These impacts on sleep and stress may entail decreased risk for mental illness, as sleep problems and stress are major risk factors for mental illness, especially depression In addition, there is growing evidence that nature experience is associated with a decreased incidence of other disorders [see 28 , 55 , 56 for reviews on the effects of green space on specific psychopathologies, including anxiety disorders 57 , attention deficit and hyperactivity disorder ADHD 58 , 59 , and depression 60 , 61 ]. Several of these associations are moderated by various contextual and individual factors, such as socioeconomic status, gender, and age In both consensus statements above, we include studies that have demonstrated significant associations, with a range of certainty regarding correlation versus causation.
It is essential that future research continues to specify and investigate underlying pathways and causal mechanisms to refine understanding of the relationships between the environment and human well-being. Over the past century, people have been increasingly concentrated in urban areas.
In many instances, modern living habits involve reduced regular contact with outdoor nature and increased time spent indoors, on screens, and performing sedentary activities 63 , This disengagement from nature may be partially driven by a negative feedback loop. As direct nature experiences become progressively unavailable to new generations, this creates an ever-narrowing spectrum of nature experiences These consensus statements underpin our conceptual model. The evidence in this arena is building to a point at which we may soon be enabled to make meaningful even if not extremely precise predictions regarding the impacts of environmental change on mental health.
Here, we propose a way forward that harnesses existing knowledge to eventually incorporate it into ecosystem service assessments. Psychological and social processes differ from bio- and geophysical processes. They exist in changing historical and cultural contexts, and even short-term changes are susceptible to multiple determinants. Given current knowledge, the model can only address average, population-level impacts, with the aim of eventually being able to distinguish and specify effects as they occur at the individual, subgroup e.
Nonetheless, our approach is broadly guided by other ecosystem service models, insofar as we trace a pathway from environment to mental health Fig. Photographs are from the public domain and free for public use.
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In step 3, we illustrate some of the crucial specifics of nature exposure i. Separately see Box 2 , we discuss what may be involved in placing a value on these benefits, in monetary or other terms. There is a considerable literature describing the monetary valuation of mental health. Analyses have focused on the avoided costs of mental illness and on the economic benefits of happiness, well-being, and thriving. A range of methods has been used in these cases, including direct market valuation, indirect market valuation avoided cost, factor income, hedonic pricing, etc. In general, mental distress and mental illness account for considerable costs, and relief of such suffering yields large benefits for society and the individuals affected , Improved learning and work productivity resultant from nature contact may also have positive economic impacts However, monetary value is only one of many ways to quantify the mental health benefits produced by nature exposure.
Many noneconomic measures of quality of life, well-being, and happiness have been developed , both in clinical settings and in sustainability science, and these may have a role in valuing mental health as an ecosystem service. One example is the DALY, now a standard currency in quantifying burden of disease and potentially suitable in ecosystem services calculations.
Another form of valuation includes a ranking approach rather than absolute values that projects the expected relative benefits of alternative scenarios of change in a specific location. These valuation approaches can help reveal the contribution of ecosystems to mental health in decision-making. With a more complete picture, decision makers can more fully consider the repercussions of losing or enhancing access to nature, in the context of urban design, including the spatial layout of built and natural environments, and proximity to workplaces and homes.
Valuation can help inform judgments of whether to invest in nature and how to do so while also considering other pressing needs. Our knowledge regarding the magnitude of mental health benefits on their own may not be enough to justify the costs associated with increasing nature within cities, but together with benefits such as water quality, flood security, urban cooling, and recreation, we can obtain a more complete picture of the impact of these types of decisions.
This step characterizes the elements of nature potentially influencing mental health and includes size total area , composition proportions of different types of natural elements , and spatial configuration e. Other relevant natural attributes may include tree canopy density, vegetation structure, species composition, or biodiversity 68 , Data can be gathered from a variety of sources e. Determining which aspects of natural features are relevant to mental health—and should therefore be considered in this step—is a key research frontier and will be informed through an iterative process via an evolution of insights and evidence regarding effects see step 3.
For example, are some tree species more beneficial than others 70? Is a diversity of tree species in a forest more beneficial than a monoculture stand 71? It is also important to note that little is known about relationships between ecological integrity or complexity and mental health benefits. It may be that places intermediate on the wild-anthropogenic spectrum, tuned to some common evolutionary-based human preferences, are associated with better mental health Our lack of certainty with respect to these and other questions regarding the relationship between nature and mental health underscores the need for future research.
