background inflammation (measured by CRP-levels) (McDade, Tallman, Madimenos, Liebert, Cepon, Sugiyama et al., 2012), and more house dust is associated with a lower prevalence of childhood atopy (Pakarinen, Hyvärinen, Salkinoja-Salonen, Laitinen, Nevalainen, Mäkelä et al., 2008).

Children and Nature

Children spend more and more time indoors, often pursuing sedentary activities, like watching television or using computers (Rideout, Foehr, & Roberts, 2010). Research from Scotland has found that children living further away from green spaces (> 20 minutes’ walking distance) displayed over 2 hours more weekly TV time as compared to children living closer to green spaces (< 5 minutes’ walking time; Aggio, Smith, Fisher, & Hamer 2015). Similar findings of the relation between green space accessibility and physical activity among children have been found in, for example, Spain (Almanza et al., 2012; Boldemann, Blennow, Dal, Mårtensson, Raustorp, Yuen et al., 2006). Spending less time outdoors and experiencing nature to a lesser extent may contribute to a disconnection from the natural world, sometimes denoted as “nature-deficit disorder” (Louv & Hogan, 2005). Apart from potentially leading to ecological illiteracy and less awareness of the value of nature (Wells & Lekies, 2006), it also has direct health implications, since exposure to nature has critical potentially positive consequences for children’s health, development, functioning, and well-being (Amoly, Dadvand, Forns, López-Vicente, Basagaña, Julvez et al., 2014; Figure 3).

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Figure 3. Free play in nature is important for children, stimulating cognitive and visomotoric development.

Photograph by Cecil Konijnendijk.

Access to natural environments is associated with a lower prevalence of obesity among children (Wolch, Jerrett, Reynolds, Mcconnell, Chang, Dahmann et al., 2011; Bell, Wilson, & Liu, 2008; Wood, Demougin, Higgins, Husk, Wheeler, & White, 2016). Playing in outdoor green spaces stimulates and improves communication skills and relationships among children (Moore, 2002), and schoolyards with high levels of vegetation seem to promote cooperation, attention skills, and creativity (Fjørtoft & Sageie, 2000; Mårtensson, Boldemann, Blennow, Söderström, & Grahn, 2009a). Children’s cognitive capacity also seem to improve by moving from a less to a more green area (Wells, 2000), and cognitive development is enhanced among children in schools situated in green areas, only partly mediated by air pollution (Dadvand, Nieuwenhuijsen, Esnaola, Forns, Basagaña, Alvarez-Pedrerol et al., 2015). Similarly, exposure to nature may reduce children’s symptoms of attention deficit hyperactivity disorder ([ADHD]; Kuo & Taylor, 2004; Taylor & Kuo, 2011).

Regarding respiratory health, the evidence is not entirely conclusive. An inverse relation between number of street trees and asthma prevalence among children has been shown (Lovasi, Quinn, Neckerman, Perzanowski, & Rundle, 2008). Particularly early life exposure (as determined by satellite-derived greenness index in the neighborhood during pregnancy) has been related to reduced prevalence of childhood asthma (Sbihi, Tamburic, Koehoorn, & Brauer, 2015). In contrast, some studies have found a slightly increased prevalence of asthma among children living closer to green areas (Dadvand, Villanueva, Font-Ribera, Martinez, Basagaña, Belmonte et al., 2014; Andrusaityte, Grazuleviciene, Kudzyte, Bernotiene, Dedele, & Nieuwenhuijsen, 2016). Such contradictory results may be due to difference is study design or potentially local plant species composition and their pollen related to allergy and related asthma incidence (Lovasi, O’Neil-Dunne, Lu, Sheehan, Perzanowski, MacFaden et al., 2013a).

Elderly

With a growing population of elderly, geriatric conditions such as dementia and the increased prevalence of anxiety require an increased focus from a public health perspective. A particular issue is that pharmaceutical interventions against anxiety often have negative side effects, such as dizziness, falls, or impaired cognitive capacity (Bénard-Laribière, Noize, Pambrun, Bazin, Verdoux, Tournier et al., 2016; Sköldunger, Fastbom, Wimo, Fratiglioni, & Johnell, 2015). This calls for alternative treatments, as well as an increased focus on improving the quality of life for people with dementia. As exposure to nature may reduce stress and restore attention, it could play a particular role in the life and care of elderly.

