Extreme-heat days – defined as days where the maximum heat index is ≥90°F and in the 99th percentile of the maximum heat index for that day – may be associated with a higher cardiovascular mortality rate, according to research published June 21 in Circulation.

Sameed Ahmed Khatana, MD, FACC, et al., studied county-level daily maximum heat index levels for all 3,108 counties in the contiguous U.S. during the summer months (May to September) between 2008 and 2017, as well as monthly cardiovascular mortality rates for adults over the age of 20, to identify the extent of the association between the two factors.

They identified 3,139,134 cardiovascular deaths during this period, with statistically significant differences in metropolitan counties across demographics such as age, sex, race, and ethnicity. Men and non-Hispanic Black adults had a significantly greater relative and absolute increase and elderly adults has a significantly greater absolute increase in cardiovascular mortality associated with extreme heat.

Further, when stratified by county metropolitan status, the association between extreme heat was statistically significant in metropolitan counties, but not in nonmetropolitan counties. In nonmetropolitan counties, the association was statistically significant among men but none of the other subgroups.

The researchers write that as climate change continues to drive an increase in extreme-heat events across the U.S., patients with cardiovascular disease will be at greater risk of morbidity and mortality. Extreme heat may also exacerbate preexisting disparities in cardiovascular health among non-Hispanic Black and non-Hispanic White communities, they add.

"Although evidence for whether certain strategies can attenuate the adverse effects of extreme heat on human health and reduce disparities is limited, studies have noted a significant association between air conditioning use and heat-related mortality and racial disparities in access to air conditioning," Khatana writes. "Other potential strategies include making cooling centers accessible to vulnerable populations and increasing tree cover in urban neighborhoods."

Keywords: Extreme Heat, Trees, Hot Temperature, Air Conditioning, Climate Change, Vulnerable Populations, Morbidity

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