Using Greenspace and Nature Exposure as an Adjunctive Treatment for Opioid and Substance Use Disorders: Preliminary Evidence and Potential Mechanisms

Abstract

The demand for opioid medication to effectively treat pain has contributed to the surging opioid crisis, which is a major source of morbidity and mortality in the U.S. More than 100,000 people begin opioid maintenance treatment (OMT) annually, the standard pharmacotherapy for opioid use disorder (OUD). Although OMT is the standard care for OUD, patients often experience or develop a heightened sensitivity to pain (hyperalgesia) as a result of the opioid medication, and also have high rates of stress and affective and anxiety-related conditions. These conditions are interactive with other behavioral and environmental correlates of opioid and other substance use disorders including impulsive decision-making (e.g., harmful opioid use associated with increased delay discounting), and a lack of alternative (i.e., substance-free) and social reinforcement. Collectively these complex and multifaceted factors constitute significant predictors of lack of adherence to OMT (and other pharmacotherapies) and relapse. There is an urgent need, therefore, to develop novel adjunctive treatments that preserve the benefits of OMT and various pharmacotherapies, and simultaneously diminish continued pain and hyperalgesia, reduce stress and anxiety-related conditions, target relevant behavioral mechanism such as impulsive choice, and also serve to enhance the value of alternative and substance free activities. Here, we discuss evidence that an environmental manipulation – access to greenspace and nature – could serve as a potential adjunctive treatment to standard pharmacotherapies by targeting multiple biological and behavioral mechanisms that standard pharmacotherapies do not address.

1. Introduction

From 2002 to 2017 in the U.S. alone, deaths from opioid overdose have more than quadrupled (National Institutes on Drug Abuse, 2018). Relatedly, pain represents the leading cause of disability in the U.S., affecting more Americans than diabetes, heart disease, and cancer combined (National Institutes of Health, 2019). The demand for opioid medication to effectively treat pain has contributed to the surging opioid crisis, which is a major source of morbidity and mortality in the U.S. More than 100,000 people begin opioid maintenance treatment (OMT) annually (SAMHSA, 2014), the standard pharmacotherapy for opioid use disorder (OUD).

Although OMT is the standard care for OUD, patients often experience or develop a heightened sensitivity to pain (hyperalgesia; Dunn et al., 2015), and also experience high rates of stress, and affective and anxiety-related comorbidities (Gros et al., 2013; Hyman et al., 2007; Sinha, 2008). These conditions are interactive with other behavioral and environmental correlates of opioid (and other substance) use disorders, including impulsive decision-making (e.g., harmful opioid use associated with increased delay discounting), and a lack of alternative (i.e., substance free) and social reinforcement. Collectively, this complex array of factors may constitute significant predictors of poor adherence to OMT and other pharmacotherapies for substance use, relapse, and ongoing harmful substance use (e.g., Karakula et al., 2016; Wilsey et al., 2008). There is an urgent need to develop novel adjunctive treatments that preserve the benefits of OMT and other pharmacotherapies, and simultaneously diminish this constellation of biobehavioral factors that complicate and preclude recovery from opioid and substance use disorders. Currently, no established adjunctive OMT treatments simultaneously target each of these mechanisms.

A promising adjunct treatment for improving the physical, affective, and behavioral correlates with OUD (hyperalgesia, stress, depression, discounting), and simultaneously enhance non-substance related reinforcement, may be exposure to greenspace (i.e., areas of grass, trees, or other vegetation). Green light -- similar to that in greenspaces -- has analgesic properties and reduces experimental pain in animal models (Ibrahim, 2017). Research shows exposure to greenspace decreases anxiety, stress, and depression, which are common comorbid conditions among individuals with opioid and other substance use disorders (SUDs; Koob & Schulkin, 2019; Kushner et al., 1990; Kushner et al., 2008). Access to greenspace is also associated with reductions in craving of various substances (Martin et al., 2019). Further demonstrating the potential of exposure to greenspace and nature as an adjunctive treatment for OUD, visual exposure to natural environments also decreases impulsive decision-making in delay discounting tasks (e.g., Berry et al., 2015), one dimension of impulsive decision-making with strong associations to addictive disorders (Mitchell, 2004 a,b; Kirby & Petry, 2004; Odum, Madden, Badger & Bickel, 2000). Relatedly, individuals living farther away from recreational outlets (including parks and greenspace) and have less access to pleasant activities have higher rates of substance use, including prescription opioid use (Leventhal et al., 2015).

In this selective conceptual review, we outline evidence that illustrates the promise of greenspace and nature exposure as a potential adjunctive treatment to OMT and other SUD pharmacotherapies (and behavioral therapies). Due to the limited literature available combining these topics, this conceptual overview broadly covers evidence that nature and greenspace exposure are relevant to OUD and SUD targets, with papers identified and included on the basis of prior knowledge, literature searches using relevant terms (e.g., green space, physical health, mental health), and references from the resulting publications. While the state of the literature warrants a more casual consideration of nature and greenspace effects, it is important to keep in mind the limitations of this approach (e.g., potential bias). We revisit these issues in the discussion.

