Heat-Related Mortality Trends Worsened from 1999 to 2023

Heat-related mortality rates in the United States experienced a notable upward trend in the 21st century, with rates increasing between 1999 and 2003, particularly in the last 7 years, according to new research.¹

Trends in heat-related mortality were examined through the Centers for Disease Control and Prevention (CDC)’s WONDER Platform, with ICD code P81 (environmental hyperthermia of newborn), T67 (effects of heat and light), or X30 (exposure to excessive natural heat) considered either the underlying or a contributing cause of death.

“Although a study using data through 2018 found a downward trend in heat-related mortality in the US, this study is the first to our knowledge to demonstrate a reversal of this trend from 2016 to 2023,” wrote the investigative team, led by Jeffrey T. Howard, PhD, an associate professor in the department of public health at the University of Texas at San Antonio.

Exposure to extreme heat has been linked to mortality, with differences by age, sex, race, and ethnicity.² In 2023, the warmest average temperature recorded since 1850 occurred, with evidence suggesting the risk of health-related mortality is growing globally.³

Although this elevated risk is notable, there is a lack of formal analyses of heat-related mortality trends in the US through 2023, according to Howard and colleagues.¹

In this study, investigators accessed data from WONDER, which combines death counts with US Census Bureau population estimates to determine mortality rates. Accounting for differences in age structure, the team extracted age-adjusted mortality rates (AAMRs) per 100,000 person-years for heat-related deaths.

For the analysis, they used joinpoint regression to assess trends in AAMRs and evaluate the “elbow points” when trends began to change to a new trajectory. These results were demonstrated as an average annual percentage change (AAPC) in rates with 95% CIs.

Overall, 21,518 deaths were recorded as heat-underlying or contributing causes of death from 1999 to 2023. This change represented an AAMR of 0.26 per 100,000 person-years (95% CI, 0.21–0.27).

In the study period, heat-related deaths increased from 1069 (AAMR, 0.38; 95% CI, 0.36–0.40) in 1999 to 2325 (AAMR, 0.62; 95% CI, 0.60–0.65) in 2023. These data represented a 117% increase in heat-related deaths and a 63% increase in the AAMR.

Specifically, 2004 marked the lowest number of heat-related deaths (n = 311), while 2023 had the highest number of heat-related deaths (n = 2325) during the study period.

After performing the joinpoint trend analysis, the AAMR increased by 3.6% per year (AAPC, 3.6; 95% CI, 0.1–7.2; P = .04) from 1999 to 2023. Overall heat-related deaths and the AAMR exhibited year-to-year variability but showed a level increase after 2016.

Further results from the analysis revealed a non-significant 1.4% decrease per year from 1999 to 2016 (AAPC, –1.4%; 95% CI, –4.7% to 2.1%; P = .42) in deaths, followed by a significant increase of 16.8% per year from 2016 to 2023 (AAPC, 16.8%; 95% CI, 6.4–28.2; P = .002).

In their conclusion, Howard and colleagues indicated these data remained consistent with site-specific data assessed in a global study, pointing to a rise in heat-related mortality worldwide.

“As temperatures continue to rise because of climate change, the recent increasing trend is likely to continue,” they wrote. “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.”

References

1. _{Howard JT, Androne N, Alcover KC, Santos-Lozada AR. Trends of Heat-Related Deaths in the US, 1999-2023. JAMA. Published online August 26, 2024. doi:10.1001/jama.2024.16386}
2. _{Lüthi S, Fairless C, Fischer EM, et al. Rapid increase in the risk of heat-related mortality. Nat Commun. 2023;14(1):4894. Published 2023 Aug 24. doi:10.1038/s41467-023-40599-x}
3. _{National Oceanic and Atmospheric Administration. 2023 Was the world’s warmest year on record, by far. Published January 12, 2024. Accessed August 28, 2024. https://www.noaa.gov/news/2023-was-worlds-warmest-year-on-record-by-far}