How green spaces benefit mental health

© 2019 Gwen Dewar, Ph.D., all rights reserved

three young children roaming together on a grassy hill - image by ajari / flickr / community commons license ccby2

Studies show that green spaces can have a protective effect on our mental well-being. We're quicker to recover from stress, and less likely to experience depression. Kids grow up with fewer psychiatric problems. Adults are less likely to commit suicide.

But not all green spaces are equal, and not everyone has equal access to safe, high-quality nature zones. Here's what every parent -- and every thinking person -- needs to know.

Improving mental well-being usually requires that we make a special effort, but here's a remedy that requires no work at all:

Experiments consistently show that we can improve our immediate outlook -- and bounce back from stress -- by merely gazing at scenes of nature.

You don't even have to go outside to unlock the effect. Researchers have demonstrated that viewing photographs  -- or looking at a window -- is enough. Scenes of nature have a restorative effect. Urban scenes, and images of buildings, usually don't.

For example, when people view nature imagery, they experience fewer negative emotions, and they are more likely to report positive moods (Golding et al 2018; McMahan and Estes 2015) et al 2017).

And studies suggest that nature scenes are more likely to activate our parasympathetic nervous system -- the system that helps us calm down and recover from stressful events (Park et al 2010; Berto 2014; van den Berg et al 2015; Hunter et al 2019).

If we reap these benefits from merely looking at nature, what happens when we spend time outdoors? 

Whether it's a 15-minute visit to a green park, or a 3-day immersive forest vacation, relaxing in green spaces can have immediate, beneficial effects on your stress physiology.

For instance, 10 to 30 minutes of quiet contemplation in nature can affect the rest of your day -- lowering your levels of the stress hormone, cortisol (Hunter et al 2019).

And compared with spending time in human-built environments, time spent in green spaces is linked with more beneficial changes in blood pressure and heart rate (Tsunetsugu et al 2010; Hansen et al 2017; Lanki et al 2017).

And nature walks appear to have a special impact on our physical and emotional well-being.

Exercise is good for mental health, but green exercise is even better. 

When people walk in natural (rather than urban) settings, they show greater evidence of stress relief. They experience lower cortisol levels. They also experience temporary reductions in blood pressure and heart rate variability (Song et al 2016; Bratman et al 2012).

Green exercise is good for mental functioning, too. As I explain in another article, kids who play and walk in green spaces show immediate, short-term improvements in their ability to focus and juggle facts. It also appears that walking in nature can help us quiet down a part of the brain that specializes in bad moods.

In an experiment led by Gregory Bratman and his colleagues, researchers scanned the brains of 38 adult volunteers using MRI. In addition, the team asked study participants about their tendencies to ruminate, or brood.

Next, the researchers randomly assigned half the participants to take a 90-minute nature walk. The other half were assigned to take a 90-minute walk along a busy, urban road.

Immediately on returning, each participant received another brain scan, and a follow-up screening for rumination. And the results?

Unlike their urban-walking counterparts, people who'd gone on nature walks experienced reductions in their tendencies to ruminate. After walking, they were less likely to agree with statements like

"My attention is often focused on aspects of myself I wish I’d stop thinking about."

And nature walkers also experienced reduced activity in the subgenual prefrontal cortex (sgPFC), a brain area that becomes more active when we're sad, withdrawn, or reflecting on negative emotional experiences.

People who'd gone on urban walks exhibited no such changes in brain activity.

So there is a lot of experimental evidence suggesting that green spaces can improve our immediate well-being. But what about long-term outcomes?

Does exposure to green space protect us from developing psychiatric disorders? Does it lower our risk of becoming depressed, or developing stress-related psychosomatic symptoms? The risk of depression? Psychosomatic symptoms? suicide? Can we assume that green 

To answer these questions definitively, we'd want to do long-term experiments. But who would volunteer for something like that? A study where you might be randomly assigned to spend years of your life deprived of nature experiences?

It wouldn't be an ethical experiment, so nobody has tried it.

Instead, researchers have opted for the next best thing:

Find out where people live, and see if the amount of local green space is correlated with mental health. Then use statistical methods to address causation.

For instance, if green space is linked with better mental health outcomes, is this merely because people who live near green spaces tend to have more wealth?

Wealth can protect people from all sorts of conditions that worsen mental health, and a study of cities in North America confirms that affluent city-dwellers have greater access to vegetation (Nesbitt et al 2019). So when researchers evaluate correlations between green space and mental health outcomes, they make statistical adjustments for the independent effect of socioeconomic factors.

