New research suggests that living in an area full of trees may be as beneficial for the heart as regular exercise.
Researchers at the University of Louisville designed a clinical trial that followed 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 cardiovascular risk changed before and after the team planted thousands of mature trees near their homes.
Results from the Green Heart Louisville Project's HEAL Study, released Tuesday, showed that people in areas with twice as many trees and shrubs had lower levels of blood markers linked to heart disease, diabetes and certain types of cancer than those who lived in areas devoid of trees.
“We're trying to see if we can reduce the rates of heart disease in a community,” said Aruni Bhatnagar, a professor of medicine at the University of Louisville who led the project.
Most previous studies showing the effects of nature on mental and physical health are observational and cannot answer whether people living in greener communities are healthier because they are wealthier and have access to better health care.
The HEAL study was set up with a control group and an intervention, meaning something measurable was shown to some participants during the study, but not before.
Bhatnagar and his team recruited about 750 people living in a 4-mile stretch of South Louisville that was cut off by a highway. Residents ranged in age from 25 to 75.
About 80% were white, and 60% were female. Half reported their median household income was $50,000.
The researchers collected blood, urine, nail and hair samples, as well as health data from each person before beginning the intervention.
Then, from 2019 to 2022, they planted about 8,500 evergreen trees, 630 deciduous trees — which lose leaves in the fall — and 45 different types of shrubs in parts of the 4-mile study area, while leaving others untouched.
Last year and this year, they collected fresh samples from residents living in both areas.
People living in the intervention areas had 13% lower levels of high-sensitivity C-reactive protein, a blood marker associated with heart disease, including stroke, coronary artery disease and heart attack. This drop was similar to that of starting a regular exercise routine, Bhatnagar said.
“I did not expect such a strong biomarker response, and it suggests that perhaps there is something truly causal in 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 new research.
How trees can improve physical health
Previous research has shown that spending time in green spaces improves mental health.
New studies have found a link between living among more trees and physical health.
Trees provide shade and keep the areas where they are planted cooler, helping to reduce the effects of urban heat that disproportionately affects low-income neighborhoods and neighborhoods of color. Hot weather increases heart disease and can cause heatstroke in people with no pre-existing conditions.
Trees also reduce noise, which is linked to higher rates of cardiovascular disease, James said.
“They provide areas 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.
“They also replace other health-harming land uses, such as industrial sites,” he added.
Because one of the major highways in the city passes through the study area, Bhatnagar and his team believe that the ability of trees to filter air pollution and prevent neighbors from constantly inhaling harmful particles could be a primary method of tree planting intervention that may be helpful in reducing inflammation symptoms among people living in green areas.
During the study, the project planted trees only in parts of south Louisville where air quality was the worst. It took air quality samples before the project, and it's still analyzing how the new trees have affected pollution. It's a complicated task, because air quality fluctuates depending on the weather — windy days can increase or decrease air pollution in some areas depending on the wind direction, and air pollution is worse on hot days.
The project plans to plant trees in control group neighborhoods over the next three or four years if the intervention neighborhoods continue to show positive results. It also seeks to determine whether tree cover improves sleep or boosts children's immune systems by encouraging play outside.
“This is not conclusive evidence. But this is the strongest evidence from any study done so far on the relationship between trees and their health,” Bhatnagar said.
Growing evidence shows that it is important to ensure an equitable distribution of green spaces around cities, which is not currently true.
Casey said it’s important that city planners, while creating more equitable access to green spaces in cities, be careful not to create “green gentrification” — that is, when places like waterfronts are restored and housing prices rise as a result, making it unaffordable for current residents to live there even after the green space is completed.
“The message here is that nature is not an amenity; green spaces are not an amenity for the rich. They are essential to us as human beings,” James said.