Researchers from the Department of Public Health, University of Texas at San Antonio, Department of Medicine, Uniformed Services University of the Health Sciences School of Medicine, Bethesda, Maryland, Department of Human Development and Family Studies, and Pennsylvania State University, State College analyzed all deaths from 1999 to 2023.

The data search was then reduced to deaths in which the International Statistical Classification of Diseases and Related Health Problems, 10th Revision code was P81 (environmental hyperthermia of newborn), T67 (effects of heat and light), or X30 (exposure to excessive natural heat) as either the underlying cause or as a contributing cause of death, as recorded in the Multiple Cause of Death file.

Data were accessed through the Centers for Disease Control and Prevention’s WONDER platform,which combines death counts with population estimates produced by the US Census Bureau to calculate mortality rates.

For each year, researchers extracted age-adjusted mortality rates (AAMRs) per 100,000 person-years for heat-related deaths. The AAMR accounts for differences due to age structures, allowing direct comparisons across time. 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. This study used publicly available, deidentified aggregate data; thus, it was not considered human subjects research.

Joinpoint version 5.2.0 (National Cancer Institute) regression6 was used to analyze AAMRs to assess trends and determine elbow points where the trend began to shift to a new trajectory. Results of joinpoint analyses are reported as average annual percentage change (AAPC) in rates with 95% CIs.

The resulting new study just published in the Journal of the American Medical Association found that heat-related mortality rates in the US increased between 1999 and 2023, especially during the last 7 years. This study is the first to the knowledge of the authors to demonstrate a reversal of this trend from 2016 to 2023.

Recent studies have found exposure to extreme heat to be associated with mortality, with variability by age, sex, and race and ethnicity. Recent research suggests that heat-related mortality risk is increasing globally, but formal analyses of heat-related mortality trends in the US through 2023 are lacking.

This study examined trends in heat-related mortality rates in the US population from 1999 to 2023. From 1999 to 2023, 21,518 deaths were recorded as heat-related underlying or contributing cause of death.

The warmest average temperature recorded since 1850 occurred in 2023. The lowest number of heat-related deaths in the study period was 311 in 2004, whereas the highest, 2325, was in 2023. The number of heat-related deaths showed year-to-year variability, with spikes in 2006 and 2011, before showing steady increases after 2016.

These results align with site-specific data analyzed in a global study that suggest increases in heat-related mortality.As temperatures continue to rise because of climate change,he recent increasing trend is likely to continue.

Study limitations include 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.

The researchers concluded by saying “Local authorities in high-risk areas should consider investing in the expansion of access to hydration centers and public cooling centers or other buildings with air conditioning.”