And yes, they end up with evidence that green spaces really do have lasting impact. Here's a pretty striking example -- a study of almost a million children growing up in Denmark.

Danish study: Kids face fewer psychiatric problems when they grow up with lots of green space

By any account, it was a massive study. Kristine Engemann and her colleagues tracked the mental health outcomes of approximately 940 thousand individuals growing up in Denmark.

At the same time, the researchers pulled government records to establish where each of these children had lived at various points in their lives.

Then the researchers consulted satellite imagery to estimate the sheer amount of greenery in local environments. How much green space would a child have encountered at a given residence?

Using a measure called NDVI (Normalized Difference Vegetation Index), Engemann's team calculated the density of vegetation with 210 meters of each home.

Next, researchers posed the question: Did a child's cumulative exposure to green space -- as measured by residential greenery -- increase his or her risk of developing a psychiatric disorder?

To answer, the researchers compared kids at either end of the "green spectrum" -- the ten percent living with the highest vegetation density versus the ten percent living with the lowest vegetation density. And there were differences.

Green-deprived kids were more likely to develop an array of psychiatric disorders.

Of course, some of the differences were due to overlap with other factors, like socioeconomic status. So the researchers made statistical adjustments, controlling for many things that might influence mental health outcomes, including

  • the age, income, education level, and psychiatric history of the children's parents;
  • neighborhood socioeconomic factors, like the average income and education level of local residents; and
  • the degree of urbanization (whether kids lived in the city, suburbs, or countryside).

After making these adjustments, some of the links disappeared. For instance, the researchers found that anorexia and bipolar disorder were no longer associated with green space deprivation.

But for other disorders, the risk estimates didn't change much, and the effect of green space was substantial.

For example, after making all adjustments, children who had grown up around the lowest levels of vegetation had an approximately 28% higher risk of developing neurotic, stress-related, or psychosomatic disorders.

The kids were also at higher risk for mood disorders (~20%), obsessive-compulsive disorder (~20%), and substance abuse (~28%).

Okay, but this is just one study, and the researchers didn't control for everything. Don't we need more studies to figure out what's going on?

Yes, that's true. But this isn't the only study of its kind. Using the satellite-imagery-and-NDVI approach, researchers around the world have looked for links between green space and mental health. And they've found some.

Large-scale studies suggest that living near green spaces can reduce your risk of depression.

Here's a case in point: A study of almost 95 thousand adults living the United Kingdom. Using NDVI, researchers estimated the density of vegetation within 500 meters of every study participant's home. And the results? People living around high levels of green space were at lower risk for major depression (Sarkar et al 2018).

Moreover, this was true even after researchers controlled for

  • economic factors (like employment status, personal income, and neighborhood SES); 
  • social factors (like participation in social events, athletic clubs, or religious groups); and
  • medical factors (like cardiovascular health, BMI, and smoking status).

The effect was strongest for people living in impoverished neighborhoods, and for individuals living in places with high population density: Green spaces -- like urban parks -- seemed particularly protective for folks coping with city hassles and poverty (Sakar et al 2018).

A similar study of almost 65 thousand people living in South Korea. After controlling for personal health factors and socioeconomic status, researchers compared the quarter of the population living with the most vegetation to the quarter of the population living with the least vegetation. The odds of developing depressive symptoms? They were approximately 23% lower for those lucky, "green" folks (Song et al 2019).

Studies conducted in the United States have also documented links between residential green space and depression.

The biggest studies have targeted older adults (Banay et al 2019; Pun et al 2018; Brown et al 2018). For instance, a study of nearly 250,000 older Americans found that people were at lower risk for depression if they lived on blocks featuring high levels of vegetation (Brown et al 2018).

But a number of smaller studies have also reported substantial effects for prime-aged adults (Cohen-Cline et al 2015) and children (Madzia et al 2019; Bezold et al 2018). And a really interesting study followed the mental health outcomes of twins.

Green spaces are linked with lower depression risk even after researchers control for genetics and childhood environmental factors

The study included 4300 pairs of adult twins, roughly half of whom were monozygotic, or "identical."

The members of each twin pair had grown up together, so they shared many childhood experiences. In addition, the monozygotic twins shared virtually all of their DNA.

But now, in adulthood, all the twins lived apart. Some individuals lived in areas with high levels of green space. Others lived in places that featured much less vegetation. Were these differences in residential green space linked with mental health?