Across the following sections, we present preliminary evidence and interim discussions on each of the mechanisms discussed above. These include (i) pain reduction properties of green light and greenspace, which may benefit individuals who experience both and painful conditions opioid use disorder, (ii) greenspace effects on mental and physical health, including reducing the burden of common comorbidities among opioid use disorder patients (e.g., stress and anxiety), (iii) acute reductions in delay discounting (associated with harmful opioid use) with visual or real-world exposure to greenspace and nature, (iv) social connection and prosocial benefits associated with greenspace, and (v) reductions in substance craving with access to greenspace. Then, we close with a brief discussion of conclusions, how behavioral and cognitive researchers and theories can contribute to (e.g., quantitative modeling of attentional mechanisms) and conceptualize (e.g., an alternative source of reinforcement/substance free activity), this research topic, limitations, and future directions. While we focus and discuss greenspace exposure as it is relevant to OUD, much of this research has implications for other substance use and SUDs as noted throughout.

2. Effects of Green Light, Greenspace, and/or Visual Nature Exposure on Pain

Evidence in animals and humans suggest both pain and addictive disorders are characterized by impaired hedonic capacity and high stress based on their shared neurobiology (Elman & Borsook, 2016). Further, both pain and addiction are associated with structural and functional changes in brain regions implicated in the regulation of affect and impulse control, as well as in reward and motivational functions (e.g., Upadhyay et al., 2010). Of fundamental importance, experiencing pain in the context of opioid and other SUDs is a significant predictor of poor treatment response and relapse for those in treatment (Chou et al., 2009; Jakubczyk et al., 2016; Tsui et al., 2016). Opioid use disorder patients report that continued pain leads to seeking additional opioids to help manage their pain. Paradoxically, continued treatment on common OMT medications (e.g., methadone) can lead to opioid-induced hyperalgesia, which is a development of heightened sensitivity to painful stimuli as a direct result of the medication prescribed to treat OUD.

A relatively new area of research that may point to an essential element in the health benefits of nature exposure is that of green light exposure and pain. In patients suffering from migraines, green light exposure (relative to other colors of light) elicits positive emotions during migraines (Noseda, Copenhagen, & Burstein, 2018), aggravates migraine symptoms significantly less than other colors of light, and reduces pain intensity in a significant number of patients (Nir et al., 2018; Noseda et al., 2016). Nature scene murals with large swaths of greenspace placed at the bedside (coupled with nature sounds in the background) during a flexible bronchoscopy procedure reduced self-reported pain compared to a control condition. This effect remained after controlling for age, gender, race, health status, and medication doses (Diette et al., 2003). It is unclear however, if this result was due to nature scenes or nature sounds, or the combination of the two. Research also shows that post-surgical patients with a view of trees compared to a brick wall were released from the hospital sooner and required fewer analgesic doses 2-5 days after surgery (Ulrich, 1984).

This pain-reducing effect of green light exposure has cross-species generality and with different types of pain. In particular, Ibrahim and colleagues (2017) demonstrated that green light exposure meant to mimic greenspace, reduced acute thermal pain in rats, and alleviated heightened pain sensitivity (hyperalgesia) and sensation of pain in response to non-painful tactile stimuli (allodynia) in a rat model of chronic neuropathic pain. The mechanisms underlying the anti-nociceptive properties of green light exposure appear complex, operating via the visual and opioid systems and more (Ibrahim et al., 2017). There is also initial evidence to suggest that greenspace exposure could moderate elements of pain catastrophizing, which is defined as ruminating on pain and feeling more helpless about painful experiences (Wells et al., 2019). This area of research may be particularly relevant for individuals who suffer from both opioid use disorder and painful conditions.

Together, these early findings on the antinocioceptive effects of green light, greenspace, and nature scenery carry hope for applied and clinical uses. That this method of producing analgesia is non-pharmacological in nature is of benefit as it may be less likely to pose safety concerns (i.e., no drug interactions with methadone or buprenorphine; side effects). For example, the origin of OMT patient pain is diverse (e.g., Dunn, Brooner, & Clark, 2014) which makes following specific clinical practice guidelines for pain treatment challenging, as many guidelines are written for specific pain conditions (e.g., lower back pain, fibromyalgia). Opioid medications, which are first-line treatments for pain, may not be appropriate for OMT patients due to cross-tolerance (i.e., decreasing analgesic efficacy), or other medication interactions (e.g., increased risk of respiratory depression) in methadone patients.