Research has suggested that being able to view, experience, or interact with nature has a positive impact on both anxiety and dementia, as well as aggressive behavior, in the elderly (Dzhambov & Dimitrova, 2014; Chapman, Hazen, & Noell-Waggoner, 2007). Incorporating safe gardens into elderly homes and rehabilitation centers seem to encourage autonomy and walking behavior (Detweiler, Sharma, Detweiler, Murphy, Lane, Carman et al., 2012; Detweiler & Warf, 2005). Experiencing nature is perceived as restorative by many elderly, who also seem to exhibit a certain feeling of familiarity with natural environments (Berto, 2007; Ottosson & Grahn, 2005). Easily accessible natural areas and even indoor plants may be particularly important for the elderly, who can be restricted in their opportunities for outdoor walks and activities (Brascamp & Kidd, 2004; Kearney & Winterbottom, 2005).

Healthy Planet Healthy People

Environmental Behavior

The natural foundation on which humans live is the planet itself. It is therefore paradoxical that human behavior, in the current Anthropocene era, is efficiently working toward crossing many planetary boundaries, biodiversity loss being one of them (Rockström, Steffen, Noone, Persson, Chapin, Lambin et al., 2009). Human behavior in interactions with nature therefore becomes crucial to addressing approaches for establishing sustainable and resilient development. This is important for the environment and indirectly for human health (Watts, Adger, Agnolucci, Blackstock, Byass, Cai et al., 2015b). Research shows that policies that encourage environmental friendly behavior (e.g., recycling and less car-use) can make substantial contributions to climate-change mitigation (Dietz, Gardner, Gilligan, Stern, & Vandenbergh, 2009). Thus, by behaving more pro-environmentally, the negative health effects of climate change may be reduced. Some theories and research suggest that exposure to nature stimulates environmental-friendly behavior automatically (Zelenski, Dopko, & Capaldi, 2015; Annerstedt van den Bosch & Depledge, 2015), which could be another argument for investing in public nature and green spaces. This might act as a mediator between natural environments and long-term health benefits. From this perspective, it is promising that research has shown that by behaving “morally correct” (e.g., altruistically or ecologically), humans feel better about themselves, sometimes denoted as “warm glow,” encouraging repeated “good behavior.” This means that a win-win could potentially be achieved, as behaving pro-environmentally would directly induce a sense of well-being, while simultaneously be beneficial from a long-term perspective (Taufik, Bolderdijk, & Steg, 2015).

Planetary Health and Ecosystem Services

A few frameworks exist to raise the awareness of the interconnection between planetary health and human health. The Rockefeller Foundation Lancet Commission on Planetary Health calls for a recognition of how the health gains made over the last century have been at the cost of Planetary health (Whitmee, Haines, Beyrer, Boltz, Capon, de Souza Dias et al., 2015). By overexploiting natural resources for immediate, short-term gains, the health of coming generations has been “mortgaged.” Therefore, all aspects of health—forest, tree, plant, and animal—must be incorporated in any human health consideration, as human health intrinsically depends on functional, natural environments.

Through the development of the concept of Ecosystem Services (Millennium Ecosystem Assessment, 2005), a possibly quantifiable view of the health effects of natural environments has emerged, while also considering the interdependence between human and nature. Ecosystem services are the benefits people obtain from ecosystems; and in the bottom-line and as a condition for the services, is biodiversity. Ecosystem services are usually conceptualized as four groups having various functions—(a) supporting services, such as nutrient cycling and soil formation, which constitute a fundament for the other services; (b) provisioning services, such as food and fresh water; (c) regulating services, such as climate regulation and water purification; (d) and cultural services, such as spiritual and educational amenities (Millennium Ecosystem Assessment, 2005).

Provisioning services include the provision of food, fuel, and natural building materials—all crucial for human health and well-being. These services are all threatened by human activities such as deforestation and poor land management. With populations increasing globally and additional pressure on, for example, food systems, provisioning services must be properly recognized and integrated in land management, environmental and health policies, and the development of cities.