It is also a reminder that the purpose of this endeavor is to create a conceptual model with which to integrate the best available evidence, wherever that may stand, as the field evolves. We must also consider how aspects of natural features result in various amounts of exposure, given the different opportunities for direct and indirect nature contact they afford.
This is addressed in the next step. Exposure is a broad term, here referring to the amount of contact that an individual or population has with nature. The proximity of people to nature is likely to be a large determinant of exposure.
A watershed located 50 km outside a city might generate considerable ecosystem services in provision of clean water to the city but not much opportunity for everyday interaction with the landscape. Conversely, the presence of a small city park may result in extensive nature exposure for neighborhood residents and commuters.
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At present, there is a limited repertoire of methods for estimating nature exposure based on geography. Ekkel and de Vries 73 have identified two principal approaches: cumulative opportunity and proximity measures. Using sources such as satellite images or land-use databases, this proportion is generally calculated as the percentage of an area of interest a zip code area, census block, etc. A cumulative exposure metric for a population can then be derived for a given spatial unit based on this composition score e.
Proximity measures typically estimate use and exposure to nature as a function of direct physical distance to the nearest nature area of a certain size, usually from a place of residence. Walking time from residence to nature has also been used as a proximity measure. These and other approaches, based primarily on estimations of average exposures, show mixed levels of reliability in their associations with health outcomes As metrics are further developed, frequency and duration of exposure should be considered, as well as aspects of the natural features themselves from step 1.
For example, the composition, spatial configuration, and other features of nature will influence the amount of exposure that a population will experience intentionally or otherwise due to resultant differences in accessibility Fig. Other characteristics of these natural spaces e. In future iterations of the model, we can look to methods used by recreation and cultural ecosystem service models for ways to isolate predictors of visitation to areas in a landscape e. Where available, primary data on actual nature exposure versus potential or opportunity for exposure can also be incorporated.
Along with other factors, spatial configuration and composition should be considered when estimating nature exposure. Different shades of green here represent different types of nature [e. Despite these differences, all panels have the same total amount of nature 34 street blocks. Vertical contrasts illustrate differences in configuration of this nature [e.
With respect to characteristics of beneficiaries, the model should eventually account for the sociodemographic, cultural, perceptual, attitudinal, and behavioral differences that influence the tendency for seeking out nature exposure 76 , Measurement approaches based on location alone can fail to account for differences in exposure that are due to factors such as access to transportation corridors, time demands, income disparities, and perceived safety. This consideration brings us to step 3. The third step in the model accounts for the experiential characteristics of nature exposure—what we term nature experience.
In moving from nature exposure to mental health effects, we need to consider these specifics. Though attuned to pragmatic considerations regarding data availability, neither cumulative opportunity nor proximity measures account for some relevant aspects of nature experience, including, for example, the sensory qualities of the exposure. Although much of the research literature defaults to eyesight as the primary modality for nature contact 79 , the auditory, tactile, and olfactory modalities are also important to consider Effective park programming can also have a substantial impact on the ways in which users interact with natural spaces, and thereby help determine how these sensory pathways are engaged Two approaches suggest ways to classify nature exposure and characterize nature experience:.
The specific ways in which people interact with nature may account for differential impacts of nature exposure on mental health Looking at water is different from swimming in water, for example. For example, walking along the edge of water and land can occur at the ocean or alongside a lake or river. To date, around human-nature interaction patterns, with photos and descriptions for many of them, have been generated and catalogued 40 , Future studies can look at specific health outcomes of people who engage not only in specific forms of interaction but also in constellations of them.
Exposure and dose can vary considerably in toxicology, if, for example, two people exposed to the same concentration of an air pollutant breathe at very different rates. A similar phenomenon may operate with respect to nature contact 83 via different levels of attention, preference, and feelings of personal connection with nature People have different levels of awareness and perceptions of natural environments 85 in their attitudes and receptivity toward nature, childhood experiences, and sense of connectedness to nature—factors that probably affect the delivered dose that results from a given exposure.
The transition from dose to effects corresponds to what economists call a production function, and what toxicologists and epidemiologists quantify using a dose-response curve. We discuss more on the multiple potential causal mechanisms below in step 4.
The fourth and final step of our conceptual model involves a characterization of the potential mental health impacts that follow from nature experience. The production of mental health benefits from nature experience may occur through multiple psychological causal mechanisms and pathways, including reduction of stress, increases in social cohesion or physical activity, or replenishment of cognitive capacities, to name just a few. In many cases, the same natural area will engage multiple mechanisms during each single experience 24 , 86 , 87 so that the cumulative effects attributed to any single pathway may be misestimated.
Current insight into each of these mechanisms is incomplete.