Continued research in this area will be imperative to understanding the exact neural mechanisms of these interventions, and ultimately how they can be employed to yield maximal analgesic benefit. Similarly, understanding the neural mechanisms of these effects will also shed light on who these interventions may be most beneficial for (e.g., how certain visual conditions may or may not preclude green light benefits; if certain types of drug use may compromise the function of necessary structures). More human research using experimental pain models to evaluate the generalizability of this effect is sorely needed (e.g., what exact “dose” of green light is required to generate analgesic effects in humans?). At this time, there is little systematized research in this area.

3. Effects of Nature on Mental and Physical Health

Both stress (see Koob & Schulkin, 2019 for review) and affective and anxiety-related conditions (Kushner, Sher, & Beitman, 1990; Kushner, Krueger, Frye, & Peterson, 2008) are significant predecessors of SUDs including OUD; and the presence of these conditions is often associated with poorer SUD outcomes (e.g., Compton, Cottler, Jacobs, Ben-Abdallah, & Spitznagel, 2003; Burns, Teeson, & O’Neill, 2005). Both correlational and experimental research support that nature exposure improves stress and anxiety. Among those in urban environments, wellbeing is positively associated, and objective (salivary cortisol) and subjective (self-reported) stress are negatively associated with levels of surrounding greenspace (White, Alcock, Wheeler, & Depledge, 2013). The association between greenspace and reduced stress has been replicated and extended to individuals dwelling in both urban and rural settings (Beyer et al., 2014), and to include additional mental health benefits such as reduced anxiety and depression (Beyer et al., 2014; Cox et al., 2017). At this time, the mechanisms behind these relations are unclear. Some have suggested these effects are driven by increased physical activity and improved social contact (Thompson, Aspinall, Roe, Robertson, & Miller, 2016). Other experimental research highlighting the benefits of nature exposure provide additional alternatives such as improvements in cognitive and/or attentional resources (Berto, 2005; Berman, Jonides, & Kaplan, 2008; Berman et al., 2012), reduced physiological arousal (Ulrich et al., 1991; Laumann, Garling, & Morten Stormark, 2003), and reduced negative affect (see Bowler et al. 2010 for meta-analysis).

Quantifying the exact effects of nature/greenspace exposure become difficult with the heterogeneity of methods used to quantify and define “greenspace” as well as varied psychological well-being measures. Houlden, Weich, Porto de Albuquerque, Jarvis, and Rees (2018) recently published a systematic review on the relationship between greenspace and mental wellbeing of adults. The review focused on several measures of greenspace, including amount of local area greenspace, greenspace types, visits to greenspace, views of greenspace, and accessibility of greenspace. The largest number of studies focused on local-area greenspace, with several studies within this category covering either life satisfaction, or general health (measured by the General Health Questionnaire, which is designed to measure psychological distress, including anxiety). For the life satisfaction measures, five studies found that life satisfaction was significantly improved with more greenspace, albeit with small linear effect sizes (.002-.003, and predominantly in urban areas, rural area associations may differ). Consistent results with the General Health Questionnaire were found, with all but one of eight studies revealing an inverse relationship between the amount of greenspace and General Health Questionnaire scores, implying reduced mental distress with more greenspace (i.e., more greenspace was associated with better mental health, and the linear regression coefficients varied considerably in this work (0.003 to 0.431).

Although effect sizes vary, results appear to me somewhat consistent. Given the influence of stress and affective and anxiety-related conditions on opioid and other SUDs, targeting these aspects of mental health in treatment may be beneficial. Perhaps nature exposure could be incorporated as an additional support to those already commonly included in OMTs (e.g., group therapy, drug screens); and in the case of the former example, even incorporated into the support itself by holding sessions outdoors. Increased exposure to nature and access to greenspace may have preventative utility by ameliorating these early predisposing factors; and could perhaps help quell the negative states thought to maintain or provoke relapse to drug use (i.e., negative affect during withdrawal and stress-induced craving; see Sinha, 2008, and Koob & Schulkin, 2019 for discussion), some of which are also associated with poorer OMT engagement and retention itself (Jaremko, Sterling, & Bockstaele, 2015). The significant associations between access to greenspace and mental health, and mental health and OUD/SUDs, the effects of nature on these processes highlights the need for further research in this area.

In tandem with findings relevant to mental health, an increasing amount of evidence has revealed physical health benefits of nature exposure. For example, greater proximity to residential greenness is associated with improved birth outcomes (Hystad et al., 2014), decreased morbidity levels (Maas et al., 2009), and decreased risk of mortality (Wang & Tassinary, 2019) when compared to urban built environments. Research suggests that individuals living closer to greenspaces are 40% less likely to be overweight and three times more likely to engage in physical activity (Ellaway, Macintyre, & Bonnefoy, 2005). Complementary to this finding, engagement in physical activity is lower, and the prevalence of obesity higher, in individuals living further away from greenspaces (Lachowycz & Jones, 2011; Toftager, et al., 2011). These physical health benefits of nature and greenspace remain even when controlling for various confounding variables such as air quality, noise pollution, and socioeconomic status.