It was a lucky, natural experiment, one that allowed researchers to control for the many genetic and early life environmental factors that can contribute to the development of mental health problems.

But of course there was more to consider. What about each individual's current socioeconomic status, and other factors that might differ between adult twins?

So after screening each study participant for mental health issues, researchers made statistical adjustments for the effects of current income, physical activity levels, and neighborhood characteristics.

The results? Researchers found no links between green space access and anxiety problems. But for depression, it was a different story.

If one twin lived in a greener environment, he or she was less likely to suffer from symptoms of depression. And this was true even for identical twins, who share virtually 100% of their DNA (Cohen-Cline et al 2015).

So NDVI correlational studies offer evidence that green spaces have a protective effect against certain psychiatric problems, including stress-related disorders, substance abuse, and depression. What about the most extreme consequences of poor mental health?

Can green spaces lower the risk of suicide?

The evidence comes from a study conducted in the Netherlands. Marco Helbich and his colleagues wanted to know if green spaces have an impact on the rate of suicide. So they calculated the density of vegetation in 398 different Dutch municipalities. Were local differences in greenery linked with local suicide rates?

Of course, the researchers knew that suicide is influenced by things that have nothing to do with green space. It's crucial to control for these factors. So Helbrich's team made adjustments for local unemployment rates, divorce rates, and the availability of medical care. They controlled for municipal differences in economic deprivation and wealth. They also controlled for whether a municipality was urban or rural, and whether it was inhabited by a high number of religious adherents.

In short, the researchers controlled for lots of things that might have contributed to suicide, yet they still found a "green" effect.

Independent of all the other factors, living near green spaces reduced the risk of suicide.

Compared with municipalities with the lowest amount of vegetation, municipalities with moderate levels of vegetation had an 8% lower rate of suicide.

Municipalities with the highest amount of vegetation had a 12% lower rate of suicide.

So what's going on? How are green spaces contributing to better mental health?

First, it bears noting: Not every NDVI study has detected the same effects. So we need more research to sort things out. But meanwhile, the pattern of evidence is suggestive.

1. Green spaces may benefit us indirectly -- by protecting us from pollution, and encouraging us to exercise. 

These factors don't explain everything -- far from it. But there's evidence that they play a role.

For instance, in a study of nearly a thousand Spanish adults, Mireia Gascon and her colleagues found that residential green space reduced the risk of anxiety and depression. So the researchers delved deeper, and discovered that up to one third of the green effect was connected with pollution.

Green spaces were linked with better outcomes -- at least in part -- because they cleaned the air, and buffered residents from irritating traffic noise (Gascon et al 2018).

Similarly, there are connections with physical exercise, albeit small ones.

Researchers in China found that daily exposure to green space improved mental well-being by encouraging people to exercise more (Zhang et al 2018). But overall, this accounted for only a small portion of the green effect (Zhang et al 2018).

Likewise, researchers in Australia found that walking was partly responsible for links between green space access and mental well-being (Sugiyama et al 2014).

2. Green spaces may also have direct effects on mental health -- providing us with relaxing distractions and natural highs.

One theory is that the sights and sounds of nature offer our brains an opportunity to slack off. We no longer have to engage in directed attention -- the kind effortful, conscious attention we pay when we're in classroom or work environment. This allows us recover from stress and mental fatigue (Kaplan 1995).

Another proposal is that human beings have an innate desire to feel connected with nature. Viewing nature -- being outdoors in a green space -- triggers positive emotions. Nature experiences are intrinsically rewarding (Ulrich 1983; Wilson 1984; Ulrich 1983).

Then there is the question of awe -- of being struck by the grandness, intricacy, or beauty of nature. Experiments confirm that dramatic nature imagery has a bigger emotional impact. An image of the Grand Canyon is more uplifting than the image of more mundane nature scene (Joie and Bolderdijk 2015). When nature inspires awe, we're less likely to dwell on our personal problems.

3. The amount of green space near your home doesn't tell the whole story. People are also affected by the green spaces they encounter elsewhere -- at work, at school, on commutes, and during recreational trips.

For example, take a recent study of 85,000 adults and teenagers in California.

Researchers measured vegetation density within a 350 meter radius around residences, and found that lots of neighborhood greenery decreased the odds of "serious psychological distress." But this was true only for teenagers and adults over the age of 65. For people in the middle, there was no effect.

Why? We might question whether prime-aged adults can benefit from green space, but other evidence -- including experimental research -- argues strongly against this. So what researchers suspect instead is that their measure -- the 350 meter zone -- is to blame.