As land is transformed into cities and provisioning services thereby threatened, urban agriculture has become a developing field, which will probably gain increasing attention as demands for food production grow with increasing populations (Eigenbrod & Gruda, 2015; Lin, Philpott, & Jha, 2015). Apart from the direct benefits of providing food, local food production in cities reduces food miles and thereby GHG emissions (Pikora, Giles-Corti, Jamrozik, & Donovan, 2003; Panter, 2015; Lee, Lee, & Lee, 2015). However, urban agriculture poses specific challenges and trade-offs, due to risks of plant uptake of air pollution and toxic contaminants in urban soil. So far, most risk assessments have shown that the health benefits of urban farming probably outweigh the risks (Brown, Chaney, & Hettiarachchi, 2016; Stewart, Korth, Langer, Rafferty, Da Silva, & van Rooyen, 2013), but the evidence-base is not well understood and the issue should be studied further.

Provisioning services also supply materials for pharmaceutical development, and modern medicine continues to rely on biodiversity, containing the raw material for biotechnology for development of new drugs (David, Wolfender, & Dias, 2015). In terms of regulating services, climate regulation from both terrestrial and marine ecosystems is important, and could indirectly prevent climate change related morbidity and mortality (Millennium Ecosystem Assessment, 2005). From a human health perspective, a few regulating services responding to certain threats may be of particular value, as described below.

Heat Regulation

Heat stress and related morbidity and mortality contribute to a substantial part of the climate-change-related health threats (Xu, Fitzgerald, Guo, Jalaludin, & Tong, 2016). Heat stress occurs when the body can no longer physiologically adapt to a high temperature, progressing to heat exhaustion, then to heat stroke and, ultimately, mortality (Kovats & Hajat, 2008). The risk is particularly high among vulnerable populations, such as children and elderly, but also people in deprived areas where air conditioning is not available (Benmarhnia, Deguen, Kaufman, & Smargiassi, 2015). People with long-term conditions, such as mental illnesses or physical disabilities, or those taking medication that compromises the body’s ability to regulate its temperature, are also particularly at risk, as are outdoor workers in hot climates (Kjellstrom, 2015).

Particularly in cities, owing to the urban heat island (UHI) phenomenon (Oke, 1973), heat stress is and will increasingly be a major public health issue as global warming continues (Ramsey, 1978: Rocklöv, Forsberg, Ebi, & Bellander, 2014; Intergovernmental Panel on Climate Change [IPCC], 2014). The UHI is due to urban structures using impervious materials that absorb and store solar heat radiation, thereby increasing the land surface temperature (Rizwan, Dennis, & Liu, 2008). Reduced evapotranspiration because of the lack of urban green and vegetated land in cities is another reason for increased ambient temperature (Taha, 1997), as well as the reduced shading from tree canopies (Armson, Stringer, & Ennos, 2012). During heat waves, the air quality is degraded not only by the temperature, but also as the concentration of air pollutants increase because the formation of many pollutants is temperature dependent (Campbell-Lendrum & Corvalán, 2007). The UHI effect can increase the ambient temperature by 7°C–12°C compared to rural surroundings (Lauwaet, Hooyberghs, Maiheu, Lefebvre, Driesen, Van Looy et al., 2015).

Heat-aware urban planning could contribute to preventing heat-related morbidity (Voskamp & Van de Ven, 2015). There is increasing evidence that urban greening, including green walls and roofs, can be an effective and cost-efficient strategy for preventing UHI, and as such it constitutes an important urban ecosystem service for public health (Bowler et al., 2010; Chen, Wang, Thatcher, Barnett, Kachenko, & Prince, 2014). Research has demonstrated that ambient temperatures are considerably lower in well-greened parts of cities (Alexandri & Jones, 2008); estimates range from an average of 1°C to 10°C temperature reduction from urban greening (Bowler et al., 2010; Zipperer, Sisinni, Pouyat, & Foresman, 1997). Urban blue spaces also have a temperature-mitigating capacity (Burkart, Meier, Schneider, Breitner, Canario, Alcoforado et al., 2015; Sun, Chen, Chen, & Lü, 2012; Sun & Chen, 2012), although fewer studies have been conducted on the impact of waterbodies on urban heat.

