It’s not unusual for people who live in big cities to seek refuge in parks or in the countryside outside the city. There’s a general sense that communing with nature and being among trees makes us feel better, whether it’s the fresher air, the quiet, the presence of wildlife, or just getting away from it all. So it’s good to see that this tree-related sense of well-being, which no one ever doubted, can be measured. Researchers from the University of Chicago have measured it, here, in Toronto, where we have many trees in our streets—over half a million—and, apparently, very good records of them.
The Chicago researchers examined the associations between the number of public trees in various neighbourhoods of the city and the health of people living among them, accounting for age, sex, education, diet, income and area income. They found that people who live in neighbourhoods with more trees on the streets feel better and in fact are healthier, reporting “significantly higher health perception and significantly less cardio-metabolic conditions.” People in greener neighbourhoods reported fewer conditions of obesity and hypertension than people in less green neighbourhoods.
What does “significantly” mean? How much better do they feel? That gets to the key difference in this study compared to others. The Chicago researchers assign a monetary value to the improved sense of well-being and good health people reported. Though their methods rely to some extent on subjective data—how people report feeling about their own health—the results are startling.
Planting just ten more trees in a city block would have a health effect comparable to increasing a person’s income by $10,000 or moving to a neighbourhood where the median income is $10,200 higher, or being seven years younger. Adding just one more tree—eleven trees to a block rather than ten—is equivalent, in terms of decreased cardio-metabolic conditions, to doubling that income gain to $20,000.
That increase in income is a prediction based on analysis of people’s perceptions of how healthy they are and a number of demographic factors. Increasing the treed area in a neighbourhood by 400 square centimetres for every square metre of neighbourhood predicts an increase of 0.04 in health perception. That amounts to ten trees per block in Toronto. So, family A, living in a neighbourhood where the median income is $10,200 higher than in Family B’s neighbourhood, are “predicted” to perceive themselves as healthier, as well as wealthier. But if Family B’s less affluent neighbourhood added ten trees to every block, the study predicts that people’s perceptions of health would rise to equal those of people living in the wealthy neighbourhood. Living in a greener area can thus compensate for the lower income.
We find that having 10 more trees in a city block, on average, improves health perception in ways comparable to an increase in annual personal income of $10,000 and moving to a neighborhood with $10,000 higher median income or being 7 years younger. We also find that having 11 more trees in a city block, on average, decreases cardio-metabolic conditions in ways comparable to an increase in annual personal income of $20,000 and moving to a neighborhood with $20,000 higher median income or being 1.4 years younger.
Toronto the green. The main image is lower in resolution than the inset section, which shows individual trees on streets. Source: Scientific Reports
The health benefits associated with having the extra ten trees per block apply only to trees planted along the streets, not those in parks or around buildings where people don’t have access. In other words, the trees that are most beneficial are the ones with which people have most contact. There are several possible reasons for this—trees help reduce air pollution, they provide shade, they reduce noise levels. There’s also a psychological aspect. A person whose windows look out on a tree-lined street may feel more inclined to go for a walk than someone who looks out on a treeless stretch of pavement and cars.
The study doesn’t establish a direct impact of trees on health, but a correlational one. Trees reduce air pollution, relieve stress and promote physical activity, and those factors together can help people become healthier.