The health benefits of nature exposure are not strictly afforded by living in close proximity to such environments. Short-term exposure to natural environments significantly reduces blood pressure, heart rate, and cortisol levels (Grazuleviciene et al., 2016), and even a simple walk in nature can be health-protective. For example, didehydroepiandrosterone (DHEA), an endogenous hormone with cardio-protective, anti-obesity, and anti-diabetic properties, is significantly elevated in the blood after walking in forested terrain, but not after walking in urban areas (Li et al., 2011). Similarly, walks in a forest, but not in urban environments, reduce pro-inflammatory cytokines in the body (Mao et al., 2012), which are associated with detrimental health effects (Haraoui, Liu, & Papp;; Zhang et al., 2012). Brief walks in a forest also reduce total hemoglobin concentrations in the right prefrontal cortex, indicating reduced activity and by proxy, increased relaxation (Park et al., 2007). Viewing forest imagery on a computer screen produces a similar calming effect (Song, Ikei, & Miyazaki, 2018), which further supports the benefit of brief exposures, and also more vicarious exposure.

Twohig-Bennett & Jones (2018) published a systematic review and meta-analysis regarding greenspace exposure and a wide range of health outcomes --- and small to medium effect sizes were generally observed in this area, however varied greatly depending on the specific measure evaluated. As in the mental health and well-being systematic review, greenspace and physical health outcomes were defined and measured in a variety of ways across studies. However, fairly consistently results showed increased green space exposure was associated with decreased salivary cortisol, heart rate, and diastolic blood pressure, among other outcomes such as decreased risk of preterm birth (Twohig-Bennt & Jones, 2018).

4. Effects of Nature Exposure (Including Greenspace) on Delay Discounting

Delay discounting refers to the decline in value of an outcome with delay to receiving that outcome (Mazur, 1987). Rapid discounting of delayed rewards is associated with a host of maladaptive behaviors, including harmful and risky opioid use and opioid use disorder, tobacco smoking, and other addiction-related behaviors (Madden Petry, Badger & Bickel, 1997; Mitchell, 2004 a,b; Kirby & Petry, 2004; Odum, Madden, Badger & Bickel, 2000).

The effect of nature exposure on impulsive choice was first demonstrated by van der Wal and colleagues (2013). They found that visual exposure to nature photographs on a computer screen resulted in less impulsive decision-making than built photographs in a monetary discounting task. That is, people discount delayed monetary rewards less (i.e., are less impulsive/more self-controlled) when visually exposed (view stimuli on a computer screen) to natural environments such as forests or mountains compared to built environments such as cities and roads. In a follow-up experiment, similar results were obtained when participants either walked through natural environments or built landscape environments and subsequently chose between receiving money now or in the future.

The effect of nature exposure on delay discounting appears robust and has been replicated by other laboratories. For example, Berry et al. (2014; 2015) showed that individuals exposed to photographs of natural environments exhibited significantly less delay discounting than those exposed to photographs of built environments using a different discounting task; and further found that the effect is potentially driven by expanded time and space perception (Berry et al., 2015; 2019; Repke et al., 2018). Another mechanism that may govern these effects on decision-making is increased attention, which may be influenced by differences in eye movements (e.g., saccades, fixations) associated with attentional demands of tracking scenes of natural versus built environments (Berto et al., 2008). Faber Taylor, Kuo, and Sullivan (2002), demonstrated that among children living in the inner city, the more natural a girl’s view from home was, the more “self-controlled” she was on a modified version of the classic marshmallow task (this same relation was not true for boys). Due to the consistent benefits conferred by nature exposure on discounting and the strong ties between discounting and OUD and SUDs, it has been previously suggested that exposure to nature should be explored as an adjunctive treatment to disorders characterized by poor impulse control (Berry et al., 2014).

The impulsivity-reducing effect of nature appears large on average (Cohen’s d = 0.89; as estimated in Rung & Madden, 2018b), and has been extended to other commodities in laboratory experiments (e.g., air quality, Berry et al., 2019). This finding is of particular importance as the hope of such an adjunctive intervention would be that it reduces impulsive decisions to use substances and not for smaller monetary outcomes per se. Recent studies employing other techniques to reduce delay discounting of money in laboratory tasks also produce reductions in drug consumption (e.g., cigarette puffs, Stein et al., 2017; see Odum, 2011 for discussion). Such findings indicate promise of generalizability, but the value of nature and greenspace exposure to reduce real-world “impulsive” decision-making remains speculative. Relatedly, and as has been done with some other manipulations of discounting, it will be important to evaluate what role (if any) demand characteristics or expectancy may have in nature effects on discounting (e.g., see Rung & Madden, 2018a; Rung & Madden, 2019 for discussions and evaluations). In sum, while studies on nature exposure on discounting have yielded consistent effects, more research is needed outlining the generality of these effects and understanding its driving mechanisms; and all the while maintaining an eye towards how their therapeutic benefit could be harnessed.