Urban green spaces are not only beneficial for adapting cities to warmer climates; they have also a mitigating function. As the needs for energy-demanding air-conditioning systems decrease in green areas, energy use diminishes, reducing GHG emissions (Parker, 1989; Charlesworth & Booth, 2012, Fryd, Pauleit, & Bühler, 2011). Therefore, green planning should be a cost-efficient mitigation method, with potentially long-term impact on the urban climate and human health.

The magnitude of evapotranspiration and shading and consequent cooling is dependent on the configuration, type, size, location, and density of vegetation (Zupancic, 2015a). The cooling effect is also influenced by seasonal and temporal variations (Renaud & Rebetez, 2009). The size of the tree canopy has a large impact, and in general, deciduous trees are more efficient that coniferous trees (Meier & Scherer, 2012). Also, small green spaces and pocket parks can contribute to cooling; for example in a 0.24-hectare park in Lisbon, the temperature was 6.9°C cooler than the surrounding areas on the hottest day (Oliveira, Andrade, & Vaz, 2011). How far the cooling effect reaches in a natural area’s built surroundings is not totally elucidated, and also depends on many factors, such as the type of infrastructure around the area, the total cooling capacity of the area itself, time of the day, and so on. A few studies have made some estimates and, for instance, a large park (around 150 ha) in Sweden contributed to a cooling effect up to 1 kilometer from the park boundary (Upmanis, Eliasson, & Lindqvist, 1998).

Although no studies have measured the direct impact of urban natural environments on heat-related morbidity, the indirect potential impact should be clear. While more research is urgently needed, it is likely that greening of cities would offset the UHI-effect and reduce heat stress.

Flooding

Increased precipitation and heavy rainfalls are other health risks related to climate change (IPCC, 2014). In cities where the level of impervious cover is high, the storm-water run-off is slow, which increases the risk for urban flooding and related health risks, such as drowning or exposure to contaminated and polluted water and water-borne diseases (IPCC, 2014). Due to poor urban drainage systems in combination with lack of adequate sanitation facilities, the threat is larger in developing countries (Mark, Jørgensen, Hammond, Khan, Tjener, Erichsen et al., 2015).

Research has demonstrated improved urban water drainage by integrating natural environments in cities (Kim, Lee, & Sung, 2016; Yao, Chen, Wei, & Sun, 2015). Trees intercept water from the canopy and stem areas, and enhance infiltration into the soil and root systems. Vegetation cover may protect areas prone to landslides during heavy rainfall, and the potential of green spaces to efficiently reduce urban flood risks has been highlighted in some notable urban water management studies (Armson, Stringer, & Ennos, 2013; Seattle, 2008). For example, a study from Seoul, Korea, found that flooding probability was reduced by over 50% in green areas (Kim et al., 2016). Wetlands and aquatic ecosystems contribute to efficient wastewater management through filtering and bacterial decomposition (TEEB, 2011). This improves water quality and decreases risk for diarrhea and other disorders related to contaminated drinking water. As with heat exposure, no studies have explored the direct health impact of improving flooding resilience or wastewater management through natural environments, but there seems to be enough evidence as a potential pathway.

Air Pollution

Air pollution is a major environmental health problem. It is estimated that two million people worldwide —more than half of them in developing countries—die every year from problems related to air pollution (WHO, 2014a). High levels of air pollution increase the risk for, for example, stroke, heart disease, lung cancer, and both chronic and acute respiratory diseases, including asthma. By replacing traffic with green spaces, one of the main sources of air pollution can be moderated, and vegetation directly improves air quality by contributing to the oxygen and carbon cycles.

It has also been suggested that trees may have the potential to reduce the levels of both indoor and outdoor air pollution (Escobedo, Kroeger, & Wagner, 2011), by absorbing gaseous pollutants, depositing particular matter (PM), and dispersing pollutants by cooling (Pugh, Mackenzie, Whyatt, & Hewitt, 2012). Larger urban green spaces and forests also create oases with better air quality. In addition, such areas may encourage behavior change, promoting active transport rather than passive and motorized air-pollution-emitting transport modes.

