From 1999 to 2023, Heat-Related Deaths in the United States More Than Doubled. Between 2016 and 2023, the Age-Adjusted Mortality Rate Increased by 16.8% Per Year. This Is Not a Slow Trend. It Is an Acceleration.

The Fourth of July weekend is one of the most dangerous of the year for heat exposure in the United States. Outdoor events, alcohol consumption, prolonged sun exposure, and physical exertion combine with summer peak temperatures to create conditions that are consistently associated with heat illness and death, particularly among older adults, outdoor workers, and people without access to air conditioning. But the risk of heat-related death in America is not simply a summer holiday problem. It is a structural and accelerating public health crisis, and this research letter published in JAMA by researchers at the University of Texas San Antonio, the Uniformed Services University of the Health Sciences, and Pennsylvania State University is one of the most methodologically rigorous characterizations of that acceleration to date. The analysis uses the same national mortality data infrastructure and Joinpoint regression methodology used in federal cancer surveillance, bringing a level of analytical precision to heat mortality trends that has rarely been applied to this cause of death. It was published in August 2024, examining data through 2023, the warmest year on record globally since 1850.

Howard et al. analyzed all deaths from 1999 to 2023 in which the International Classification of Diseases, 10th Revision (ICD-10) codes P81, environmental hyperthermia of newborn; T67, effects of heat and light; or X30, exposure to excessive natural heat, appeared as either the underlying or contributing cause of death, as recorded in the Multiple Cause of Death file accessed through the CDC WONDER platform. Age-adjusted mortality rates (AAMRs) per 100,000 person-years were calculated for the full study period and for demographic subgroups. Joinpoint regression analysis was used to identify statistically significant inflection points in the AAMR trend and to calculate the average annual percentage change (AAPC) for each identified segment.

From 1999 to 2023, a total of 21,518 deaths were recorded as heat-related in the United States, with an overall AAMR of 0.26 per 100,000 person-years. The number of annual heat-related deaths more than doubled during this period, rising from 1,069 in 1999 to 2,325 in 2023. Joinpoint analysis identified two distinct trend segments. From 1999 to 2016, the AAMR showed a nonsignificant decrease of 1.4% per year (AAPC −1.4%, 95% CI −4.7% to 2.1%, P = 0.42), with year-to-year variability including spikes in 2006 and 2011. From 2016 to 2023, the AAMR increased by a statistically significant 16.8% per year (AAPC 16.8%, 95% CI 6.4% to 28.2%, P = 0.002). The overall AAPC for the full 1999 to 2023 period was 3.6% per year (95% CI 0.1% to 7.2%, P = 0.04).

• —The analysis relies on ICD-10 coded death certificate data, which is subject to reporting variability across certifiers and jurisdictions. Heat-related deaths are known to be systematically undercounted because heat frequently contributes to cardiovascular, respiratory, and renal deaths without being recorded as the primary cause.
• —The study uses a research letter format, which limits the depth of subgroup analysis and methodological detail that can be presented. Demographic breakdowns by race, ethnicity, age group, and geography are referenced but not fully elaborated in the main paper.
• —Joinpoint regression identifies inflection points in a trend but cannot establish causation. The association between the post-2016 acceleration and intensifying climate conditions is plausible and supported by external evidence but not directly established by this analysis.
• —The study period ends at 2023. 2024 data, which would reflect what was described as an even more extreme heat year globally, is not included.

Heat is killing more Americans every year, and the rate at which deaths are increasing has accelerated sharply since 2016. The Fourth of July falls squarely within the peak of heat mortality season. The people most at risk are not randomly distributed. They are older, lower income, outdoor workers, and residents of rapidly warming regions without adequate infrastructure to manage extreme heat. The United States has invested heavily in road safety, vehicle safety standards, and emergency cardiac care, and mortality from those causes has fallen substantially over the same period this paper covers. Heat mortality has gone in the opposite direction. The tools to reduce it exist. The political will to deploy them at scale has not kept pace with the acceleration in deaths.