Living in a tree-filled area may have just as many heart benefits as regular exercise, according to a new study.
Researchers at the University of Louisville designed a clinical trial to follow hundreds of people living in six low- to moderate-income neighborhoods in South Louisville, Kentucky. They used blood and other samples to better understand how their risk of heart disease changed before and after the research team planted thousands of mature trees near people's homes.
Results from the Green Heart Louisville Project's HEAL study released Tuesday found that people living in areas with twice as many trees and shrubs had lower levels of blood markers linked to heart disease, diabetes and some cancers compared to people living in areas with fewer trees.
“We're trying to see if we can reduce the incidence of heart disease in the community,” said Aruni Bhatnagar, a professor of medicine at the University of Louisville, who led the project.
Most of the research to date showing the effects of nature on mental and physical health is observational, and cannot answer whether people who live in greener areas are healthier because they are wealthier and have better access to health care.
The HEAL study consisted of a control group and an intervention group, meaning that some of the participants were exposed to something measurable during the study but not before the study.
Bhatnagar and his team recruited about 750 people, ages 25 to 75, who live in a four-mile stretch of South Louisville that's divided by a highway.
Approximately 80% were white and 60% were women. Half reported a median household income of $50,000.
Before starting the intervention, the researchers collected blood, urine, nail and hair samples, as well as health data from each person.
Then, between 2019 and 2022, the team will plant about 8,500 evergreen trees, 630 deciduous trees (which lose their leaves in the fall) and 45 shrubs in parts of the four-mile study area, while leaving other parts alone.
New samples were taken from residents living in both areas last year and this year.
People living in the intervention areas saw a 13 percent drop in levels of high-sensitivity C-reactive protein — a blood marker associated with cardiovascular disease such as stroke, coronary artery disease and heart attacks — which was the same drop as when they started regular exercise, Bhatnagar said.
“We didn't expect the biomarker responses to be as strong as they were, which suggests there may be something genuinely causal about the effects of trees on health,” said Peter James, director of the Center for Occupational and Environmental Health at the University of California, Davis School of Medicine, who was not involved in the study.
How trees improve your physical health
Previous studies have shown that spending time in green spaces can improve mental health.
A new study shows a link between living among more trees and better physical health.
Trees provide shade and cooling for the areas where they are planted, helping to mitigate the effects of urban heat, which disproportionately affects low-income communities and people of color. Hot weather can exacerbate heart disease and cause heatstroke in people without pre-existing conditions.
Trees also help to reduce noise, which is linked to higher rates of cardiovascular disease, James said.
“Parks provide places for people to relax, exercise, and perhaps more importantly, socialize,” Joan Casey, an environmental epidemiologist and associate professor of environmental and occupational health sciences at the University of Washington, said in an email.
“It also displaces other land uses that are detrimental to health, such as industrial sites,” she said.
As one of the city's major arterial roads crosses the study area, Bhatnagar and his team believe that the ability of trees to filter air pollution and prevent nearby residents from constantly inhaling harmful particles could be the main reason behind tree planting intervention lowering inflammatory markers in people living in green areas.
During the study, the project planted trees only in areas of South Louisville where air quality was the worst. Air quality samples were taken before the project and analysis is still ongoing to see how the new trees affected pollution. This is a complex endeavor because air quality fluctuates with weather. Windy days can increase or decrease air pollution in certain areas depending on wind direction, and hotter days can make air pollution worse.
If results in the treatment areas continue to be positive, the project plans to plant trees in the control areas after another three to four years, and also wants to see whether tree cover encourages outdoor play and improves sleep and children's immunity.
“There's no smoking gun,” Bhatnagar said, “but this is the strongest evidence yet that there's been about the relationship between trees and health.”
There is growing evidence showing the importance of ensuring equitable distribution of green spaces around cities, but the current situation is not such that the case.
Casey said it's important that urban planners be careful not to create “green gentrification” when creating more equitable access to urban green space — that is, restoring spaces like waterfronts so that housing prices rise and current residents can no longer afford to stay there once the green space is completed.
“The key message here is that nature is not an amenity. Green space is not a perk for the wealthy. It is essential for us as humans,” James said.