The direct quantification of the role of trees and the natural environment in air-quality improvement is complex due to the high number of influencing variables involved. However, various tools and deposition models have been developed for estimating impact of public green spaces on air pollution removal (Selmi, Weber, Rivière, Blond, Mehdi, & Nowak, 2016). Using such modeling, the average tree cover of a city can be related to the potential for air-pollution removal. For example, in a study of Strasborg with about 28% tree cover of total city area, the annual removal of air pollution (CO, NO2, O3, PM10, PM2.5, and SO2) was estimated to 88.23 tons (Selmi et al., 2016). The proportion of total emissions varied between 0.03% (CO) to 6.6% (PM10). Although the benefit may appear small, the potential public health impacts could be considerable given the high burden and costs of air-pollution-related morbidity. Several other modeling studies suggest similar positive impacts in other cities across the world (Nowak, Hirabayashi, Bodine, & Greenfield, 2014; Baró, Chaparro, Gómez-Baggethun, Langemeyer, Nowak, & Terradas , 2014; Vailshery, Jaganmohan, & Nagendra, 2013; Jeanjean, Monks, & Leigh, 2016).

These estimates must be considered with caution, though, as they are bound to carry a high level of uncertainty. A large number of various factors influence the capacity of trees to remove air pollutants, such as urban infrastructure and form, tree canopy size and species, leaf area index, length of the growing season, general air flow, season, local weather, and climate. For example, the potential for reducing air pollutants seems to be less pronounced in northern climates (Setala, Viippola, Rantalainen, Pennanen, & Yli-Pelkonen, 2013). In certain physical conditions, such as in street canyons with dense tree lining and tree canopies covering the street, pollutants may be trapped and even worsen the air quality locally (Pugh et al., 2012; Tiwary & Kumar, 2014). In general, there is no scientific consensus on whether trees contribute to the removal of air pollutants or not. Only a small fraction of pollutants are absorbed by the tree; most are washed off onto the ground, and thus the effect is only temporary. However, this improves the air quality during pollution peaks, and at least the particulates become less harmful from a respiratory perspective. Regardless, the design and choice of urban vegetation is crucial if using vegetation as an ecosystem service for air-quality improvements, to avoid pollutant trapping in street canyons, and to maximize deposition and dispersion effects (Janhäll, 2015).

Noise

The disease burden related to noise pollution in 2011 was estimated to be around 1.0–1.6 million disability adjusted life years (DALYs), a measure of the years lost to ill-health, disability, or early death (WHO, 2011). Noise is mostly caused by traffic and industrial activity and with urbanization people are likely to be exposed to more noise. The health impacts of noise are not only auditory, such as hearing loss and tinnitus; annoyance, stress, sleep disturbance, cardiovascular diseases, and children’s cognitive development are also affected (Basner, Babisch, Davis, Brink, Clark, Janssen et al., 2014).

More access to quiet areas, such as larger parks, is important to reduce negative noise effects in cities. Urban greenery, including lower bushes and hedges, can act as noise barriers (Fang & Ling, 2003). Some cases show that a 15-meter-deep tree belt can reduce noise with up to 6dBA at a 50 meter distance; and a 30-meter- deep belt, up to 10dBA (Nilsson, Bengtsson, & Klaeboe, 2014). Vegetation reduces noise through the redistribution and absorption of sound energy. The plant root system also makes the soil porous and acoustically soft, which reduces especially low-frequency noise (Van Renterghem, Hornikx, Forssen, & Botteldooren, 2012). Natural soundscapes, such as bird song or streaming water, may buffer the negative perception of noise (Coensel, Vanwetswinkel, & Botteldooren, 2011). Green walls and roofs also reduce noise levels (Wong, Kwang Tan, Tan, Chiang, & Wong, 2010).

Natural spaces can reduce the annoyance associated with noise (Gidlöf-Gunnarsson & Öhrström, 2007), as well as related stress symptoms. The natural environment’s capacity to reduce noise depend on physical factors like tree height, width and form, species, design, and vegetation density (Fang & Ling, 2003). For example, vegetation with many branches and thick, fleshy leaves absorbs sound particularly well (Fang & Ling, 2003).

