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Deaths due to heat-related illness in the U.S. more than doubled over the past quarter-century, researchers reported.

From 1999 to 2023, the number of deaths with heat listed as an underlying or contributing cause increased from 1,069 to 2,325, with corresponding age-adjusted mortality rates (AAMRs) increasing from 0.38 (95% CI 0.36-0.40) to 0.62 (95% CI 0.60-0.65) per 100,000 person-years, according to Jeffrey Howard, PhD, of the University of Texas at San Antonio, and colleagues.

This translated to a 117% increase in the number of heat-related deaths and a 63% increase in the AAMR, they wrote in a research letter in .

"This study found that heat-related mortality rates in the U.S. increased between 1999 and 2023, especially during the last 7 years," they wrote. "Although a study using data through 2018 found a downward trend in heat-related mortality in the U.S., this study is the first to our knowledge to demonstrate a reversal of this trend from 2016 to 2023."

"As temperatures continue to rise because of climate change, the recent increasing trend is likely to continue," they said.

Although recent research suggests that heat-related mortality risk is increasing globally, "formal analyses of heat-related mortality trends in the U.S. through 2023 are lacking," the authors noted in their introduction.

According to a joinpoint trend analysis, over the entire period from 1999 to 2023 the AAMR increased by 3.6% per year (P=0.04). From 1999 to 2016, there was a nonsignificant decrease of 1.4% per year (P=0.42), followed by a significant increase of 16.8% per year in the AAMR from 2016 to 2023 (P=0.002).

"In places like New Mexico we feel what the data in this study are showing," Paul Charlton, MD, an emergency physician in Gallup, New Mexico, and board member for Healthy Climate New Mexico, said in an email to 麻豆传媒. "New Mexico is getting hotter. We've had record temperatures, with longer stretches of high temperatures than have been experienced in anyone's lifetime."

In terms of specific health outcomes, "we see more burns from hot pavement, children doing normal activities presenting with heat exhaustion and heat stroke, athletes and workers (cowboys, indoor warehouse workers, restaurant workers) experiencing exertional heat stroke at practice and at work, and increased cardiovascular disease acute events ([myocardial infarction], heart failure exacerbations) during heat events," Charlton said, adding that heat-related deaths may be undercounted due to the way they are coded on death certificates.

"Physicians, medical examiners, and coroners may not be coding in a way that captures the causal contributions from heat," he said. "They might not know for certain whether heat was a contributing factor, particularly if the death did not occur during a well-publicized heat wave."

"Heat-response measures make a positive difference in decreasing death rates," Charlton added. "These include early warning and surveillance systems, access to indoor cooling, increased access to cooling centers, workplace heat standards, healthcare system readiness, public education, infrastructure standards, and air quality management."

Bethany Carlos, MD, MPH, a pediatrician with Children's National Hospital in Washington, D.C., and board member of the American Academy of Pediatrics Council on Environmental Health and Climate Change, said in an email that she was surprised to see such a consistent rise in heat-related deaths over the last three decades. "However, I do recognize the stated bias of the increased awareness could have led to more accurate identification," she wrote. "I would be curious to see more data stratified by geographic regions, ethnicities, and other social determinants of health categories."

"At this rate, the patients I am seeing today will be more likely to have a heat-related mortality than their parents and grandparents," she said. "It raises the importance of mitigating the effects of climate change and reducing our carbon emissions. We need to engage our parents, educators, community leaders, and schools in strategizing on solutions."

The investigators concluded by urging local authorities in high-risk areas to "consider investing in the expansion of access to hydration centers and public cooling centers or other buildings with air conditioning."

Researchers gathered data from 1999 to 2023 in the CDC's WONDER (Wide-ranging Online Data for Epidemiologic Research) platform, which combines death counts with population estimates produced by the Census Bureau to calculate mortality rates.

All deaths from 1999 to 2023 in which either "environmental hyperthermia of newborn," "effect of heat and light," or "exposure to excessive natural heat" was listed in the Multiple Cause of Death file as either the underlying cause or as a contributing cause of death were analyzed. This turned up a total of 21,518 heat-related deaths, with an AAMR of 0.26 per 100,000 person-years (95% CI 0.24-0.27).

The researchers extracted AAMRs for each year per 100,000 person-years for heat-related deaths. "The AAMR accounts for differences due to age structures, allowing direct comparisons across time," they said. "The approach of analyzing cause-specific mortality rates rather than excess mortality is warranted because the excess mortality methodology is subject to confounding from the COVID-19 pandemic from 2020 to 2023."

Study limitations included the potential for misclassification of causes of death, leading to possible underestimation of heat-related mortality rates; potential bias from increasing awareness over time; and lack of data for vulnerable subgroups.

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