Prime-aged adults have more mobility than teenagers and elderly people. They venture outside their home neighborhoods more often, and these trips may exposure them to green spaces elsewhere. As a result, we might expect prime-aged adults to be less dependent on residential green space for their mental health.

So a better measure of green space exposure would account for all the greenery that people encounter each day -- not just at home, but wherever they go. And when researchers have taken this more fine-grained approach, they've confirmed links between green space exposure and mental well-being. This includes studies of prime-aged adults, as well as teenagers and school children (Zhang et al; Mennis et al ; Amoly et al 2014).

4. What about the least active among us? People who -- for a variety of reasons -- are less likely to venture beyond their own private gardens or yards? In these cases, it's probably not neighborhood vegetation that matters most, but rather the view out the window.

In support of this idea, a study of older adults in China found that residential green space density -- measured by satellite photos and NDVI -- was not linked with mental health. Instead, what made a difference were the immediate surroundings of the home: Older people were less likely to be depressed if they were lucky enough to have a street view of vegetation (Helbich et al 2019).

5. The quality of green space matters.

NDVI studies tell us only about the density of vegetation. They don't tell us how local people feel about that vegetation. Some spaces are inviting, serene nature spots, or beautiful urban parks with lots of amenities. Other green spaces might be inaccessible (e.g., someone else's private property) or stressful (e.g., hotspots for illegal dumping or crime).

When researchers have delved deeper -- going beyond simple measure of residential NDVI -- they've found that it isn't the sheer quantity of residential green space that's important, but the quality. For instance, children experience fewer psychological difficulties when they live near high quality green spaces and urban parks (Francis et al 2012; Balseviciene et al 2014).

6. Residential greenery can be socially isolating, which may counteract the mental health benefits of green spaces. For this reason, some people might find urban parks -- high quality, safe, green spaces that foster a sense of community -- to be more protective of their mental well-being.

In a study of neighborhoods in Chicago, researchers found that neighborhood vegetation helped reduce people's stress levels. But at the same time, individuals living in very green areas often experienced lower levels of social support.

So it wasn't the sheer amount of green space that helped people the most, but a specific kind of green space -- urban parks.  Presumably, that's because these parks provided the benefits of nature, while still permitting people to feel connected with other members of their community (Fan et al 2011).

7. Privilege and social injustice loom large.

We've seen that nature spots and green spaces benefit people regardless of their socioeconomic status. But of course we don't all have equal opportunities to visit high quality green spaces -- or enjoy them once we're there.

The advantages of wealth are obvious. But other forms of privilege -- like white privilege -- are crucial as well.

For example, in the United States, African Americans have faced a long history of racism, harassment, and violence in public parks and green spaces (Lee and Scott 2016 Lee and Scott 2017). How restorative can a nature visit be, if you have to worry about being harassed, wrongfully arrested, or worse? You might live next to a beautiful public green space, yet lack true access. 

8. And it's a sure bet that individual differences matter.

Research also suggests that the benefits depend on your emotional connection to nature. Do you feel part of the natural world? Do you feel connected to other creatures and life forms? If so, you will probably perceive green spaces to be especially restorative (Berto et al 2018).

And certain populations might respond differently to the same natural features. For example, a study of more than 55,000 American kids found that children with autism spectrum disorder (ASD) were more likely to develop anxiety if they lived near green spaces with lots of trees. For normally-developing children, there was no such link (Larson et al 2018).

What should we do?

Access to nature is a fundamental component of well-being. It's a right that we must fight for. 

So in addition to getting our kids outdoors, we need to ensure there are safe, welcoming places to play. We need to design school grounds with green space, add green space to our urban environments, and change a mentality that regards green space as a perk for the privileged few.

For more information on this topic, see my article about the benefits of outdoor learning. And read about the benefits of nature for our cities. In addition to improving public health, green spaces help mitigate the effects of climate change. They help reduce the heat island effect in cities, and release more moisture into the atmosphere.

This page from the U.S. Environmental Protection Agency offers strategies and resources for adding green space to our urban environments.

Finally, put pressure on politicians and urban planners to add green space to your community, and protect our natural environment. Make sure your voice is heard. Vote for a better future.


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Title image of children roaming on the grass by ajari/flickr

satellite image of Denmark by European Space Agency

image of toddler and father by Ian D. Keating / flickr

image of child hugging globe by woodleywonderworks / flickr