Few studies have investigated the health impact of natural spaces from a noise perspective, but Bodin, Björk, Ardö, and Albin (2015) showed that people with windows facing a yard, water, or a green space had less noise-related concentration problems. The efficiency of nature’s capacity to reduce noise is not yet determined, and evidence is unclear. However, most studies seem to conclude that tree trunks and forest floors can have a relatively high impact on noise, if they are adequaltely planned for the purpose, with, for example, enough density and depth of tree belts (Van Renterghem, 2014).

Environmental Hazards and “Ecosystem Disservices”

Natural environments can also be associated with negative health outcomes. Environmental disasters such as tsunamis, earthquakes, storms, floods, and volcanic eruptions all emanate from nature. However, while these events are labeled as “hazards,” they are only natural agents that transform a vulnerable human condition into a disaster (Noji, 2000). Factors that influence the transformation of natural events into disasters are mostly anthropogenic, for example, poverty and social inequality, which influence vulnerability; environmental degradation due to poor land use; rapid population growth; and climate change (Noji, 2000). Thus, although they emanating from nature, it is inappropriate to say that the negative health outcomes of natural disasters are caused by nature. Preventative actions must therefore be directed toward reducing the vulnerability of populations rather than preventing the natural events. Such actions include, for example, disaster-aware city planning, the strengthening of buildings, putting in place early warning systems, education, and establishing efficient evacuation measures (Toya & Skidmore, 2015).

Other harmful effects of nature, sometimes called “ecosystem disservices,” are vector-borne diseases, allergies from pollen, natural emissions of volatile organic compounds (VOC), falling branches, and even the fear of crime in urban green areas (Gatersleben & Andrews, 2013; Dunn, 2010; Lyytimäki, Petersen, Normander, & Bezák, 2008). Like natural disasters, most of those disservices are related to anthropogenic interactions with the environment.

Vector-Borne Diseases

Vector-borne diseases, such as malaria, dengue fever, and Lyme disease, are real threats connected with nature. The greatest burden occurs in tropical and developing countries with poor sanitation and hygiene (WHO, 2014b, 2015b). Vector-borne diseases are also public health problems in developed countries, of increasing severity due to climate change and the associated changing patterns of spread and intensity (Chevalier, Courtin, Guis, Tran, & Vial, 2016). In most cases, exposure to vector-borne pathogens can be greatly reduced through education and simple interventions, such as wearing protective clothing, using non-toxic insect repellents, and avoiding outdoor activities during the times of day when vector activity peaks. While raising awareness of risks with vector-borne diseases and providing publically available information about protection methods is important, another main target for protecting health should be to prevent climate change and thus reducing the related increases in vector species and associated pathogens and parasites (Gibbons & Vaughn, 2002; Githeko, Lindsay, Confalonieri, & Patz, 2000).

Allergenic Pollen

Air-borne pollen grains cause much suffering due to related allergies and allergenic asthma. Grass, trees, and weeds are the most common sources of pollen (Singh & Hays, 2016). Since the late 20th century, there has been a remarkable increase in allergies (Platts-Mills, 2015). Allergies due to airborne pollen are increasing with air- pollution and climate change in combination with Westernized lifestyles and impaired immune function (possibly partly due to the lack of contact with nature; Singh & Hays, 2016; Baldacci, Maio, Cerrai, Sarno, Baïz, Simoni, Annesi-Maesano et al., 2015). Climate change has led to earlier and prolonged blooming periods; pollen allergenicity has been affected, and invasive allergenic species are spreading over new areas (Frank & Ernst, 2015; Smith, Cecchi, Skjøth, Karrer, & Šikoparija, 2013). Allergy incidence could be reduced by careful urban planning, selecting tree species and tree sex without allergenic potential (Ogren, 2004).

Biological Volatile Organic Compounds

VOCs can be both human-made and naturally occurring organic chemicals. Through a chemical reaction with nitrogen oxide in the air, VOCs form ground-level ozone, which is also a human health hazard. Natural VOCs are not only harmful; they also play a critical role in atmospheric chemistry and air-quality regulation. Studies on various tree species show that it is possible to reduce the emissions of VOCs and optimize air-pollution absorption in cities through careful planning, site-specific management, and the selection of low-emitting species (Donovan, Stewart, Owen, Mackenzie, & Hewitt, 2005; Curtis, Helmig, Baroch, Daly, & Davis, 2014). A study in Denver suggested that the planting of one million low-VOC-emitting trees would be equivalent to removing almost 500,000 cars from inner-city traffic in terms of reduced air pollution (Curtis et al., 2014).