5. Effects of Nature and Greenspace Exposure on Social Connection and Prosocial Behavior

Opioid and other SUDs is associated with social isolation for some, which can contribute to continued harmful substance use (Polenick, Cotton, Bryson, & Birditt, 2019; Stein et al., 2007). Exposure to natural environments and/or greenspace can influence feelings of, or desire for social connectedness (i.e., desire for close relationships with others). For instance, Weinstein, Przybylski and Ryan (2009) evaluated the effect of nature exposure on prosocial values and behavior in four experiments. Participants either viewed images of nature or built environments on a computer screen; or sat in the lab with or without plants. Across all experiments and regardless of the type of exposure, participants in the nature condition showed greater value of intrinsic aspirations (i.e. desire for close relationships with others and the community), lower value of extrinsic aspirations (i.e. acquire fame and material wealth); and in a generosity and trust task, more participants assigned to the nature condition chose to donate money to a peer instead of keeping money for one’s self. These changes in participants’ values and decision-making were mediated by increased connectedness to nature and feelings of autonomy. The benefits of nature on prosocial behavior have also been replicated in cooperative situations with groups (see Zelenski et al., 2015), and there is evidence of additional prosocial behavior resulting from nature exposure in naturalistic settings outside of the laboratory (see Guéguen & Stefan, 2016 for an example of the effects of immersion in nature on helping behavior).

Interestingly, in the study conducted by Weinstein et al. (2009), exposure to nature images was accompanied by an instructional script, which aimed to increase immersion in the scene by directing participants’ attention to environmental details (e.g., colors and textures, the air, smells, and sounds of the environment). This aspect of the exposure can be considered an attentional manipulation, the implications of which were not thoroughly discussed by the authors. Although enhancing attention to stimuli may bolster the effect of nature exposure in the lab setting, this directed advertency does not necessarily occur spontaneously outside of the lab, and thus may not reflect what one may anticipate from increasing the prevalence of greenspace alone (e.g., planting trees around the neighborhood). The former may be an important first step, with the requisite that educating people to be mindful and attentive is key to producing a robust greenspace benefit.

The potential importance of attention to nature in these types of exposure manipulations may be gleaned from Passmore and Holder (2017), who found similar increases in prosocial behavior and values in the context of a 2-week intervention. Their nature exposure manipulation consisted of orienting participants’ attention to nature without directly increasing exposure to nature, and their findings are in agreement with those of Weinstein et al. (2009): attending to nature produced relatively higher intrinsic (vs. extrinsic) valuation and generosity (proportion of money allocated to self and other), although this increase was only significant relative to those attending to built scenes and not those in a “do nothing” control (findings which may be more relevant for those in urban environments). This increased attention not only shifted prosocial behavior and values, but also positive emotions during the intervention and positive affect and connectedness (to others, nature, and life) at the end of the intervention. These findings support the idea that it is not time in, or excess availability of nature per se that is important, but attention to it. That nature benefits can be conferred without directly increasing its presence underscores its feasibility and broadens its accessibility as a potential therapeutic tool.

Complementary to the experimental research showing the benefits of nature on prosocial values and behavior are the findings of correlational studies indicating the deleterious effects of nature deprivation on behaviors. In a longitudinal study following participants from middle childhood to adolescence, Younan et al. (2016) found a negative relationship between the amount of greenspace around participants’ homes and parent-reported aggressive behavior; this relation held when controlling for neighborhood quality, sociodemographics, and climate factors. In a similar study, Kuo and Sullivan (2001) found that higher levels of vegetation around buildings predicted less property and violent crimes while controlling for other variables (e.g., number of apartments in the building).

In sum, there is converging evidence for prosocial benefits of increased (and drawbacks of decreased) nature exposure using various empirical methodologies. Determining whether or not nature and greenspace may directly facilitate (or indirectly, by changes in values) social contact would be of benefit, given that certain types of social support may be important in SUD treatments (Cucciare, Han, Curran, & Booth, 2016; Dobkin, De Civita, Paraherakis, & Gill, 2001; Zhou et al., 2017). That this manipulation appears to positively impact related values and motivational processes, and that deprivation may impede them, makes this possibility appear promising but it remains speculative without direct data.

