Our frenetic modern lives can be calmed by the experience of natural spaces, brimming with greenish light, the smells of soil and the silent fluttering of leaves in the breeze. It’s as if our very cells can exhale and relax our bodies and minds when surrounded by nature. Some people try to maximize the alleged therapeutic effects of contact with the unbuilt environment by taking forest bathing sessions, slowing down and being carefully immersed in nature. But green spaces are shrinking in a rapidly urbanizing world as our cities grow and grow. Scientists work to understand how green spaces can affect our mental health or how their lack can affect them.
A study published in the journal PNAS on Monday details what the scientists say is the association’s largest research between green spaces and mental health. Researchers at Aarhus University in Denmark found that growing close to vegetation is associated with a reduced risk of adult mental health disorders of up to 55 percent. The biologist who led the study, Kristine Engemann, combined decades of satellite imaging with extensive Danish population health and demographic data to investigate the mental health effects of growing near greenery.
“The scale of this study is quite something,” stated Kelly Lambert, a neuroscientist at the University of Richmond who studies the psychological effects of natural spaces. Smaller studies have suggested that lack of green space increases the risk of mood and schizophrenia disorders and may even affect cognitive development.
But more practical factors, such as socioeconomic status, mental illness family history, and urbanization can also have significant effects on mental health. For example, wealthier families might be able to afford more access to nature in neighborhoods and have access to other wealth-related resources that could enhance childhood development. A large and rich set of data is needed to isolate the effects of nature from so many potential confounding factors. That’s just the Danish Civil Registration System.
Created in 1968, the system assigns each Danish citizen a personal identification number and records the gender, place of birth and PINs of parents. A PIN links people across multiple databases, including records of mental health, and is updated with residence changes. “It’s an incredibly rich data source,” Engemann says. The final data set of the researchers included nearly 1 million Danes born between 1985 and 2003 for whom they had longitudinal records of mental health, socioeconomic status and place of residence. Researchers were able to calculate the vegetation density around each residence by satellite data dating back to 1985. Unfortunately, these data can not distinguish between an old-growth forest and an overgrown field, but the more greenery that is packed into a plot of land, the higher the density of vegetation.
Armed with these data, the researchers compared the risk of developing 16 different mental health disorders in adulthood with how much green space surrounded each child’s residence. And because they had yearly income, work history and education level, they could weigh the relative contribution of green space against socioeconomics of the parents and neighborhood.
After taking into account these potential confounding factors, the researchers found that growing up close to green space was associated with a lower risk of developing psychiatric disease in adulthood from 15% to 55%, depending on the specific disease. For instance, alcoholism was most strongly associated with a lack of green space, and green space was not associated with the risk of developing an intellectual disability.
The strength of the association between green space and the risk of psychiatric disorder was similar to other factors known to influence mental health, such as socioeconomic status. It is estimated that approximately 20 percent of the Danish adult population will suffer from poor psychiatric health within a given year, making these slight changes in risk potentially important, according to Engemann.
“Green space seemed to have an association that was similar in strength to other known influences on mental health, like history of mental health disorders in the family, or socioeconomic status,” says Engemann. What’s more, the effect of green space was “dosage dependent” — the more of one’s childhood spent close to greenery, the lower the risk of mental health problems in adulthood.
Engemann cautions that the study does have limitations: “It’s purely correlational, so we can’t definitively say that growing up near green space reduces risk of mental illness.” Establishing cause and effect for variables like these is incredibly difficult, according to Engemann.
Still, the breadth and depth of data used for this analysis add to the circumstantial evidence linking green space and mental health. “The effect is remarkable,” adds Lambert. “If we were talking about a new medicine that had this kind of effect the buzz would be huge, but these results suggest that being able to go for a walk in the park as a kid is just as impactful.”