Fear of Nature

It is worth considering why a concept such as “ecosystem disservices” has emerged, given that humans are actually dependent on ecosystems for their survival. One reason for an increasing susceptibility toward nature may be the gradual disconnection from nature that has happened over recent generations (Louv & Hogan, 2005). As people become less familiar with natural environments, the perception of risks and potential hazards may become disproportionate compared to the actual benefits that can be gained (Slovic, 2000, 2010). Considering that many of the disservices encountered are often caused by anthropogenic interference with ecosystems in the first place and that proper management and education could decrease these risks substantially, it is unlikely that the disservices would outweigh the many health benefits from natural environments (Van den Bosch & Nieuwenhuijsen, 2016; Villa, Bagstad, Voigt, Johnson, Athanasiadis, & Balbi, 2014; McPherson, Simpson, Peper, Maco, & Xiao, 2005).

Putting It All Together

Considering the existing evidence on human interactions with nature, we can conclude that health and well-being are influenced positively by access and exposure to various kinds of natural environments. It is also likely that those positive effects are mediated by a broad range of different factors, probably often working in synergy and accumulatively but sometimes working more or less in isolation, and sometimes trade-offs must be considered. Different studies have connected various mediating factors with respect to health outcomes (De Vries et al., 2013a; Dadvand et al., 2016), but no conclusive research has managed to define the relative effects of each factor in relation to each health outcome. We know that natural environments (usually defined in studies as level of greenness in a neighborhood) have a positive effect on all-cause mortality (Gascon, Triguero-Mas, Martínez, Dadvand, Rojas-Rueda, Plasència et al., 2016; Villeneuve, Jerrett, Su, Burnett, Chen, Wheeler et al., 2012), blood pressure (Grazuleviciene, Dedele, Danileviciute, Vencloviene, Grazulevicius, Andrusaityte et al., 2014), prevalence of obesity (Lovasi, Schwartz-Soicher, Quinn, Berger, Neckerman et al., 2013b; Dadvand et al., 2014), depression, and cardiovascular mortality (Reklaitiene et al., 2014; Tamosiunas, Grazuleviciene, Luksiene, Dedele, Reklaitiene, Baceviciene et al., 2014; McEachan, Prady, Smith, Fairley, Cabieses, Gidlow et al., 2016). However, we do not yet know which of the potentially mediating factors account for the effects or to what degree they function in different populations and regions. It is plausible that effects such as the prevention of heat exposure, stress recovery, and increased physical activity act together to reduce the prevalence of cardiovascular events, but many other pathways are possible as well.

Similarly, research has shown associations between natural environments and improvements in several health-related outcomes, such as pregnancy outcomes (Dadvand et al., 2012b; Grazuleviciene, Danileviciute, Dedele, Vencloviene, Andrusaityte, Uždanaviciute et al., 2015), general physical and mental health (Triguero-Mas et al., 2015; Alcock et al., 2014), longevity (Takano, Nakamura, & Watanabe, 2002), and behavioral and cognitive development (Dadvand et al., 2015; Markevych , Tiesler, Fuertes, Romanos, Dadvand, Nieuwenhuijsen et al., 2014; Amoly et al., 2014). Again, several mediators or pathways are likely to contribute to these outcomes, but so far, we are unable to define their relative impacts.

In addition, most of the studies relating natural environments to specific health outcomes have defined nature broadly, often by satellite indicators of greenness, where the resolution can limit the specificity of exposure measures and various aspects of the quality of the green spaces (including biodiversity and other ecological indicators) cannot be assessed. While attempts have been made to specify aspects of the environment, for instance, blue spaces (Wheeler, White, Stahl-Timmins, & Depledge, 2012), perceived restorative qualities (Annerstedt, Ostergren, Bjork, Grahn, Skarback, & Wahrborg, 2012), and patterns of nature (Hagerhall, Laike, Taylor, Küller, Küller, & Martin, 2008), we still know little about which specific landscape or nature features influence specific health outcomes.

