The greener the neighborhood, the lower the stroke risk, according to a new study led by researchers at the University of Miami Miller School of Medicine.

As part of the study, researchers matched images of the greenness in residential street blocks gathered from space with health data from residents. “There’s a lot of evidence that our natural environment does influence health, and we wanted to look at it particularly with stroke,” said study co-author William Aitken, M.D., a cardiology fellow and public health scientist at the University of Miami and UM/Jackson Memorial Hospital. Dr. Aitken is presenting the findings during the 2021 International Stroke Conference, taking place virtually from March 17-19.

The study used records from more than 249,000 Medicare beneficiaries, ages 65 and older, who lived in Miami-Dade County in 2010 and 2011. The records were matched against satellite images of specific residential street blocks to measure the level of greenness in the residential block – “whether that be trees or shrubs or grasses or other greenness,” Dr. Aitken said.

Researchers adjusted for factors such as gender, income, and race and ethnicity. They also took into account whether residents had health factors – such as diabetes, high blood pressure, or high cholesterol – that would affect their risk of having a stroke.

When compared with people living in the least-green street blocks, those living in the most-green blocks had a 20 percent overall lower risk of a stroke or transient ischemic attack (also known as a TIA or “mini-stroke”).

Specifically, the greenest blocks correlated with 26% lower odds of TIA and 16% lower odds of ischemic stroke, the most common types of stroke. The odds of hemorrhagic stroke were not reduced by a statistically notable amount.

Overall, the apparent effect of greenery was noteworthy, Dr. Aitken said. He estimated that the increased stroke risk of living in the least-green neighborhoods as compared with the most-green would be comparable to the risk of stroke someone would have if they developed diabetes.

Dr. Aitken collaborated with public health researchers from a program of research on greenness and health in the Miller School’s Department of Public Health Sciences, including Joanna Lombard, M.Arch.; Kefeng Wang, M.S.; and Abraham K Parrish, M.A., M.L.S.

The study builds on the research team’s previous findings that link green space to reduced risk for heart disease and heart attacks.

José Szapocznik, Ph.D., professor of public health sciences and founder of the research team that led the study, said he was surprised by the huge impact that tree canopy can have on serious chronic diseases such as cardiovascular disease.

“Our research shows that in Miami-Dade County, large populations live in low-greenness blocks, often in the poorest neighborhoods with the most vulnerable populations,” he said. “If we can improve their cardiovascular health by 20 percent by planting trees – even without requiring major lifestyle changes which are so difficult to achieve – we could have a remarkable impact in our neediest populations.”

For the stroke study, Dr. Aitken said he and his colleagues could not account for how much time people spent outside or how they interacted with the environment. But he said there are several possibilities.

Dr. Aitken said the study could help leaders and policymakers think about the potential of fighting stroke in large swaths of people at once, instead of just individuals.

It can be tough to convince large numbers of people to get regular exercise, quit smoking, and watch their diet, blood sugar, and cholesterol, he said. But nudging cities to incorporate more green space and providing encouragement for people to spend “a little more time in the environment, maybe that would affect everybody living in that area.”

Scott C. Brown, Ph.D., research associate professor of public health sciences, who was the principal investigator on the study, noted that the findings that greenery may be associated with reduced risk of stroke may be important for informing ongoing greening interventions.

“Miami and other cities in the U.S. are currently undergoing tree-planting initiatives, such as Million Trees Miami,” said Dr. Brown. “To the extent that research can identify which aspects of greenery and parks may be associated with lower rates of stroke and other chronic diseases, this work may be able to help policymakers optimize their investment in tree-planting and parks programming in ways to achieve the best health and economic benefits in the most at-risk neighborhoods.”