July 31, 2023

“As far as we know, this is the first study to determine that among people at high risk for heat-related cardiovascular problems, people living in urban heat islands are especially vulnerable.”

Dr. Stephanie Cleland

Dr. Stephanie Cleland

Stephanie Cleland, PhD, a 2023 alum of the UNC-Chapel Hill Gillings School of Global Public Health, describes a key finding from a new study published in the journal Environment International. The study was part of her PhD dissertation research into how extreme heat and urban heat island intensity affect cardiovascular health, and it included data on more than 48 million hospitalizations in 120 United States cities over 18 years.

An urban heat island describes how a city can experience much hotter temperatures than nearby rural areas — often because common city surfaces like asphalt, brick and glass absorb and retain far more heat than rural ground cover like grass, trees and crops.

A large body of research has already shown that extreme heat events — a series of several unusually hot days in a row — are linked to increased hospitalizations and deaths for cardiovascular disease. It is also known that the populations most at risk from high temperatures are older adults, females and those with chronic health conditions.

This new study adds evidence that when members of these vulnerable populations live in areas with higher urban heat island intensity, they are more likely to be hospitalized than their counterparts living outside of cities.

Specifically, 78% of study areas with high urban heat island intensity saw increases in cardiovascular disease hospitalization during periods of extreme heat, compared to 60% of areas with low urban heat island intensity.

In other words, there is a geographic as well as demographic element to calculating people’s risk of heat-related illness and death.

“This problem is not going away any time soon,” Cleland emphasizes. “The world is becoming increasingly urbanized, and cities are expanding. Climate change is driving up average temperatures and making extreme heat events more common.”

Cleland adds that much prior research has investigated the link between green space and heat-related illness, but that’s not a perfect model. In the southwestern U.S., for example, rural areas don’t have a lot of asphalt — but they don’t have much green, either.

One of the biggest remaining questions specific to urban heat islands is what can best protect the people who live within them. If, for example, the answer is to ride out hot days in an air-conditioned space, then — realizing that not all people can afford reliable air conditioning — public health interventions might focus on designating free ‘cooling centers’ in libraries and other public buildings that anyone can access during a heat wave.

“But that’s just one hypothesis and one possible solution,” Cleland cautions. “I hope to see more research that really teases out the drivers of heat-related illness in urban areas. Once we know which factors are most protective, we can design solutions that work for the most people.”

Cleland’s co-authors on this study are Jason West, PhD, professor of environmental science and engineering at the Gillings School; and William Steinhardt, Lucas Neas and Ana Rappold with the U.S. Environmental Protection Agency — Rappold also is an adjunct associate professor in Gilling’s Department of Environmental Sciences and Engineering.

Contact the UNC Gillings School of Global Public Health communications team at sphcomm@unc.edu.

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