While some specifications remain elusive, the health benefits of natural environments have started to enter the domains of policy and broad recommendations. In the Parma Declaration (WHO, 2010b), the European Union member countries committed themselves to providing children with healthy and green environments in which to play and be physically active. An urban green space indicator was developed, recommending that each inhabitant should have no more than 300 meters distance to a green space of at least 1 hectare (Annerstedt van den Bosch et al., 2016; WHO, 2016). The distance and size specifications are based on previous research on accessibility and on a common praxis in urban green planning. However, the specific recommendations are of minor value in comparison to the mere recognition by the world’s largest health organization that natural environments constitute important health resources.

Depending on which use the research is aimed the relative limitation in knowledge has more or less importance. If the plan is to use the evidence for designing a hospital garden for mentally ill, it should be of particular importance to know which features of the environments have specific effects on mental states. However, if the main goal is to prevent heat stress or reduce air pollution, a general tree-canopy indicator could be of suboptimal yet sufficient value. Although certain tree species are more efficient in reducing heat or air pollutants, it would at least be a good start from a policy and planning perspective to simply increase the urban green cover. This could be monitored relatively easily with, for example, satellite imaging, though much more detailed but localized analysis would be required for a hospital garden.

While it is likely that making investments in urban natural spaces would improve the health of a population, particularly in socially deprived areas, further studies should confirm relationships and define the types of nature as well as their relative importance for various risk factors and health outcomes. This would enable evidence-based health policies and public health actions and optimize the use of existing resources. It could also help to direct investments to the areas where the needs are most daunting, and where natural environments could be most efficient in providing solutions. Deciding where to locate green or blue spaces can sometimes be important, not only in terms of making them accessible to populations, but also so that they have the most impact on the health of the environment.

A particular feature of so-called nature-based solutions for urban health is the potential for co-benefit. By investing in natural spaces and preserving vegetation, a multitude of benefits to both environment and health, as well as climate, can be achieved, and sometimes even win-win-win scenarios can be expected since solutions tend to be cost-efficient (Bone & Nurse, 2010). Lining a road with trees, for example, not only improves air quality and thermal comfort but also aids in stress recovery— contributing to better health and thus reducing health-service costs. This is in contrast to technological solutions, such as cool roofs for reducing heat (Dabaieh, Wanas, Hegazy, & Johansson, 2015), which rarely provide any co-benefits.

In an increasingly urbanized world, much of the research focus has been on urban green spaces and urban populations, especially in the Western world. The health benefits in the great outdoors or other nonurbanized areas have been less scrutinized scientifically. The mental health values of nature and of other, more cultural ecosystem services to nonurban populations are, for example, less well explored.

Studies on indigenous populations living in close relation with the surrounding environment and natural resources are rare, and mostly qualitative methods have been applied (Lewis & Sheppard, 2006), making it difficult to quantify any health impact. It seems likely, though, that the values ascribed to and the benefits achieved from interactions with nature vary a great deal, depending on the context and to what extent the view of nature is mostly functional (for example, through timber production or direct livelihood) or more holistic, including cultural values (Buijs, Elands, & Langers, 2009).

The evidence for the fundamental value of natural environments and ecosystems for human health and survival is indisputable. This is not only demonstrated by how human health and well-being are negatively influenced by environmental degradation and climate change (Wang & Horton, 2015). As a health resource for urbanized societies, natural environments may also fullfil the needs for recreation, stress recovery, and social cohesion, while simultaneously regulating the urban climate. To optimize “nature’s health services,” scientific research must be pursued to improve cost-efficiency, exploring such factors as the adequate selection of vegetation, structure, and distribution. The effect sizes of natural interventions should also be explored and compared with other public health interventions, while simultaneously quantifying and taking into account potential co-benefits.

Regardless of their particular features, shapes, and forms, natural areas must be maintained and protected in and outside cities, safeguarding biodiversity and ecosystem services, not only for the sake of the environment, but equally for the sake of future human health and well-being across the planet.

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