6. Effects of Green Space on Craving

While particularly relevant to addiction-related processes, research on the effects of nature and greenspace on craving is lacking. In the only known study to examine this topic, Martin, Pahl, White, and May (2019) asked a sample of health organization and university employees to indicate a good/substance they most commonly experienced cravings for (either food, chocolate, caffeine, nicotine, alcohol, or other) and to complete a measure quantifying the frequency and strength of cravings for the indicated good/substance. Self-reported proportion of greenspace comprising the view from one’s home and access to a private garden were associated with less craving; both of these relations were mediated by reductions in negative affect, although the association between garden access and craving frequency was only partially mediated. Almost a third of participants reported they most frequently craved substances with psychoactive compounds (30%), and all analyses controlled for whether the craving target included a psychoactive compound (which was not a significant predictor in any statistical models). Notwithstanding the typical limitations of correlational research, these findings illustrate that nature exposure may be relevant in maintenance and relapse to substance abuse by way of protective effects against craving and negative affect.

The effects of greenspace on nicotine/tobacco craving are highly relevant for tobacco use disorder and craving in general, but also relevant for individuals in opioid maintenance treatment. For example, 80% or more of methadone maintenance patients also smoke (Cooperman, Richter, Bernstein, Steinberg, & Williams, 2015), yet few receive smoking cessation interventions. This issue corresponds to recent calls for nicotine/tobacco use and opioid use to be co-treated as the standard of care (Morris & Garver-Apgar, 2020). These results also highlight the potential broad-ranging benefits of an adjunctive treatment such as nature or greenspace exposure, with potential to target multiple aspects of craving and associated phenomenon (e.g., negative affect) across different drug classes with relevance for polysubstance use.

Additional research must be undertaken to understand the causal value of nature and greenspace exposure in the domain of craving and what other mediators may dictate. Complementary to the discussion of nature exposure on delay discounting above, the authors proposed self-control as one such potential mediator that may act in concert with negative affect. Perhaps most importantly in the context of the present review, the association between craving and greenspace needs to be expanded to other substances, including opioids. The findings of Martin and colleagues are the first of their kind and enlightening, but much work remains to be done.

7. Conclusions, Limitations, and Future Directions

The present narrative and conceptual review synthesized and discussed several biological, behavioral, and environmental targets that could simultaneously be addressed by increasing greenspace and nature exposure in the context of adjunctive treatments for OUD and other SUDs. These targets include (i) pain reduction, (ii) mental and physical health benefits (e.g., stress and anxiety reduction), (iii) reductions in delay discounting, (iv) social connection and prosocial benefits associated with greenspace, and (v) reductions in substance craving with access to greenspace. Greenspace and nature exposure is proposed in addition to standard pharmacotherapies and other components commonly included in SUD treatments (e.g., motivational enhancement therapy, cognitive behavioral therapy, counseling, group meetings), particularly in the context of OUD, but with potential for wider application to other SUDs. Although this area of research shows promise across numerous biobehavioral and environmental processes, there is a need for much more data to inform how this adjunct treatment might be applied.

Human and non-human behavioral scientists and their philosophical perspectives would be particularly well-suited to address outstanding empirical questions pertaining to nature and green space exposure; and could make unique and strong contributions to this developing field. For example, behavioral economic theory suggests that substance use is influenced by the availability and value of alternative sources of substance-free reinforcement (i.e., activities or environments that compete with substance use; Vuchinich & Tucker, 1988). Hence, high rates of drug use are more likely in environments without substance-free sources of reinforcement (Carroll et al., 2009; Volkow et al., 2003) and drug use tends to decrease if access to alternative reinforcers is increased (Higgins et al., 2004; Heinz et al., 2012). Because substances are readily available and easy to access in many communities (Komro et al., 1999), individuals with less access to alternative reinforcers (i.e., parks, recreational outlets) due to neighborhood deprivation or financial restrictions may be more likely to choose substances as a means of deriving pleasure (Audrain-McGovern et al., 2004; Murphy et al., 2005; 2005; Leventhal et al., 2015). As reviewed previously, other alternative reinforcers such as social interactions may also be positively influenced through nature/greenspace exposure. Nature and greenspace exposure may help to shift motivational processes over time. While these general principles apply to the use of opioids, they have not yet been examined within the context of nature or greenspace access and exposure.

Although data specific to opioid use and nature exposure is limited, there is evidence that the physical environment may influence substance use (NSDUH, 2008; Barbeau et al., 2004; Galea et al., 2004). For example, studies measuring access to environmental resources have indicated that proximity and density of substance use outlets alone (liquor stores, bars) are associated with increased substance use (Halonen et al., 2013) and substance-related consequences (Livingston, 2008; Jennings et al., 2014). Relatedly, individuals who live farther away from recreational outlets (including parks and greenspace) and have less access to pleasant activities have higher rates of substance use, including prescription opioid use (Leventhal et al., 2015). With exposure to greenspace being related to improved health, it may also serve as an alternative reinforcer to substance use via access to parks and recreational centers (Mennis et al., 2016) but has yet to be empirically tested. Additionally, quantifying behavioral and cognitive mechanisms of these effects would allow more precise characterization of the contributing underlying processes at play (e.g., quantitative modeling of effects of nature on drug demand, delay discounting, relapse, timing). Hence, more theoretically and quantitative based research in this area is needed.

