It’s a complicated relationship to figure out, because variables that affect how many trees you see each day could also affect your health. The population of a concrete, inner-city apartment complex may have socioeconomic differences, for example, from the population of a leafy, well-tended suburb. University of Chicago psychologist Marc Berman and his colleagues used a detailed analysis to try to tease out the impact of trees themselves.
They started by going to Canada. In a country with universal health care, they figured, access to doctors isn’t as much of a variable as in the United States.Since socioeconomic status can still affect how people use doctors, the authors also gathered information on their subjects’ income and education. And rather than comparing people from multiple areas, they focused only on the city of Toronto.
From a large-scale, ongoing project called the Ontario Health Study, the authors collected data on over 31,000 adult residents of Toronto. In addition to household income and years of education, they looked at subjects’ sex, diet (self-reported servings of fruits and vegetables per day), and neighborhood. The Ontario Health Study questionnaires also asked subjects whether they’d ever been diagnosed with various physical and mental health conditions.
The final measurement was health perception: how healthy do subjects feel they are, on a scale from 1 to 5? It sounds vague, but this measurement has been found to strongly predict actual health, the authors write.
Subjects in the study came from about 3,200 neighborhoods. (These neighborhoods are geographical units that Canada calls dissemination areas, with 400 to 700 residents each.) To count the trees in each neighborhood, the authors combined satellite imagery of Toronto with data from a survey of over half a million trees on Toronto public land. The researchers weren’t really interested in parks. Instead, they wanted to know about trees along the roads—the trees neighborhood residents would see every day.
As you might expect, several variables were linked to subjects’ perceived health. These included income, age, diet, and even sex (males perceived themselves as slightly healthier). But when the researchers controlled for all of these variables, removing their effects, they still found that trees mattered. Ten extra trees per city block increased subjects’ health perception as much as $10,200 in extra income, or being 7 years younger.
Trees were also linked to gains in actual health measurements. People in neighborhoods with more trees reported significantly fewer “cardio-metabolic” diagnoses such as high blood pressure, heart disease, or diabetes.
The boost in health from 10 extra trees per block was only a difference of about 1% on the 1-to-5 scale of perceived health. The decrease in disease was also about 1%. Even though it’s a small difference, Berman says the strength of these results was surprising. “To get those equivalent health benefits monetarily required giving each household in that city block over 10K,” he says, or shipping everyone to a wealthier neighborhood. Or making everyone seven years younger—which, as he points out, is “obviously something that cannot be done.”
Although the study shows a strong relationship between trees and health, Berman says, it can’t prove cause and effect. Another limitation is the self-reported health data, which might not be accurate.
Assuming trees do lead to better health, researchers don’t know exactly why. Is it because trees create cleaner air? A more attractive environment? Better motivation to exercise? “My guess is that a few different mechanisms are at play,” Berman says. He hopes to get some answers in future research.
Meanwhile, he says, we can all take advantage of whatever trees are available to us. “When individuals are fatigued and need a break, we find that brief walks in nature can improve memory and attention by 20%,” Berman says, citing earlier studies. And beautifying our homes or neighborhoods by planting trees, shrubs or gardens might improve our physical health. “While the onus may lie on cities to make these improvements on a large scale,” Berman says, “we as individuals can also take ownership.”