Trees — not grass and other greenery — associated with lower heart disease risk in cities

A multi-institutional study led by the University of California, Davis, finds that living in urban areas with a higher percentage of visible trees is associated with a 4% decrease in cardiovascular disease. By comparison, living in urban areas with a higher percentage of grass was associated with a 6% increase in cardiovascular disease. Likewise, a higher rate of other types of green space, like bushes or shrubs, was associated with a 3% increase in cardiovascular disease.

The new research was published in Environmental Epidemiology.

"Our findings suggest public health interventions should prioritize the preservation and planting of tree canopies in neighborhoods," said Peter James, first author of the study. James is an associate professor in the UC Davis Department of Public Health Sciences and director of the Center for Occupational and Environmental Health.

"Urban forestry initiatives and policies that protect mature trees are likely to yield greater cardiovascular health benefits compared to investments in grass planting," James said.

In 2023, more than 900,000 people died from cardiovascular disease in the U.S. That's the equivalent of 1 in every three deaths, according to the Centers for Disease Control and Prevention.

Cardiovascular diseases are serious conditions that affect the heart and blood vessels. This includes problems with the heart - such as arrhythmia, heart valve disea

se and heart failure - and problems with blood vessels, like blood clots (including stroke) and peripheral artery disease.

Study uses machine learning and street-level images

Previous research using satellite imagery has shown that urban green spaces can be beneficial to public health. But satellite imagery can classify a wide range of vegetation as green space.

"Satellite imagery has allowed for important new understandings about how the landscape - built and natural - can influence human health. But because the view is from far, far above, and lumps all types of vegetation into one category, it can mask differences that may be significant," James explained.

James is a prominent researcher known for innovative use of street-level imagery and satellite imagery in environmental health research. Street-level imagery, most commonly derived from platforms such as Google Street View, captures neighborhood environments photographed from the vantage point of what a pedestrian would see and has been increasingly used in public health research.

The new study was among the first to create a street-level assessment of how trees, grasses and other green spaces could impact cardiovascular health.

The researchers used deep learning to analyze more than 350 million street view images to estimate how much of each neighborhood was covered by trees, grass, or other green space.

They then linked those findings to nearly 89,000 women in the ongoing Nurses' Health Study. They determined the type and percentage of greenery - trees, grass or other green space - within about 500 meters of each participant's home address.

They then compared the types and percentages of greenery with 18 years of health data - including medical records and death certificates - to determine which study participants developed cardiovascular disease.

Findings show a lower incidence of cardiovascular disease

The researchers found that higher percentages of visible trees were associated with a 4% lower incidence of cardiovascular disease. Street views with a higher percentage of visible grass were associated with a 6% increase in cardiovascular diseases, and other green space types were associated with a 3% increase.

The association between more visible trees and a lower incidence of cardiovascular disease was consistent even when stratified by factors such as population density, census region, air pollution and neighborhood socioeconomic status.

The researchers were surprised by the potentially harmful associations with grass and other green spaces. But the surprise findings highlighted the importance of disaggregating greenspace data to better understand cardiovascular effects.

The authors suggest there may be multiple reasons for the unexpected negative association, including an increased use of pesticides, air-quality impacts from mowing, lower cooling capacity compared to trees, and lower capacity to filter noise and air pollution. Additional research may help identify the unique environmental factors that can adversely impact cardiovascular risk.

The study had several limitations. First, the cohort is all female and primarily white, which limits generalizability due to the relatively homogeneous racial and socioeconomic backgrounds of participants. Also, the researchers used the nurses' residential addresses, which don't capture how they spent their time (for example, at work) or how they interacted with other green spaces. And street-view images only represent a snapshot in time and cannot see private green spaces that may influence health, such as backyard gardens.

Despite the limitations, the findings offer significant insights.

"The research opens a promising new avenue: improving cardiovascular health through community-level environmental changes rather than relying solely on individual lifestyle choices," said Eric B. Rimm, co-author of the study.

Rimm is a professor of medicine at Harvard Medical School and of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health.

"Heart disease has such an enormous impact on the Western world that even moving the needle slightly towards earlier prevention can make a meaningful difference," Rimm said.

A complete list of authors and funders appears in the paper.

Resources:

• Read the study
• UC Davis Center for Occupational and Environmental Health
• UC Davis Department of Public Health Sciences