Several particular limitations currently exist on this topic and need additional systematized research. First, the vast majority of the work outlined here and that exists in the literature more generally has not tested the effects of greenspace and nature exposure within those with OUD, let alone any SUD population. In fact, we are not aware of a single study that tests the effects of nature exposure on any of these mental health or behavioral targets in the context of formally diagnosed SUDs. It is possible that there is limited generality for the reductions in pain, improved mental health, reduced delay discounting, and increased social connection afforded by exposure to greenspace/nature. Given this possibility, it will be a highly important area of future research to determine the effects of greenspace/nature exposure on these biobehavioral and environmental outcomes among individuals with SUDs directly. Further, due to the limited literature available combining these topics, this conceptual overview covers evidence that nature and greenspace exposure are relevant to OUD and SUD targets, with papers identified and included on the basis of prior knowledge, literature searches using relevant terms (e.g., green space, physical health, mental health), and references from the resulting publications. While the preliminary state of the literature warrants a more conceptual consideration of nature and greenspace effects as provided in this manuscript, it is important to keep in mind the limitations of this approach (e.g., potential bias, potentially neglecting null results). As more evidence accumulates, a systematized review will be most beneficial.

A recent review (Wendelboe-Nelson et al., 2019) synthesizing the literature regarding mental health and well-being benefits of greenspace suggests that the great diversity in study design, definitions used, outcome measures assessed, and reporting of results makes aggregating evidence to identify potential causal mechanisms difficult. In particular, there is a limited amount of research using randomized controlled crossover designs and mixed methods (Wendelboe-Nelson et al., 2019). Rigorous methodologies afforded by behavioral sciences could supplement the existing literature to better answer questions associated with greenspace and nature in the context of SUDs (e.g., behavioral economic demand, delay discounting, biologically verified urinalysis results of nonmedical prescription opioid use).

Second, the “dose effects” of nature are not well understood. That is, how long or what type of nature exposure results in benefits (e.g., Hoyle et a., 2017)? What role do perceptions play (i.e., individuals may judge greenspace qualitatively differently, Hoyle et al., 2017; Seresinhe et al., 2015)? And is the same time or type of greenspace/nature exposure that is required for pain reducing properties the same as that for reductions in delay discounting? Although firm conclusions are difficult regarding the ability to establish dose effects based on current evidence, some data exist to inform this question. Data across a large-scale study conducted in England suggest that at least 2 hours per week in nature could serve as a benchmark to begin the discussion (White et al., 2019). The “dose effects” and other effects discussed in this review may also vary as a function of how connected one feels to nature, and other individual differences in affinity towards greenspace and nature (Cervinka et al. 2012; Lin et al., 2014; Perrin & Benassi, 2009). However, the range in methodologies and exposure methods outlined in the present conceptual review makes it impossible to gauge the generalizability of this duration across targets.

Opioid maintenance treatment is in some cases paired with other adjunctive treatments including group counseling, or cognitive behavioral therapy techniques, to try to address psychological and social problems associated with OUD and other SUDs. Although generally viewed as positive, results are mixed as to the efficacy of these treatments. For example a randomized trial has shown little difference in reduction of substance use overall between physician management only and physician management plus cognitive behavioral therapy (Fiellin et al., 2013). However, results may differ based on the type of primary opioid used. Specifically, primary prescription opioid use patients assigned to physician management plus cognitive behavioral therapy showed more than twice the mean number of weeks of abstinence for all substances tested, than those assigned to physician management only, while primary heroin use patients did not differ by treatment (Moore et al., 2016). These and other findings suggest that additional research aimed at predicting responses to behavioral interventions and treatments is warranted (Moore et al., 2016), and the same is likely true for an intervention such as exposure to greenspace (e.g., outcomes may differ based on how connected one is to nature). Greenspace exposure may serve as a complement to other behavioral aspects of treatment, and should not replace existing treatments. One benefit of greenspace exposure, is the passive nature of the intervention. While other interventions may take focus, continued work and education on behalf of the patient (e.g., as within cognitive behavioral therapy) rendering these interventions susceptible to lack of participation or attrition, the benefits of greenspace exposure occur with little to no effort on behalf of the patient (assuming some greenspace is nearby/accessible). The benefits of greenspace can be experienced with no prior training, education, or required meetings, and can likely easily fit into a daily or weekly schedule. This would also be a very cost-effective treatment provided there is nearby nature or natural features (e.g., trees) to attend to.

