A new study to examine the relationship between extreme-heat events and cardiovascular mortality found that the hottest days can indeed be deadly.
To make the determination, three researchers from the University of Pennsylvania looked at the number of “extreme-heat days” in more than 3,000 counties across the United States, running the numbers against monthly cardiovascular mortality rates. Each additional extreme-heat day in a given month was associated with a 0.12% higher cardiovascular mortality rate for that month, adding up to nearly 6,000 additional deaths between 2008 and 2017. The group shared its analysis in Circulation.
“As the climate continues to warm and extreme-heat days increase in frequency, the role that extreme heat plays in the cardiovascular health of US adults will continue to increase,” wrote first author Sameed Ahmed M. Khatana, MD, MPH, and coauthors.
An “extreme heat day” was defined as one where the maximum heat index met two conditions: it was 90 degrees Fahrenheit or higher, and it was in the 99th percentile of the maximum heat index from a baseline period of 1979 to 2007.
On these days, the authors posit, biological factors contributing to increased cardiovascular death may include an increase in cardiac strain, attributable to increases in heart rate and more forceful contractions of the heart muscle. Additionally, some people may experience an upregulation of inflammatory mediators, which could increase the risk of thrombosis.
Most of the additional deaths were attributed to ischemic heart disease as opposed to stroke.
The researchers also aimed to understand how the relationship between cardiovascular mortality rates and extreme-heat days differed in subgroups based on age, sex, race and ethnicity. They concluded that the burden of excess mortality indeed fell disproportionately on certain subgroups.
“Men and non-Hispanic Black individuals had a significantly greater relative and absolute increase and elderly adults had a significantly greater absolute increase in cardiovascular mortality associated with extreme heat,” the authors wrote.
While the results shed light on a relationship that will become increasingly important given climate change projections, the study’s authors also note some of the limitations surrounding the data that they used for the study.
“Because only monthly mortality data were available, it is not possible to attribute cardiovascular deaths in a particular month directly to the extreme-heat days that occurred in that month because a death may have occurred either before or significantly after the occurrence of an extreme-heat day," they wrote.
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