The FINANCIAL — Cardiovascular-related deaths due to extreme heat(link is external) are expected to increase between 2036 and 2065 in the United States, according to a study supported by the National Institutes of Health. The researchers, whose work is published in Circulation(link is external), predict that adults ages 65 and older and Black adults will likely be disproportionately affected.
While extreme heat currently accounts for less than 1% of cardiovascular-related deaths, the modeling analysis predicted this will change because of a projected rise in summer days that feel at least 90 degrees. This heat index, which factors in what the temperature feels like with humidity, measures extreme temperature. Older adults and Black adults will be most vulnerable because many have underlying medical conditions or face socioeconomic barriers that can influence their health – such as not having air conditioning or living in locations that can absorb and trap heat, known as “heat islands(link is external).”
“The health burdens from extreme heat will continue to grow within the next several decades,” said Sameed A. Khatana, M.D., M.P.H., a study author, cardiologist, and assistant professor of medicine at the University of Pennsylvania, Philadelphia. “Due to the unequal impact of extreme heat on different populations, this is also a matter of health equity and could exacerbate health disparities that already exist.”
To generate these predictions, researchers evaluated county-level data from the contiguous 48 states between May and September of 2008–2019. More than 12 million deaths related to cardiovascular disease occurred during that time. Using environmental modeling estimates, they also found that the heat index rose to at least 90 degrees about 54 times each summer. Researchers linked the extreme temperatures that occurred during each summer period to a national average of 1,651 annual cardiovascular deaths. Some areas, such as the South and Southwest, were affected more than others, such as the Northwest and Northeast.
Using modeling analyses to forecast environmental and population changes, the researchers looked to 2036–2065 and estimated that each summer, about 71 to 80 days will feel 90 degrees or hotter. Based on these changes, they predicted the number of annual heat-related cardiovascular deaths will increase 2.6 times for the general population — from 1,651 to 4,320. This estimate is based on greenhouse gas emissions, which trap the sun’s heat, being kept to a minimum. If emissions rise significantly, deaths could more than triple, to 5,491.
For older adults and Black adults, the projections were more pronounced. Among those ages 65 and older, deaths could almost triple, increasing from 1,340 to 3,842 if greenhouse gas emissions remain steady — or to 4,894 if they don’t. Among Black adults, deaths could more than triple, rising from 325 to 1,512 or 2,063.
In comparing current and future populations, the researchers accounted for multiple factors, including age, underlying health conditions, and where a person lived.
Most people adapt to extreme heat, as the body finds ways to cool itself, such as through perspiration. However, people with underlying health conditions, including diabetes and heart disease, can have different responses and face increased risks for having a heart attack, irregular heart rhythm, or stroke.
“The number of cardiovascular events due to heat affects a small proportion of adults, but this research shows how important it is for those with underlying risks to take extra steps to avoid extreme temperatures,” said Lawrence J. Fine, M.D., a senior advisor in the clinical applications and prevention branch, in the Division of Cardiovascular Sciences at the National Heart, Lung, and Blood Institute (NHLBI), part of NIH.
The authors described cooling approaches that some cities are using – planting trees for shade, adding cooling centers with air conditioning, and using heat-reflective materials to pave streets or paint roofs. However, more research is necessary to understand how these approaches may impact population health.
What are Heat Islands?
Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. These pockets of heat are referred to as “heat islands.” Heat islands can form under a variety of conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, and in any season.
A review of research studies and data found that in the United States, the heat island effect results in daytime temperatures in urban areas about 1–7°F higher than temperatures in outlying areas and nighttime temperatures about 2–5°F higher. Humid regions (primarily in the eastern United States) and cities with larger and denser populations experience the greatest temperature differences. Research predicts that the heat island effect will strengthen in the future as the structure, spatial extent, and population density of urban areas change and grow.[1]
Causes of Heat Islands
Heat islands form as a result of several factors:
Reduced Natural Landscapes in Urban Areas. Trees, vegetation, and water bodies tend to cool the air by providing shade, transpiring water from plant leaves, and evaporating surface water, respectively. Hard, dry surfaces in urban areas – such as roofs, sidewalks, roads, buildings, and parking lots – provide less shade and moisture than natural landscapes and therefore contribute to higher temperatures.
Urban Material Properties. Conventional human-made materials used in urban environments such as pavements or roofing tend to reflect less solar energy, and absorb and emit more of the sun’s heat compared to trees, vegetation, and other natural surfaces. Often, heat islands build throughout the day and become more pronounced after sunset due to the slow release of heat from urban materials.
Urban Geometry. The dimensions and spacing of buildings within a city influence wind flow and urban materials’ ability to absorb and release solar energy. In heavily developed areas, surfaces and structures obstructed by neighboring buildings become large thermal masses that cannot release their heat readily. Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects.
Heat Generated from Human Activities. Vehicles, air-conditioning units, buildings, and industrial facilities all emit heat into the urban environment. These sources of human-generated, or anthropogenic, waste heat can contribute to heat island effects.
Weather and Geography. Calm and clear weather conditions result in more severe heat islands by maximizing the amount of solar energy reaching urban surfaces and minimizing the amount of heat that can be carried away. Conversely, strong winds and cloud cover suppress heat island formation. Geographic features can also impact the heat island effect. For example, nearby mountains can block wind from reaching a city, or create wind patterns that pass through a city.
Characteristics of Heat Islands
Heat islands are usually measured by the temperature difference between cities relative to the surrounding areas. Temperature can also vary inside a city. Some areas are hotter than others due to the uneven distribution of heat-absorbing buildings and pavements, while other spaces remain cooler as a result of trees and greenery. These temperature differences constitute intra-urban heat islands. In the heat island effect diagram, urban parks, ponds, and residential areas are cooler than downtown areas.
Surface temperatures vary more than atmospheric air temperatures during the day, but they are generally similar at night. The dips and spikes in surface temperatures over the pond area show how water maintains a nearly constant temperature day and night because it does not absorb the sun’s energy the same way as buildings and paved surfaces. Parks, open land, and bodies of water can create cooler areas within a city. Temperatures are typically lower at suburban-rural borders than in downtown areas.
In general, temperatures are different at the surface of the earth and in the atmospheric air, higher above the city. For this reason, there are two types of heat islands: surface heat islands and atmospheric heat islands. These differ in the ways they are formed, the techniques used to identify and measure them, their impacts, and to some degree the methods available to cool them.
Surface Heat Islands. These heat islands form because urban surfaces such as roadways and rooftops absorb and emit heat to a greater extent than most natural surfaces. On a warm day with a temperature of 91°F, conventional roofing materials may reach as high as 60°F warmer than air temperatures.[2] Surface heat islands tend to be most intense during the day when the sun is shining.
Atmospheric Heat Islands. These heat islands form as a result of warmer air in urban areas compared to cooler air in outlying areas. Atmospheric heat islands vary much less in intensity than surface heat islands.
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