Although greenspace exposure is accessible in theory, many individuals living in urban areas have significantly less access to greenspace than those living in suburban or rural environments. This may be especially true during the recent COVID-19 pandemic, under conditions of physical distancing and stay-at-home orders. It is currently unclear what impacts lack of greenspace exposure for those living in more urban areas may have during quarantine measures. In addition, lack of greenspace may be experienced in combination with other relevant quarantine related factors (e.g., isolation, reduced structure in schedules, difficulty accessing treatment/pharmacotherapy). However, it appears that at least some of these COVID-19 related factors have led to increased adverse mental health conditions and substance use for some (Czeisler et al., 2020), but also increased use of urban greenspace (Ventor, Barton, Gundersen, Figari & Nowell, 2020). It is possible that access to greenspace during a pandemic could buffer some of the adverse mental health conditions experienced (e.g., stress, anxiety, depression), or offer an alternative to substance use (e.g., outdoor recreation), highlighting the importance of greenspace access. Relatedly, a recent review found that although real-world nature exposure is preferred, virtual reality (VR) nature exposure may also provide some physical and mental health benefits (White et al. 2018). Specifically, virtual reality nature has been examined in the context of anxiety and pain, with generally promising results, however, more controlled research is needed. The current prevalence of remaining at home during the pandemic may also present an opportunity to test the effects of virtual reality nature exposure in predominantly built environments, particularly in the context of OUD and SUDs, but also individuals who may have difficulty with mobility.

Overall, these findings have implications for city and building planning. Increasing focus of city planning toward enhancing biologically diverse greenspace and parks could have far-reaching benefits, particularly around urban substance use disorder treatment facilities, and urban spaces more generally. Incorporating green spaces may be especially important as researchers and community and city planners work towards reducing environmental and health disparities, both for individuals with SUDs and in general. For example, research has shown that there is less availability and access to green space in lower income communities compared to more affluent communities (Astell-Burt, Feng, Mavoa, Badland, & Corti, 2014). Less greenspace availability and access could in turn influence or further exacerbate existing health disparities (see Jennings & Gaither, 2015). As noted by Jennings & Gaither (2015), critical features of the natural environmental have not yet been fully integrated into the conversation on health disparities, and more research is needed. Although anecdotal, it is also worth noting that very expensive substance use treatment facilities are often located within natural spaces/on waterfronts, however, these treatments are not affordable to many. Further, where incorporating such space outside and around buildings may be less practical, architects may work to incorporate spaces that can bring the outside in (e.g., atriums, courtyards). Of course, less “green” characteristics of such architectural features must also be taken into consideration and ideally met with appropriate materials and design (i.e., choosing energy-efficient glass and minimizing windows, as increased window space increases energy expenditures).

While this lack of “real world” access may also be mitigated by the use of photos of nature, there have yet to be studies that methodically compare types of nature exposure while controlling for the richness of nature. This latter point is particularly relevant because recent research shows that increased biodiversity in greenspace enhances the psychologically restorative effects (Woods et al., 2018). Thus, while images of nature have certainly demonstrated benefit for some of the targets discussed herein, the relative merits of in-person vs. pictorial exposure to nature while controlling for other factors are unclear. If nature and greenspace effects are uniformly governed by exposure to green light in particular, then comparative studies on nature immersion vs. pictorial exposure may be a moot point. However, whether green light can substitute for nature for establishing effects on targets that are not primarily governed by the opioid system, like delay discounting, is also unclear.

Next, the effects of distraction while in greenspace are unknown. While increasing attention to nature/greenspace without increasing exposure is sufficient to produce increases in positive affect and prosociality, it has yet to be demonstrated if the converse is true. For example, Is there benefit to running outside as opposed to indoors, on a treadmill if an individual does not simultaneously appreciate his or her surroundings? Does frequent or prolonged distraction by a cell phone while in greenspace negate or reduce greenspace benefits? Manipulation of both exposure and attention within the same study will help outline the relative contributions of these two components, how small changes to individual’s typical routines could be made to garner the benefits of exposure (opting to stroll outside vs. in front of the television on the treadmill), and the obstacles that may preclude intervention efficacy.

As noted throughout, there is a substantial need to intersect all of these questions with substance use disorder populations. Relying on convenience samples or healthy populations provides basis and impetus for further research, but the effects of nature and greenspace may be moderated by a multitude of factors that are different among those with OUD and other SUDs, and these populations can in some cases experience different barriers that will need to be addressed (e.g., likely lower income and less transportation, access to technology, etc.). We believe that the available research aptly speaks to the potential that greenspace and nature exposure has to improve a variety of difficulties associated with OUD and SUDs; and call for investigators to continue working toward, and begin this directed translational endeavor in the hopes of reducing the morbidity, mortality, and burden associated with opioid and other SUDs.